Fluke 6060B - Option -651 Low-Rate Fm

Option
-651
Low-Rate
Fm
f|
m
651-1,
INTRODUCTION
The Low-Rate FM (Option
-651),
extends the FM
3-dB bandwidth
to a lower frequency
(approximately 0.5 Hz instead of
20
Hz). This
option makes the Generator useful for
testing
FM
radios
that use sub-audio
tones
or low-rate digital techniques.
The option
consists of a small printed
circuit
assembly A2A9 mounted atop
the
Synthesizer
PCB. When installed,
certain components
are removed from the
Synthesizer
PCB, the
option switch
is set on the Controller, and
a coupling capacitor is shorted on
the
Output
PCB.
A DIP switch
on
the
Low-Rate
FM PCB allows the Generator to be
configured for Low-Rate FM or
standard
(normal) operation (except that the coupling
capacitor on the Output PCB is shorted in both states). The DIP
switch settings
are listed
in Table 65 M.
651-2.
OPERATION
Operation
of
the Generator with the
Low-Rate FM option is the same
as
operation
of
a
standard instrument except that the maximum FM
deviation is limited to 9.99 kHz.
651-3. CIRCUIT DESCRIPTION
The
Low-Rate FM option
allows
frequency modulation at very low
rates
for
use
in digital
modulation testing.
In the
low-rate
mode, ac coupling capacitor A2A1
Cl6
is shorted
to
prevent any
dc
current
from entering the loop amplifier (A2A I
U27) and changing the
phase-detector operating point. The voltage at the
output
of the audio integrator (A2A1
U41-1) is kept at zero volts.
Keeping the voltage at the output of A2A1 U41 at zero volts
is
accomplished with a
voltage-zeroing loop consisting of A2A9 Ul. The
positive terminal of this op-amp
monitors the output voltage of the audio
integrator
through A2A9 R8. The negative
terminal
is
connected to ground at
A2A9
R9. By feeding current through A2A9 R7 back
to
the negative input of the audio
integrator
(A2A 1 U41), its output voltage
is
kept at
OV.
This is
a
lead-lag
circuit
with
a low frequency break of approximately
0.3
Hz.
Potentiometer A2A9 R4 adjusts the compliance
of
the
voltage-zeroing loop. Part of the
active high-pass filter ( A2A9 U4
1 )
is disabled
by connecting the previous stage directly to
A2A 1 C
11
4. Since
the
maximum deviation is limited to
9.99
kHz, the gain
of
the audio
integrator and the VCO summing network
(A2A1
R88, C117, CI46) is decreased by a
factor of ten. The Generator can
be reconfigured back to the normal mode by
programming the DIP switches on the option
board
(See Table
651-1).
651-4.
PERFORMANCE
TEST
The Generator is externally frequency modulated
with a low-frequency square wave
signal. The droop of the demodulated
signal
is measured using a spectrum analyzer as an
FM demodulator (slope detection is
used).
651-1