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Model 8902A
Operation
FM Output
Do not apply greater lhan
10
Vpk (a(*
+
dc) into (he rem-pancl FM OUTPUT
conncclor or damagc
lo
thc instrrrrncnt may r t w l t .
DESCRIPTION
The rear-panel FM OUTPUT (dc-coupled, 10
kCl
output impedance) provides an auxiliary outpiit
for the FM demodulated from the RF input signal. This output enables monitoring of FM during
display of other measurements (such as AM, QM, or audio signals)
o r
when the modulation rate is
very low.
The output signal comprises both
a
dc voltage, related to the counted IF frequency, and an ac voltage
(bandwidth 16 kHz, one pole) proportional to the FM deviation.
With the input signal centered in the IF, the nominal dc offset voltage at FM OUTPUT is
approximately 0.8 Vdc for the 1.5 MHz IF and -5.6 Vdc for the 455 kHz IF. The FM sensitivity is
typically 6 V/MHz or 6 mV/kHz. The d c voltage at FM OUTPUT can be calculated as follows:
(K) (FREQ ERROR)
1000
V d c = V,JJ
-
where:Vdc=representation of the dc voltage at FM OUTPUT volts,
V,,J=representation
of
the dc offset voltage for the IF used volts,
K=the FM sensitivity in mV/kHz, o r V/MHz, and
FREQ ERROR=the kHz error displayed when FREQ ERROR is selected.
Measure V,,ff when the frequency error is 0 kHz. The sensitivity can be measured
using the following procedure:
PROCEDURE
To determine the sensitivity of the FM OUTPUT, apply an unmodulated
2 MHz carrier at the R F
INPUT. (Select 3.2 SPCL, which uses the 1.5 MHz IF described in Filters, R F and IF on page 3-259).
Measure the dc voltage at the output with a dc voltmeter. Move the carrier to
1
MHz
and again
note the dc voltage, The FM sensitivity
(K
in V/MHz or mV/kHz) is:
K
= (Vdc
*
2
MHz)
-
(Vdc
f
1
M H z )
EXAMPLE
A
2
MHz unmodulated signal is applied to the RF INPUT. A d c voltmeter connected to FM
OUTPUT measures +3.89 Vdc. When the carrier is set to I MHz, -2.272 Vdc is measured. The
FM sensitivity is:
(3.890)
-
(-2.272) = 6.162
V/MHZ
OT
6.162
mV/kHz
3-191
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