Sect. II Page 8
unfiltered pulses allows the user to filter the re- rate and that it is desired to investigate this f-m.
sponse as desired.
When the frequency-modulated waveform is applied
The PULSE output consists of one negative volt- to the frequency meter, the panel meter will in-
age pulse for every cycle of the inputsignal. These dicate the average frequency of 50 KC. For each
pulses have an amplitude of -35 volts peak and a cycle of the applied frequency, a voltage pulse
constant width such that a full scale meter reading will be available from the PULSE terminal, as
(unexpanded) produces an average voltage of -20 shown in Figure 2B-5. Since the amplitude and
volts. The pulse width is about 6/10 of the period width of these pulses are constant, and since the
corresponding to a full scale frequency. pulses are negative from ground, their short-time
average value will vary in exact accordance with
For example, on the 100 KC range every input the frequency modulation they contain. The original
cycle produces an-output pulse about 6 micro- modulating waveform can therefore be recovered
seconds wide and -35 volts high. This pulse is if the pulses are averaged with a suitable low-
presented at a resistive output impedance of about pass filter.
23,0009. If the terminal is shunted by a capacity
of .01 microfarads, a filter time constant of RC Not only can the waveform be recovered, but the
= 200 microseconds mill result. This will produce amount of deviation in the signal can readily be
a maximum peak-to-peak ripple of (6/200) x 35 measured, because the peak-to-peak amplitude of
volts = 1.05 volts, which is 5.25 percent of full the variations in the short-time average level will
scale. This maximum ripple only occurs at fre- be exactly proportional to the deviation. Since the
quencies which correspond to readings at the very amplitude and width of the output pulses is such
low end of the meter scale. The ripple will de- as to give a d-c output level of approximately -20.
crease to about 4/10 of this maximum-for full volts open circuit for a full-scale reading on @e
scale readings. Thus, at 100 KC the peak-to-peak
ripple would be about 2 percent of full scale if
a .01 mfd shunt condenser was used to produce a
filter time constant of 200 microseconds. This cor-
responds to a high frequency cut-off of 1/2 RC = 800
cps.
ONE CYCLE OF DIEVIATION FREQUENCY '9 !
This frequency limit can be extended by using a
shorter time constant if more ripple can be toler-
ated. Although the single shunt condenser is the
simplest filter for this application, much better 0 v--
results can be obtained with a multi-section filter
properly designed to reject signals at the input
frequency and higher.
-jjv---- . 1 ---1.1.. - 1 w w I I 8 . . 8 . '\
\
The particular filter to be used depends upon the 1
AVERAGE VOlTAGE (-20V MAX. FOR Full SCALE FREQ.)
application. For example, if it is desired to
measure the deviation of a 60 KC signal which is 500 B OUTPUT PULSES BEFORE FILTERING
being frequency modulated at 1000 cps, a 1000
cps bandpass filter can be used to select to 1000
ov---
cps component from the output terminal andpresent
it to an ac voltmeter which can be accurately
calibrated in deviation (20 volts = 100 KC on top
range.)
OUTPUT VOLTAGE AFTER FILTERING
(PEAK-PEAK DfflATlOIv EQUAL TO FULL SCALE
2B-13 PULSE OUTPUT APPLICATIONS
GIVES 20V P-P A-C OUTPUT)
How the PULSE output of the frequency meter
proves valuable in measurements can be described
by assuming that a frequency of 50 KC is to be
measured. Assume further that this frequency con- Figure 2B-5. Demodulated FM Signal Recovered
tains a i 5 KC frequency modulation swing at a 1 XC from Pulse Output Jack
0002 2 -2 |
ACEC
