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A modification of the instantaneous frequency meter for low frequency signals
Journal of Neuroscience Methods, 10 (1984) 247-248 247
Elsevier Letter to the Editor
A MODIFICATION OF THE INSTANTANEOUSFREQUENCYMETER FOR LOW FREQUENCY SIGNALS.
S. Obara Department of Physiology, Teikyo University, School of Medicine, Itabashi-ku, Tokyo 173, Japan
Kaga 2-11-I,
Key words: instantaneous frequency - reciprocal t i m e - i n t e r v a l - sample-hold IC This modification of the instantaneous
frequency meter (reference I) is an
a l t e r n a t i v e choice of the control logic pulses, and a c t u a l l y involves a simple change in wiring.
This, however, ensures more stable zero-output level in the
face of prolonged absence of input, and hence w i l l be useful f or those working in signal frequency range much lower than our IO - 400 Hz. An i n v e s t i g a t o r had the device made f o r recording single unit EMG, with the display time extended to 20 ms f o r a pen recorder.
Although the system works
s a t i s f a c t o r i l y , he is annoyed by a gradual s h i f t in zero-output level which he claims may amount to some - 200 mV in 30 min in the absence of inputs ! Such a practice has been unexpected, but the r e s u l t should be quite r e a d i l y predicted.
The zero-output level is given
by a base l i n e a f t e r each display.
The device's output is that of a sample-hold IC (LF398),
and is e s s e n t i a l l y a
buffered voltage across the hold capacitor Ch which is normally open-circuited a f t e r the short reset pulse P4.
In such a s i t u a t i o n ,
minimize the leakage around Ct as well (reference I ) .
"care i~ust be taken to
as Ch and the analog switch terminals"
The slow DC s h i f t in question is c l e a r l y caused by c;~arging of
Ch with a leakage current along the c i r c u i t board from a negative power supply line.
In our device, small such DC s h i f t also does occur, but to the p o s i t i v e
d i r e c t i o n , which r a r e l y exceeds the trace of tens of seconds.
width of CRO in our s i l e n t i n t e r v a l
The responsible leakage is presumably derived from within
the IC's, since Ch in our case is insulated by a Teflon standoff. For lower frequency applications such as the above then, we suggest that Ch be normally shunted to ground,
and open-circuited only
f or the display time.
The silaplest method is shown by block diagrams in Fig. 1 (references I and 2). The reset pulse P4 at the logic
input of the analog switch (pin 13, MC14066B)
is changed to Im3 (pin 9 in Mono. 3, MCI4538B). i t is normally high,
This switch section across fo r ~
~
is a negative l o g i c ,
and turns to low level only f o r
Ch (pin l & 2) is then normally closed,
duration, as requi~ed.
i.e.
the display time of P-~. and opens
In our device, this simple change resulted in a
0165-0270/84/$03.00 (~ 1984 Elsevier Science Publishers B.V.
248
A,
B,
ANALOGSW (I/t) (~14066B) CIRCUIT
~ : discharge arrest
I
P2: Ct charge
~ > - ~
~ : Rt disconnect P3: display time (not in use) ~ : S-H reset
iol
SAMPLE-HOLD (LF398) i~___I OUIPUI
ct'
j,6_~] :~>-~4~_~ 19 T -~113~~z~L_,-
P~
R
P4: replaced by ~ (not in use) ~ : not in use
P1 ~
SAMPLE ~
-
Fig. I . Modification of the instantaneous frequency meter (reference I) for a steady zero-output l e v e l . A: Assignments for the p o s i t i v e and negative logic pulses available in the o r i g i n a l c i r c u i t . B: Block diagrams showing only the relevant parts. The reset pulse to MCI4066B (analog SW IC) is changed from P4 to !~-~, as marked. The block diagram of LF398 (sample-hold IC) is also shown. steady zero-output level of about 0.5 mV at the F-V conversion of IO0 Hz/l V, even without DC-zeroing (pin 2, LF398).
A possible problem may arise from the
fact that in t h i s modification the output charging Ch (pin 6) is being shunted only with 300 ~ inside the IC (and plus ON resistance of MCI4066B) at the very s t a r t of the sample period,
thus loading LF398 (reference 2).
The time lag,
however, is the transmission time among the IC's, perhaps only in the order of tens of nsec.
This is short enough compared with the time constant of 3 ~sec
for charging Ch in
the sample mode, and hence w i l l be
the IC's power d i s s i p a t i o n . inputs, has been tested,
n e g l i g i b l e in terms of
A worst case assumption, sustained high frequency
but without any n o t i c i a b l e d e t e r i o r a t i o n in device's
performance. References: I) Obara, S. and Nagai, T. (1983) An improved instantaneous frequency meter for use with a m u l t i - t r a c e CRO: re-examination of the p r i n c i p l e s involved. J. Neurosci. Meth., 7: 89-98. 2) Linear Databook (1980) National Semiconductor Corp. CMOS Integrated C i r c u i t s (1978) MOTOROLAINC. (Received March 23rd, 1984) (Accepted March 23rd, 1984)
Elsevier Letter to the Editor
A MODIFICATION OF THE INSTANTANEOUSFREQUENCYMETER FOR LOW FREQUENCY SIGNALS.
S. Obara Department of Physiology, Teikyo University, School of Medicine, Itabashi-ku, Tokyo 173, Japan
Kaga 2-11-I,
Key words: instantaneous frequency - reciprocal t i m e - i n t e r v a l - sample-hold IC This modification of the instantaneous
frequency meter (reference I) is an
a l t e r n a t i v e choice of the control logic pulses, and a c t u a l l y involves a simple change in wiring.
This, however, ensures more stable zero-output level in the
face of prolonged absence of input, and hence w i l l be useful f or those working in signal frequency range much lower than our IO - 400 Hz. An i n v e s t i g a t o r had the device made f o r recording single unit EMG, with the display time extended to 20 ms f o r a pen recorder.
Although the system works
s a t i s f a c t o r i l y , he is annoyed by a gradual s h i f t in zero-output level which he claims may amount to some - 200 mV in 30 min in the absence of inputs ! Such a practice has been unexpected, but the r e s u l t should be quite r e a d i l y predicted.
The zero-output level is given
by a base l i n e a f t e r each display.
The device's output is that of a sample-hold IC (LF398),
and is e s s e n t i a l l y a
buffered voltage across the hold capacitor Ch which is normally open-circuited a f t e r the short reset pulse P4.
In such a s i t u a t i o n ,
minimize the leakage around Ct as well (reference I ) .
"care i~ust be taken to
as Ch and the analog switch terminals"
The slow DC s h i f t in question is c l e a r l y caused by c;~arging of
Ch with a leakage current along the c i r c u i t board from a negative power supply line.
In our device, small such DC s h i f t also does occur, but to the p o s i t i v e
d i r e c t i o n , which r a r e l y exceeds the trace of tens of seconds.
width of CRO in our s i l e n t i n t e r v a l
The responsible leakage is presumably derived from within
the IC's, since Ch in our case is insulated by a Teflon standoff. For lower frequency applications such as the above then, we suggest that Ch be normally shunted to ground,
and open-circuited only
f or the display time.
The silaplest method is shown by block diagrams in Fig. 1 (references I and 2). The reset pulse P4 at the logic
input of the analog switch (pin 13, MC14066B)
is changed to Im3 (pin 9 in Mono. 3, MCI4538B). i t is normally high,
This switch section across fo r ~
~
is a negative l o g i c ,
and turns to low level only f o r
Ch (pin l & 2) is then normally closed,
duration, as requi~ed.
i.e.
the display time of P-~. and opens
In our device, this simple change resulted in a
0165-0270/84/$03.00 (~ 1984 Elsevier Science Publishers B.V.
248
A,
B,
ANALOGSW (I/t) (~14066B) CIRCUIT
~ : discharge arrest
I
P2: Ct charge
~ > - ~
~ : Rt disconnect P3: display time (not in use) ~ : S-H reset
iol
SAMPLE-HOLD (LF398) i~___I OUIPUI
ct'
j,6_~] :~>-~4~_~ 19 T -~113~~z~L_,-
P~
R
P4: replaced by ~ (not in use) ~ : not in use
P1 ~
SAMPLE ~
-
Fig. I . Modification of the instantaneous frequency meter (reference I) for a steady zero-output l e v e l . A: Assignments for the p o s i t i v e and negative logic pulses available in the o r i g i n a l c i r c u i t . B: Block diagrams showing only the relevant parts. The reset pulse to MCI4066B (analog SW IC) is changed from P4 to !~-~, as marked. The block diagram of LF398 (sample-hold IC) is also shown. steady zero-output level of about 0.5 mV at the F-V conversion of IO0 Hz/l V, even without DC-zeroing (pin 2, LF398).
A possible problem may arise from the
fact that in t h i s modification the output charging Ch (pin 6) is being shunted only with 300 ~ inside the IC (and plus ON resistance of MCI4066B) at the very s t a r t of the sample period,
thus loading LF398 (reference 2).
The time lag,
however, is the transmission time among the IC's, perhaps only in the order of tens of nsec.
This is short enough compared with the time constant of 3 ~sec
for charging Ch in
the sample mode, and hence w i l l be
the IC's power d i s s i p a t i o n . inputs, has been tested,
n e g l i g i b l e in terms of
A worst case assumption, sustained high frequency
but without any n o t i c i a b l e d e t e r i o r a t i o n in device's
performance. References: I) Obara, S. and Nagai, T. (1983) An improved instantaneous frequency meter for use with a m u l t i - t r a c e CRO: re-examination of the p r i n c i p l e s involved. J. Neurosci. Meth., 7: 89-98. 2) Linear Databook (1980) National Semiconductor Corp. CMOS Integrated C i r c u i t s (1978) MOTOROLAINC. (Received March 23rd, 1984) (Accepted March 23rd, 1984)