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A high-speed gated baseline restorer and its implementation in a blood cell volume analyser
PII: S1350-4533(96)00061-6
ELSEVIER
A high-speed gated baseline restorer and its implementation in a blood cell volume analyser E. W. Abel*,
S. In@
*, Y. Liu*, J. J. F. Belch+ and A. Koop$
“School of Biomedical Engineering, University of Dundee, Dundee, UK; tDepartment of Medicine, Ninewells Hospital & Medical School, Dundee, UK; TDepartment of Medical Physics, Leicester Royal Infirmary, Leicester, UK
ABSTRACT Arrumte measurement of the height of individual fn~1se.rin a ,signal that is a.~.-coupled is dependent uj~on rorrerling a changing baseline to a constant value. A new fast baseline restorer, using internally triggered gated feedback with gain, has been designed and a version optimized Jor Gaussian-shaped pulses. Prrfbrmancu is dependent on the mrqnitudes of both thrfilard and the feedback gains. The design presented can achieve a 0.5% acrumcv in pul,tr height for a minimum pulse width of 2.5 p r or j&r 5 ps overla&tGng pulses as long as the overlap i.5.1ec.cthan 50 70. 7jlpiral applications for the baseline restorer include blood rell volume measurement .systems,scintillation rountetx and radiation spectroscopy. 0 1997 l
Baseline
Med.
Phys..
Eng.
restorer, 1997.
Vol.
flow
cytometry,
19, 267-272,
pulse
analysis
April
1. INTRODUCTION A.C.-coupled amplifier systems in pulsed signal applications suffer from baseline variations, which cause problems for accurate measurement of pulse amplitude. Baseline restorers are designed to restore the baseline of an a.c.-coupled signal to an arbitrary d.c. level. This enables pulse height to be measured, which is required for applications such as blood cell-sizing and electrocardiography. For bipolar signals, such as EEG, the baseline is relatively well-defined, and restoration using a simple high-pass filter is sufficient. However, for monopolar pulsed signals, the baseline varies according to the magnitude of the pulses, and a more complex baseline restoration technique must be used to restore it to a constant value. The baseline restorer acts as an adaptive nonlinear filter that operates according to the slew rate of a signal such that it affects the varying baseline, not tvhe pulse itself’. Examples of’thi wide uses for baseline restoration include electrocardiogranhv’,“, radiation snectrosconv”. the restorat& *of frame synchro‘nization bulses in televisions-‘, optical studies of reactive chemicals in stopped flow apparatus and pulse amplifier stabilization in scintillation counters incorporating photomultiplier tubes”. An important use for baseline restoration is in electronic flow cytometry for blood cell-sizing, in which cells are categorized (:orrespor~Ir~~~ 10:E. W. Ahrl.
height
into different populations according to their volume, which is *reiated to the height-of the pulses generated as cells flow through the orifice of the cytometer. The ideal baseline restorer would act fast enough to restore the baseline at or immediately after the end of the occurrence of a pulse. This would enable the height of the hext pulse to be measured accurately without having to ensure a minimum interval between pulses, allowing its use in situations where pulse intervals cannot be readily controlled, such as flow cytometers. This work demonstrates the limitations in performance of existing baseline restorers and describes the design and optimization of a new technique that comes close to meeting the ideal requirements. The performance of a working system, designed for use in flow cytometric sizing of blood platelets, is illustrated. 2. BASELINE RESTORATION TECHNIQUES AND PERFORMANCE LIMITATIONS The fastest baseline restorers for monopolar signals have usually been based on a technique developed by Robinson 7. The basic configuration is illustrated in I;igure I(a). When no signal is applied, each diode conducts, and the output voltage is the same as the reference voltage. Input nulses larger than the diode breakover voltage Krause Dl”to become non-conducting and thee capacitor C discharges with a current 1,-I, during
A high-speed baseline restomr: E. W. Abel et al.
Figure I (a) had a restoration time of 50 ps for a 1% error in the baseline and less than 1% error in pulse amplitude, while that in Figure I(b) took 25 ps. The faster of these restorers therefore would require a minimum pulse mark:space ratio of 1:2.5 to restore a lo-ps pulse, which is inadequate for many applications, including flow cytometry. The restoration time was found to be highly dependent upon the value of capacitor Cl, which needs to be large enough to allow the steady state baseline voltage to be effectively constant yet small enough to allow the circuit to return to its quiescent state quickly after each pulse. These two conflicting requirements limit the performance of this type of restorer.
+V 11 + 6
II =o-, DlIr
Buffer
AL1
D2~r
3. DEVELOPMENT RESTORER
” 0 u
lnverter
D3TW
AL2
D4lr
0
y-
D5w
Vout
Clipping Circuit
=@-J
12
-v
of the Robinson restorer’. (b) ModifiFiie 1 (a) Configuration cation with amplifiers and limiters (AL1 and AL2) by De B&chop” for use in a particle counter
the pulse. After pulse termination the output voltage returns to the baseline and the capacitor is recharged. The baseline offset is dependent upon the ratio of I2 to I12.‘. A blood cell-sizing apparatus, developed by Taylor*, incorporated a Robinson restorer but could only restore the signal for count rates lower than 100 per second which is only about l-5% the count rate of modern cellsizing systems. Small refinements to the Robinson technique have been described’, the main advance being the incorporation of amplification stages with limiters’ to increase the speed of restoration and the recovery time when the amplifier saturates, as shown in Fipre I(b). Here, the amplifiers are configured to produce near-ideal diode characteristics, including the removal of their inherent voltage drop. The output clipping circuit is used to remove negative undershoots that occur during the restoration process. The above baseline restorers were considered for use in a flow cytometer system currently being developed for blood cell volume measurement. Their performance has not been fully reported previously, so the baseline restoration times of the two circuits in Figure 2 were analysed using a circuit simulator (PSPICE, MicroSim Corp., Irvine, CA, USA). The pulse used was Gaussian-like in shape and created from a half-rectified sine wave (20 kHz, 25-p.s pulse width), low-pass-filtered and with a negative step of 35% of the amplitude of the sine wave superimposed at the end of the pulse, simulating an instantaneous. change in the baseline. The simulation used electronic components with ideal characteristics. The restorer in
268
OF THE
NEW BASELINE
The new baseline restorer proposed here is a development of a gated feedback technique that has been used for the suppression of high-amplitude, low-frequency interference”*5J0, shown schematically in Figure 2(a). This uses an external trigger, such as a photosensitive transistor device, to sense the start of the pulse. The signal is fed back only if the external trigger voltage is below a set threshold voltage, so that it operates only on the changing baseline and undershoot of the pulse. The feedback signal is amplified by a signal-conditioning stage (K), designed to increase the speed of restoration. This sta e has, in previous designs, included integration B, digital feedback5 and nonlinear amplification3. During the occurrence of the pulse itself, the feedback loop is open circuit, and no restoration occurs. The gated feedback restorer does not suffer from the capacitor problems of the Robinson type. Many applications, such as electronic flow cytometers, do not have access to an external trigger, so the gate must be triggered by the pulse itself.
External Trigger I
-I/
-1
Vin
HP Filter
+ t-$-J-+ Summing
Gain G
-
Junction
Threshold
Q+Y--EF
Gain H Figrue 2 (b) Block triggering
LP
Filter Switch SW
(a) Basic gated baseline restorer with external triggering. diagram of the new gated baseline restorer with internal
The new baseline restoration technique proposed here, shown schematically in Figure Z(b), uses a high-speed internally triggered gate in the feedback loop and incorporates feedback gain to restore the pulse quicker than would be possible without amplification. The signal containing the changing baseline is initially high-pass-filtered, to remove the d.c. level and unwanted low frequencies, shifting the signal to a zero average level. The signal is then amplified by a factor C, and the output Fated into the feedback loop, via an analogue switch SW, when its value is lower than that of a preset threshold, i.e. when it approaches the region of the baseline. The gain, G, is incorporated into the restorer to provide faster restoration. The gated signal is appropriately low-pass-filtered to leave only the frequencies associated with the changing baseline. The filter is placed after the switch to remove the high-frequency discontinuities that occur due to switching. The feedback signal is then amplified by a factor H and subtracted from the original signal to restore it to the desired baseline. When the signal rises above the threshold, the switch SW is opened and a zero level is fed back to the subtractor stage. The threshold is set slightly above the zero level in case of any superimposed noise in the signal, but is well below the level of the smallest pulse in the range of interest, to ensure that the amplitude of the pulse is unaffected by the restorer. At the output of the main amplifier, the signal is amplified by a factor of 1 /C. restoring the original pulse height. The complete restorer therefore can be placed into an existing circuit to restore the baseline without affecting the amplitude or offset of the signal
4. DESIGN
OPTIMIZATION
The main parameters influencing the performance of the restorer in Fjprp Z(b) are the cut-off frequency, the order and type of the high-pass (HP) and low-pass (LP) filters used, the gain C; and the gain H. The effect of changing these parameters was modelled using MATLAB (Mathworks Inc., Natick, MA, USA) and its block diagram simulation toolbox SIMULINK. Optimum values of each parameter were selected to minimize restoration time. The test pulse selected was the same as for the earlier simulation. The Gaussian-like shape was selected because this shape of pulse increases .gradually from the baseline and therefore requires more careful selection of cut-off frequencies than square or triangular pulses, which have well-defined spectral components. The model tested the effects of using two different types of filter: the Butterworth filter for its good amplitude response and the Bessel fitter for its linear phase characteristics. Tub& 1 illustrates how varying each filter parameter affects the restoration time and shows how the progressive selection of the best values for each parameter results in a reduction of the restoration time of the baseline restorer. The initial filter parameters used were a HP cL[t-off frequencv of 600 Hz, a first
order LP cut-off frequency of 800 Hz, a gain (3, of 10 and unity feedback gain. The best parameters for the high-pass filter were selected first acd, using these, the best parameters for the lowpass filter, the gain and the number of feedback loops were selected and updated into the model sequentially, moving from left to right in Table 1, resulting in a progressively shorter restoration time. The restoration times (To,,,,) given are the times taken to restore the baseline, and hence the peak height of the following pulse, to less than a 0.5% error in its final value, representing an S-bit accuracy of measurement. The optimal parameter value for each step in the process is shown in italics in the table. The Bessel filter restores the signal to the baseline faster than the Butterworth filter, the inferior performance of the Butterworth filter being due to overshoot and oscillations in the restored signal. The oscillation is accentuated by incorporating higher-order filters. A HP filter cut-off frequency (f;) of about 600 Hz was found to be optimal for the test pulse. At this frequency, the filter restores the baseline with less than 0.5% error in peak height. Increasing J attenuates the low frequency components of the signal, gradually reducing pulse height, while decreasing J; attenuates less of the baseline frequencies, which increases restoration time. A first-order LP filter gave the best performance, higher order filters in the feedback loop increasing the restoration time dramatically by driving the restorer into oscillation. It is’important to keep JI for the LP filter higher than that for the HP filter, but, not so high that the pulse height decreases as a result of’ frequency components in the pulse itself being fed back and removed by subtraction. This effect produces an increased error (u,, in Tabk I) in the pulse height. The optimal cut-off’ frequencies are, of course, tailored to the shape and frequency of the pulse and can be determined from a spectral analysis of the pulse and a knowledge of the characteristics of the Bessel filter.
5. CIRCUIT
DESCRIPTION
The selected parameter values from the simulation process have been implemented into an electronic circuit for the baseline restorer circuit, which is shown in F@TP 3. High-speed video amplifiers are used for all the operational amplifiers in the forward branch of the circuit. The inverting amplifier configuration provides signal linearity and stability during subsequent amplification. The double pole changeover switch for the gate is configured from a DG403 high-speed analogue switch (IC7). The high-pass filter is followed by an inverting summing amplifier, IC2, with a variable gain set by potentiometer Pl. The output of IC2 is gated into the feedback circuit and compared with a variable threshold level, set by potentiometer P5, to take into account any slight deviation of the baseline above the zero level. The AD790 comparator, IC8. was chosen for its fast response time of 45 ns, its wide input voltage range of fl.5 V. its driving caparily and a suitable
269
A high-speed baseline restorer: E. W. Abel et al. Table 1 Progressive reduction the simulated restorer
in restoration
G-5, W) Bessel/2 Butter/2 Bessel/4 Butter/4 Bessel/8 Butter/8
60 1360 680 1050 560 1580
times
T 0 ._5S by optimal
selection
of filter
parameters,
LPorder
7&% (PS)
LPf, (Hz)
7;!i% (PS)
es (%)
1
60 1310 2500
200 400 600 800
2210 1460 1320 60 56 55
0.08 0.09
200 400 600 800
350 180 60 980
1000 1200
985
1000
735
1200
2 3
2K6
forward
gain (G) and feedback
Gain ((3
7; 5cS, (P)
10
60 30 17
25 50
0.1 0.11
100
0.5
1
(H)
in
Gain (H) 1
II Unstable
1000
gain
8 4 5 3 1.85 1.65
2 3 4
5 6
1K
Pl
HA3-2641-5
115v1 5v+ r 10K BZY60
-l-r 15’BZY66
1 P4
IOK
,
= 112
10K
+/ 5Vl
/+ ’
-
1OK ’ BZY66
15’+ 5Vl
1
,[fF6
DG403
1 &
*
15v-
15v-1-L
Y
=
Figure
3
Circuit
for the new baseline
restorer
switching level to operate the analogue switches. The comparator alters the state of the gate via the control line. When a +5-V level is applied to the control line of IC7, the baseline of the signal is fed back to the summing junction of IC2. This feedback signal is low-pass-filtered and amplified by the inverting am lifier, IC5, with a variable gain set by P3. An o Ffset adjustment is incorporated into IC3 to adjust out any d.c. offsets that may be present at the IC2 gain stage that could drive the baseline above the threshold level of the comparator. Amplifier IC3 has an inverting configuration to correct the voltage sign and level after the summing amplifier IC2. The EL2044C video amplifier used for IC2 and IC3 provides sufficient driving current to IC2 to ensure rapid baseline restoration. At the output of IC2, the signal is inverted and attenuated as required, to correct the previous inversion and to obtain the desired pulse height. The negative pulse undershoot produced during restoration is substantially removed by the clipping diode Dl at the output of IC3. The performance of the prototype baseline restorer was tested with a low-amplitude, Gaussianshaped pulse, as used in the earlier simulations.
270
The output signal was monitored using a computer based storage oscilloscope (PC99, Amplicon Ltd, Brighton, UK). The analysis was performed using feedback gains from H=l to H=6. The effect of varying G and His shown in Figure 4 for both the actual restorer and the computer simulation. The curves are very similar, confirming the validity of the model and its use for
,
of 0
Fiie4 model W)
1
Comparison of the baseline
2 3 4 Feedback Gain (H) of restoration restorer (0)
5
6
times for the mathematical and the final electronic circuit
selecting the filter parameters. The difference at low gains is attributed to the fact that the restoration time is very sensitive to small changes in gain, so the tolerances in values of the passive components used in the amplifiers would be expected to produce an error in the predicted performance. The minimum pulse width for less than a 0.5% error in both the baseline and pulse height was 2.5 ps for a pulse mark:space ratio of 1:l. By decreasing the space between pulses, even by overlapping them, it was found that this accuracy could be maintained as long as the interval between pulse peaks was no less than 5 ps, i.e. a minimum pulse width of 5 pus. Accuracy was nearly independent of the pulse width for pulses as long as about 120 /.Ls, above which the filter cut-off frequencies used were no longer suitable for the test pulse. Finally, in order to confirm the operation of the baseline restorer on real data, it was implemented with a Coulter counter (a Thrombocytometel Model C with a modified preamplifier to increase its bandwidth to 140 kHz). The gain, G, was limited to 25 to prevent saturation due to the magnitude of the output signal from the instrument. Usin.g &~re 5, a feedback gain of 10 was selected to give a restoration time of about 4.2 ~LS for a pube height error of 0.5%. A sample of the raw and restored signals from Coulter B31 latex calibration particles particles (2.08 ,um mean diameter, 4.71 fl volume) is shown in Figuw 6. 6. DISCUSSION I;igur~ 5 demonstrates that, by implementing feedback gain, it is possible to obtain faster restoration times than would be possible with unity gain. It also shows that the feedback gain can be selected to suit the maximum desired or usable forward gain in a particular application, as was done for the restorer in the Coulter counter system, where the maximum gain without amplifier saturation was 35. In situations when the signal gain must be restricted to prevent an amplifier going into saturation, the potential loss of performance can therefore be overcome by increasing the feedback gain. For example, to obtain a restoration time of 3.5 ps with 1% error in the pulse height, it would
0-I 0
0.5 Time
6
z4 8 cz >02
r F
0, 0 Figure 6 particles lmrestorrd
1 (ms)
Examples flowing signal.
0.5 Time
, 1 (ms)
of signals obtained from through a Coulter flow (b) An unrestorrd signal
B31 latex c-ytomrter.
calibration (a)
An
be equally satisfactory to use a forward signal gain C~l00 and a feedback gain H=3, or f+3 and H=lOO. This versatility of using two linear gain stages, as opposed to the variety of different signal conditioning stages used in earlier designs, allows the designer to tailor the restorer to the minimum pulse width and pulse:space ratio required for a particular application. There are practical limitations to the gain selection in that increasing the overall gain higher than about 600 will progressively introduce an offset to the restored signal. Figures 4 and 5 show clearly that increasing the GH product gives only small improvements at high gain values, while the improvements made at lower gains are more substantial. The mam factors limiting the operational bandwidth of the system are the speed of the comparator and the analogue switch, so restoration times could be reduced if required by incorporating faster components. Decreasing the filter cut-off frequencies of the low- and high-pass filters will allow an increase in the maximum operational pulse width, but this should be done with a knowledge of the spectral content of the pulse.
1 100
200
300
400
500
600
GxH Figure 5 Restoration of the final baseline mathematical model
times to reach within for diffcrcnt gain-feedback of the baseline restorer
0.1% (0) and loop products
1%
(0) in the
7. CONCLUSION This new baseline restorer employs internally gated feedback to give fast restoration times without the need to trigger the feedback element from
271
A high-speed baseline restorer: E. W Abel et al.
an external source. Restoration times of 2.5 ps and a pulse height accuracy of 0.5% can be achieved. Overlapping of pulses does not adversely affect pulse height accuracy, provided the interval between the peaks of adjacent pulses is greater than 5 us. The linear forward and feedback gain stages can be set to suit the signal gain requirements, to provide the required restoration time for a specified interval between pulses. Filter parameters can be optimized for the frequency and shape of the pulses being analysed. The result is a baseline restorer with a much improved performance over previously reported methods and which is not limited by circuit design, only by the speed of the devices used. The restorer gives a high pulse-height accuracy with a short restoration .time, which makes it suitable for many high-speed pulse-height measurement systems. ACKNOWLEDGEMENTS The authors wish to express their thanks to the Sir Samuel Scott of Yews Trust for their financial support for this work. REFERENCES 1. Henry, Electronic
272
P., Baseline Design
News,
restorer is voltage 1989, 33(l), 191.
programmable.
2. Arbel, A. F., Analog Signal Processing and Instrumentation. Cambridge University Press, Cambridge, 1980. 3. Ohkawa, S. and Husimi, K., Elimination of a low frequency large amplitude noise using the improved baseline restorer employing gain switching. I&!% Trunsactions on Nuclear Science, 1986, 33(l), 415-418. 4. Patchett, G. N., Television (colour & monochrome) Part III. Norman Price Ltd, Romney, Kent, 1974, p. 140. 5. Grudic, M., Gated baseline restorer without ‘droop’. Reviau of Scientz$c Instruments, 1987, 58(6), 1104-l 105. 6. Patzelt, R., Improved baseline stabilization for pulse amplifiers. Nuclear Instruments and Methods, 1968, 59, 283-288. 7. Robinson, L. B., Reduction of baseline shift in pulse amplitude measurements. Review of Scientijic Instruments, 1961, 34, 1057. 8. Taylor, W. B., A versatile cell detector for cell volume measurements. Medical & Biological l%@zeering, 1970, 8, 281-290. 9. De Bisschop, F., Lambert, H. and Demey, G., Improved electronic gate technique, for particle counting and sizing in liquids. Medical & Biological Engkeering &? Cornputing, 1991, 29(6), 49-54. 10. Kuwata, M., Maeda, H. and Husimi, K., New baseline restorer based on feedforward differential compensation. ILEE Transactions on Nuclear Science, 1994, 41(4), 12361239.
ELSEVIER
A high-speed gated baseline restorer and its implementation in a blood cell volume analyser E. W. Abel*,
S. In@
*, Y. Liu*, J. J. F. Belch+ and A. Koop$
“School of Biomedical Engineering, University of Dundee, Dundee, UK; tDepartment of Medicine, Ninewells Hospital & Medical School, Dundee, UK; TDepartment of Medical Physics, Leicester Royal Infirmary, Leicester, UK
ABSTRACT Arrumte measurement of the height of individual fn~1se.rin a ,signal that is a.~.-coupled is dependent uj~on rorrerling a changing baseline to a constant value. A new fast baseline restorer, using internally triggered gated feedback with gain, has been designed and a version optimized Jor Gaussian-shaped pulses. Prrfbrmancu is dependent on the mrqnitudes of both thrfilard and the feedback gains. The design presented can achieve a 0.5% acrumcv in pul,tr height for a minimum pulse width of 2.5 p r or j&r 5 ps overla&tGng pulses as long as the overlap i.5.1ec.cthan 50 70. 7jlpiral applications for the baseline restorer include blood rell volume measurement .systems,scintillation rountetx and radiation spectroscopy. 0 1997 l
Baseline
Med.
Phys..
Eng.
restorer, 1997.
Vol.
flow
cytometry,
19, 267-272,
pulse
analysis
April
1. INTRODUCTION A.C.-coupled amplifier systems in pulsed signal applications suffer from baseline variations, which cause problems for accurate measurement of pulse amplitude. Baseline restorers are designed to restore the baseline of an a.c.-coupled signal to an arbitrary d.c. level. This enables pulse height to be measured, which is required for applications such as blood cell-sizing and electrocardiography. For bipolar signals, such as EEG, the baseline is relatively well-defined, and restoration using a simple high-pass filter is sufficient. However, for monopolar pulsed signals, the baseline varies according to the magnitude of the pulses, and a more complex baseline restoration technique must be used to restore it to a constant value. The baseline restorer acts as an adaptive nonlinear filter that operates according to the slew rate of a signal such that it affects the varying baseline, not tvhe pulse itself’. Examples of’thi wide uses for baseline restoration include electrocardiogranhv’,“, radiation snectrosconv”. the restorat& *of frame synchro‘nization bulses in televisions-‘, optical studies of reactive chemicals in stopped flow apparatus and pulse amplifier stabilization in scintillation counters incorporating photomultiplier tubes”. An important use for baseline restoration is in electronic flow cytometry for blood cell-sizing, in which cells are categorized (:orrespor~Ir~~~ 10:E. W. Ahrl.
height
into different populations according to their volume, which is *reiated to the height-of the pulses generated as cells flow through the orifice of the cytometer. The ideal baseline restorer would act fast enough to restore the baseline at or immediately after the end of the occurrence of a pulse. This would enable the height of the hext pulse to be measured accurately without having to ensure a minimum interval between pulses, allowing its use in situations where pulse intervals cannot be readily controlled, such as flow cytometers. This work demonstrates the limitations in performance of existing baseline restorers and describes the design and optimization of a new technique that comes close to meeting the ideal requirements. The performance of a working system, designed for use in flow cytometric sizing of blood platelets, is illustrated. 2. BASELINE RESTORATION TECHNIQUES AND PERFORMANCE LIMITATIONS The fastest baseline restorers for monopolar signals have usually been based on a technique developed by Robinson 7. The basic configuration is illustrated in I;igure I(a). When no signal is applied, each diode conducts, and the output voltage is the same as the reference voltage. Input nulses larger than the diode breakover voltage Krause Dl”to become non-conducting and thee capacitor C discharges with a current 1,-I, during
A high-speed baseline restomr: E. W. Abel et al.
Figure I (a) had a restoration time of 50 ps for a 1% error in the baseline and less than 1% error in pulse amplitude, while that in Figure I(b) took 25 ps. The faster of these restorers therefore would require a minimum pulse mark:space ratio of 1:2.5 to restore a lo-ps pulse, which is inadequate for many applications, including flow cytometry. The restoration time was found to be highly dependent upon the value of capacitor Cl, which needs to be large enough to allow the steady state baseline voltage to be effectively constant yet small enough to allow the circuit to return to its quiescent state quickly after each pulse. These two conflicting requirements limit the performance of this type of restorer.
+V 11 + 6
II =o-, DlIr
Buffer
AL1
D2~r
3. DEVELOPMENT RESTORER
” 0 u
lnverter
D3TW
AL2
D4lr
0
y-
D5w
Vout
Clipping Circuit
=@-J
12
-v
of the Robinson restorer’. (b) ModifiFiie 1 (a) Configuration cation with amplifiers and limiters (AL1 and AL2) by De B&chop” for use in a particle counter
the pulse. After pulse termination the output voltage returns to the baseline and the capacitor is recharged. The baseline offset is dependent upon the ratio of I2 to I12.‘. A blood cell-sizing apparatus, developed by Taylor*, incorporated a Robinson restorer but could only restore the signal for count rates lower than 100 per second which is only about l-5% the count rate of modern cellsizing systems. Small refinements to the Robinson technique have been described’, the main advance being the incorporation of amplification stages with limiters’ to increase the speed of restoration and the recovery time when the amplifier saturates, as shown in Fipre I(b). Here, the amplifiers are configured to produce near-ideal diode characteristics, including the removal of their inherent voltage drop. The output clipping circuit is used to remove negative undershoots that occur during the restoration process. The above baseline restorers were considered for use in a flow cytometer system currently being developed for blood cell volume measurement. Their performance has not been fully reported previously, so the baseline restoration times of the two circuits in Figure 2 were analysed using a circuit simulator (PSPICE, MicroSim Corp., Irvine, CA, USA). The pulse used was Gaussian-like in shape and created from a half-rectified sine wave (20 kHz, 25-p.s pulse width), low-pass-filtered and with a negative step of 35% of the amplitude of the sine wave superimposed at the end of the pulse, simulating an instantaneous. change in the baseline. The simulation used electronic components with ideal characteristics. The restorer in
268
OF THE
NEW BASELINE
The new baseline restorer proposed here is a development of a gated feedback technique that has been used for the suppression of high-amplitude, low-frequency interference”*5J0, shown schematically in Figure 2(a). This uses an external trigger, such as a photosensitive transistor device, to sense the start of the pulse. The signal is fed back only if the external trigger voltage is below a set threshold voltage, so that it operates only on the changing baseline and undershoot of the pulse. The feedback signal is amplified by a signal-conditioning stage (K), designed to increase the speed of restoration. This sta e has, in previous designs, included integration B, digital feedback5 and nonlinear amplification3. During the occurrence of the pulse itself, the feedback loop is open circuit, and no restoration occurs. The gated feedback restorer does not suffer from the capacitor problems of the Robinson type. Many applications, such as electronic flow cytometers, do not have access to an external trigger, so the gate must be triggered by the pulse itself.
External Trigger I
-I/
-1
Vin
HP Filter
+ t-$-J-+ Summing
Gain G
-
Junction
Threshold
Q+Y--EF
Gain H Figrue 2 (b) Block triggering
LP
Filter Switch SW
(a) Basic gated baseline restorer with external triggering. diagram of the new gated baseline restorer with internal
The new baseline restoration technique proposed here, shown schematically in Figure Z(b), uses a high-speed internally triggered gate in the feedback loop and incorporates feedback gain to restore the pulse quicker than would be possible without amplification. The signal containing the changing baseline is initially high-pass-filtered, to remove the d.c. level and unwanted low frequencies, shifting the signal to a zero average level. The signal is then amplified by a factor C, and the output Fated into the feedback loop, via an analogue switch SW, when its value is lower than that of a preset threshold, i.e. when it approaches the region of the baseline. The gain, G, is incorporated into the restorer to provide faster restoration. The gated signal is appropriately low-pass-filtered to leave only the frequencies associated with the changing baseline. The filter is placed after the switch to remove the high-frequency discontinuities that occur due to switching. The feedback signal is then amplified by a factor H and subtracted from the original signal to restore it to the desired baseline. When the signal rises above the threshold, the switch SW is opened and a zero level is fed back to the subtractor stage. The threshold is set slightly above the zero level in case of any superimposed noise in the signal, but is well below the level of the smallest pulse in the range of interest, to ensure that the amplitude of the pulse is unaffected by the restorer. At the output of the main amplifier, the signal is amplified by a factor of 1 /C. restoring the original pulse height. The complete restorer therefore can be placed into an existing circuit to restore the baseline without affecting the amplitude or offset of the signal
4. DESIGN
OPTIMIZATION
The main parameters influencing the performance of the restorer in Fjprp Z(b) are the cut-off frequency, the order and type of the high-pass (HP) and low-pass (LP) filters used, the gain C; and the gain H. The effect of changing these parameters was modelled using MATLAB (Mathworks Inc., Natick, MA, USA) and its block diagram simulation toolbox SIMULINK. Optimum values of each parameter were selected to minimize restoration time. The test pulse selected was the same as for the earlier simulation. The Gaussian-like shape was selected because this shape of pulse increases .gradually from the baseline and therefore requires more careful selection of cut-off frequencies than square or triangular pulses, which have well-defined spectral components. The model tested the effects of using two different types of filter: the Butterworth filter for its good amplitude response and the Bessel fitter for its linear phase characteristics. Tub& 1 illustrates how varying each filter parameter affects the restoration time and shows how the progressive selection of the best values for each parameter results in a reduction of the restoration time of the baseline restorer. The initial filter parameters used were a HP cL[t-off frequencv of 600 Hz, a first
order LP cut-off frequency of 800 Hz, a gain (3, of 10 and unity feedback gain. The best parameters for the high-pass filter were selected first acd, using these, the best parameters for the lowpass filter, the gain and the number of feedback loops were selected and updated into the model sequentially, moving from left to right in Table 1, resulting in a progressively shorter restoration time. The restoration times (To,,,,) given are the times taken to restore the baseline, and hence the peak height of the following pulse, to less than a 0.5% error in its final value, representing an S-bit accuracy of measurement. The optimal parameter value for each step in the process is shown in italics in the table. The Bessel filter restores the signal to the baseline faster than the Butterworth filter, the inferior performance of the Butterworth filter being due to overshoot and oscillations in the restored signal. The oscillation is accentuated by incorporating higher-order filters. A HP filter cut-off frequency (f;) of about 600 Hz was found to be optimal for the test pulse. At this frequency, the filter restores the baseline with less than 0.5% error in peak height. Increasing J attenuates the low frequency components of the signal, gradually reducing pulse height, while decreasing J; attenuates less of the baseline frequencies, which increases restoration time. A first-order LP filter gave the best performance, higher order filters in the feedback loop increasing the restoration time dramatically by driving the restorer into oscillation. It is’important to keep JI for the LP filter higher than that for the HP filter, but, not so high that the pulse height decreases as a result of’ frequency components in the pulse itself being fed back and removed by subtraction. This effect produces an increased error (u,, in Tabk I) in the pulse height. The optimal cut-off’ frequencies are, of course, tailored to the shape and frequency of the pulse and can be determined from a spectral analysis of the pulse and a knowledge of the characteristics of the Bessel filter.
5. CIRCUIT
DESCRIPTION
The selected parameter values from the simulation process have been implemented into an electronic circuit for the baseline restorer circuit, which is shown in F@TP 3. High-speed video amplifiers are used for all the operational amplifiers in the forward branch of the circuit. The inverting amplifier configuration provides signal linearity and stability during subsequent amplification. The double pole changeover switch for the gate is configured from a DG403 high-speed analogue switch (IC7). The high-pass filter is followed by an inverting summing amplifier, IC2, with a variable gain set by potentiometer Pl. The output of IC2 is gated into the feedback circuit and compared with a variable threshold level, set by potentiometer P5, to take into account any slight deviation of the baseline above the zero level. The AD790 comparator, IC8. was chosen for its fast response time of 45 ns, its wide input voltage range of fl.5 V. its driving caparily and a suitable
269
A high-speed baseline restorer: E. W. Abel et al. Table 1 Progressive reduction the simulated restorer
in restoration
G-5, W) Bessel/2 Butter/2 Bessel/4 Butter/4 Bessel/8 Butter/8
60 1360 680 1050 560 1580
times
T 0 ._5S by optimal
selection
of filter
parameters,
LPorder
7&% (PS)
LPf, (Hz)
7;!i% (PS)
es (%)
1
60 1310 2500
200 400 600 800
2210 1460 1320 60 56 55
0.08 0.09
200 400 600 800
350 180 60 980
1000 1200
985
1000
735
1200
2 3
2K6
forward
gain (G) and feedback
Gain ((3
7; 5cS, (P)
10
60 30 17
25 50
0.1 0.11
100
0.5
1
(H)
in
Gain (H) 1
II Unstable
1000
gain
8 4 5 3 1.85 1.65
2 3 4
5 6
1K
Pl
HA3-2641-5
115v1 5v+ r 10K BZY60
-l-r 15’BZY66
1 P4
IOK
,
= 112
10K
+/ 5Vl
/+ ’
-
1OK ’ BZY66
15’+ 5Vl
1
,[fF6
DG403
1 &
*
15v-
15v-1-L
Y
=
Figure
3
Circuit
for the new baseline
restorer
switching level to operate the analogue switches. The comparator alters the state of the gate via the control line. When a +5-V level is applied to the control line of IC7, the baseline of the signal is fed back to the summing junction of IC2. This feedback signal is low-pass-filtered and amplified by the inverting am lifier, IC5, with a variable gain set by P3. An o Ffset adjustment is incorporated into IC3 to adjust out any d.c. offsets that may be present at the IC2 gain stage that could drive the baseline above the threshold level of the comparator. Amplifier IC3 has an inverting configuration to correct the voltage sign and level after the summing amplifier IC2. The EL2044C video amplifier used for IC2 and IC3 provides sufficient driving current to IC2 to ensure rapid baseline restoration. At the output of IC2, the signal is inverted and attenuated as required, to correct the previous inversion and to obtain the desired pulse height. The negative pulse undershoot produced during restoration is substantially removed by the clipping diode Dl at the output of IC3. The performance of the prototype baseline restorer was tested with a low-amplitude, Gaussianshaped pulse, as used in the earlier simulations.
270
The output signal was monitored using a computer based storage oscilloscope (PC99, Amplicon Ltd, Brighton, UK). The analysis was performed using feedback gains from H=l to H=6. The effect of varying G and His shown in Figure 4 for both the actual restorer and the computer simulation. The curves are very similar, confirming the validity of the model and its use for
,
of 0
Fiie4 model W)
1
Comparison of the baseline
2 3 4 Feedback Gain (H) of restoration restorer (0)
5
6
times for the mathematical and the final electronic circuit
selecting the filter parameters. The difference at low gains is attributed to the fact that the restoration time is very sensitive to small changes in gain, so the tolerances in values of the passive components used in the amplifiers would be expected to produce an error in the predicted performance. The minimum pulse width for less than a 0.5% error in both the baseline and pulse height was 2.5 ps for a pulse mark:space ratio of 1:l. By decreasing the space between pulses, even by overlapping them, it was found that this accuracy could be maintained as long as the interval between pulse peaks was no less than 5 ps, i.e. a minimum pulse width of 5 pus. Accuracy was nearly independent of the pulse width for pulses as long as about 120 /.Ls, above which the filter cut-off frequencies used were no longer suitable for the test pulse. Finally, in order to confirm the operation of the baseline restorer on real data, it was implemented with a Coulter counter (a Thrombocytometel Model C with a modified preamplifier to increase its bandwidth to 140 kHz). The gain, G, was limited to 25 to prevent saturation due to the magnitude of the output signal from the instrument. Usin.g &~re 5, a feedback gain of 10 was selected to give a restoration time of about 4.2 ~LS for a pube height error of 0.5%. A sample of the raw and restored signals from Coulter B31 latex calibration particles particles (2.08 ,um mean diameter, 4.71 fl volume) is shown in Figuw 6. 6. DISCUSSION I;igur~ 5 demonstrates that, by implementing feedback gain, it is possible to obtain faster restoration times than would be possible with unity gain. It also shows that the feedback gain can be selected to suit the maximum desired or usable forward gain in a particular application, as was done for the restorer in the Coulter counter system, where the maximum gain without amplifier saturation was 35. In situations when the signal gain must be restricted to prevent an amplifier going into saturation, the potential loss of performance can therefore be overcome by increasing the feedback gain. For example, to obtain a restoration time of 3.5 ps with 1% error in the pulse height, it would
0-I 0
0.5 Time
6
z4 8 cz >02
r F
0, 0 Figure 6 particles lmrestorrd
1 (ms)
Examples flowing signal.
0.5 Time
, 1 (ms)
of signals obtained from through a Coulter flow (b) An unrestorrd signal
B31 latex c-ytomrter.
calibration (a)
An
be equally satisfactory to use a forward signal gain C~l00 and a feedback gain H=3, or f+3 and H=lOO. This versatility of using two linear gain stages, as opposed to the variety of different signal conditioning stages used in earlier designs, allows the designer to tailor the restorer to the minimum pulse width and pulse:space ratio required for a particular application. There are practical limitations to the gain selection in that increasing the overall gain higher than about 600 will progressively introduce an offset to the restored signal. Figures 4 and 5 show clearly that increasing the GH product gives only small improvements at high gain values, while the improvements made at lower gains are more substantial. The mam factors limiting the operational bandwidth of the system are the speed of the comparator and the analogue switch, so restoration times could be reduced if required by incorporating faster components. Decreasing the filter cut-off frequencies of the low- and high-pass filters will allow an increase in the maximum operational pulse width, but this should be done with a knowledge of the spectral content of the pulse.
1 100
200
300
400
500
600
GxH Figure 5 Restoration of the final baseline mathematical model
times to reach within for diffcrcnt gain-feedback of the baseline restorer
0.1% (0) and loop products
1%
(0) in the
7. CONCLUSION This new baseline restorer employs internally gated feedback to give fast restoration times without the need to trigger the feedback element from
271
A high-speed baseline restorer: E. W Abel et al.
an external source. Restoration times of 2.5 ps and a pulse height accuracy of 0.5% can be achieved. Overlapping of pulses does not adversely affect pulse height accuracy, provided the interval between the peaks of adjacent pulses is greater than 5 us. The linear forward and feedback gain stages can be set to suit the signal gain requirements, to provide the required restoration time for a specified interval between pulses. Filter parameters can be optimized for the frequency and shape of the pulses being analysed. The result is a baseline restorer with a much improved performance over previously reported methods and which is not limited by circuit design, only by the speed of the devices used. The restorer gives a high pulse-height accuracy with a short restoration .time, which makes it suitable for many high-speed pulse-height measurement systems. ACKNOWLEDGEMENTS The authors wish to express their thanks to the Sir Samuel Scott of Yews Trust for their financial support for this work. REFERENCES 1. Henry, Electronic
272
P., Baseline Design
News,
restorer is voltage 1989, 33(l), 191.
programmable.
2. Arbel, A. F., Analog Signal Processing and Instrumentation. Cambridge University Press, Cambridge, 1980. 3. Ohkawa, S. and Husimi, K., Elimination of a low frequency large amplitude noise using the improved baseline restorer employing gain switching. I&!% Trunsactions on Nuclear Science, 1986, 33(l), 415-418. 4. Patchett, G. N., Television (colour & monochrome) Part III. Norman Price Ltd, Romney, Kent, 1974, p. 140. 5. Grudic, M., Gated baseline restorer without ‘droop’. Reviau of Scientz$c Instruments, 1987, 58(6), 1104-l 105. 6. Patzelt, R., Improved baseline stabilization for pulse amplifiers. Nuclear Instruments and Methods, 1968, 59, 283-288. 7. Robinson, L. B., Reduction of baseline shift in pulse amplitude measurements. Review of Scientijic Instruments, 1961, 34, 1057. 8. Taylor, W. B., A versatile cell detector for cell volume measurements. Medical & Biological l%@zeering, 1970, 8, 281-290. 9. De Bisschop, F., Lambert, H. and Demey, G., Improved electronic gate technique, for particle counting and sizing in liquids. Medical & Biological Engkeering &? Cornputing, 1991, 29(6), 49-54. 10. Kuwata, M., Maeda, H. and Husimi, K., New baseline restorer based on feedforward differential compensation. ILEE Transactions on Nuclear Science, 1994, 41(4), 12361239.