Fm Output - HP 8901B Operation And Calibration Manual

Model 8901B
Operation
FM Output
Do not apply greater than
10 Vpk (ac
+
dc) into the rear-panel FM
OUTPUT
connector or damage to the instrument may result.
DESCRIPTION
The rear-panel FM OUTPUT (dc-coupled, 10 kR output impedance) provides
~ I I
auxiliary output for
the FM demodulated from the
RF'
input signal. This output enables monitoring of FM during display
of other measurements (such as AM, @M, or audio signals) or 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 dc voltage at FM OUTPUT can be calculated as follows:
( K ) (FREQ ERROR)
1000
Vdc
=
V,ff -
where:Vdc=representation of the dc voltage at
FM OUTPUT volts,
V,ff=representation of the dc offset voltage for the
IF
used volts,
K=the FM sensitivity in mV/kHz, or 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
RF
INPUT. (Select 3.2 SPCL, which uses the 1.5 MHz
IF
described in Filters, RF and
IF
on page 3-229).
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 f 2 M H z ) - (Vdc f 1 M H z )
EXAMPLE
A
2
MHz unmodulated signal is applied to the RF INPUT.
A
dc voltmeter connected
to
FM OUTPUT
measures +3.89 Vdc. When the carrier is set to 1 MHz, -2.272 Vdc is measured. The FM sensitivity
i s :
(3.890)
- (-2.272)
=
6.162 V/MHz or 6.162 mV/kHz
FM Output
3-163