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
~II
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
MHz)
-
(Vdc
f
1
MHz)
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
is:
(3.890)
-
(-2.272)
=
6.162
V/MHz
or
6.162
mV/kHz
FM Output
3-163
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