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Iff= B/4 (i.e., 2 f = B/2), the component fr-2 f is attenuated by 3 dB which introduces
a meter error of 1 ,65 dB.
If f = B/2 (i.e., 2 f = B), the component fr-2 f is attenuated by 18 dB which introduces
a meter error of 0,1 dB.
Finally, if f = B, the attenuation of the fr-2 f component is greater than 40 dB, and the
error in the meter reading is negligible.
4.8. EFFECT OF RIPPLE IN VCO CONTROL VOLTAGE
Ripple in the external voltage, applied via the FREQUENCY CONTROL VOLTAGE "In"
socket to tune the 2010, can produce unwanted modulation of the variable frequency fv
produced by the VCO.
The frequency, fv. is mixed in the first modulator with the incoming signal for analysis
(V sin(27Tft)). Sum and difference frequencies are produced. The difference frequency is
removed by filtering. Therefore, only the sum frequency is important, where
If ripple, V m, of frequency f m, is present in the control voltage, side bands are produced
in the sum frequency.
vsum
v sin (2
1T
(f
+
fv)t)
+
0,5 m v sin (2
1T
(f
+
(fv
+
fm ))t)
+
0,5 m V sin (2
1T
(f
+
(fv- fm
))t),
where m is the modulation index
and
m
The amplitudes of the side bands are 0,5 m. V
1 6f
Y2Vm df
0,5
m
0,5
dV
2 fm
fm
df
=
VCO sensitivity.
where
dV
The maximum permissible ripple voltage which will produce side bands more than 80 dB
below the required signal can be calculated for different ripple frequencies and scale
ranges.
For 80 dB suppression of the side bands 0,5 m must be less than, or equal to, 10- 4 .
~
df
1o-4
-
~
V2tm
dV
vm
~
v'2tm
. lo-4
df/dV
55
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