Fluke 6060B - Frequency Modulation; Software Operation

THEORY OF
OPERATION
3-9. Frequency
The
0.01-MHz to
1050-MHz
frequency coverage is
divided into the
following three
bands:
Low-band 0.01
MHz to 245
MHz
Mid-band
245
MHz to 512
MHz
High-band
512 MHz to 1050
MHz
The
high and
mid
bands are
derived directly from
a
voltage-controlled
oscillator
(VCO)
followed
by a
binary
divider that
is
part of the
main
phase-locked
loop
(PLL).
This
PLL
synthesizes
the
245-
to
5
12-MHz band using a
modified
N-divider loop
with
a
single-sideband
mixer
(SSB) in the
feedback path. The
reference
frequency
for the loop
is
1 MHz,
which would
normally provide
1-MHz steps in a
conventional
N-diyider loop.
However, this
Generator
provides 0.02-MHz steps by
using a
modified
N-divider
circuit
with
pulse
deletion
controlled by a
rate multiplier.
Additional
resolution is
gained
by
introducing a
signal
from the
sub-synthesizer
circuit
into the main
PLL through the SSB
mixer in the feedback
path. This
signal
provides
internal
frequency steps of 5 Hz.
The sub-synthesizer
consists of a
14-bit rate
multiplier
followed by a
divide-by-1000.
Since
the main PLL
bandwidth varies
with the
programmed
frequency
(due
to N
changing and
variations in the VCO
tuning coefficient),
the Controller uses
compensa-
tion
to program the
phase
detector gain via the
KN
DAC to
maintain constant
loop
bandwidth. By
keeping the loop
bandwidth
constant, loop
stability and
modulation
transfer is
controlled,
thus ensuring
accurate, wideband
FM.
3-10. Frequency
Modulation
Frequency
modulation
is achieved by
applying the
modulation signal
simultaneously to
the PLL
VCO and
the
Phase
Detector.
Both are necessary
because modulating
either the
VCO or
the Phase
Detector alone
results
in FM with a
high-pass filter
characteristic, or
phase
modulation with a
low-pass filter
characteristic. The
filter
characteristic
cutoff
frequencies
are equal
to the PLL
bandwidth.
The
modulating
signal
applied to the VCO
and the Phase
Detector is
adjusted
in
amplitude by
the
KV DAC to
compensate for
variations
in the VCO
tuning
coefficient.
This
compensation is
done
automatically by
the Controller using
factory
calibration
data
measured on
the VCO in
each
Generator. This
compensation data is
stored in the
VCO
Calibration
EPROM.
By
integrating the
modulation
signal
applied to the
Phase
Detector and
simultaneously
applying
the
modulation
signal to the VCO,
the two effects
are
complementary
and result
in a flat
FM
response.
3-1
1 .
SOFTWARE
OPERATION
.
The
Generator
software is
executed on
a Texas
Instruments
TMS
9995
micr^rocessor
in
the
A2A7
Controller
assembly.
The
instrument
program
is stored
m
48K-bytes
oi
ROM,
two
scratch
pad RAM,
2K-bytes
off-chip
and
250-bytes
on-chip
RAM,
Three
2K-byte
EPROMs
contain
the
individual
Generator
calibration
data.
The
software
provides
the
following
general
functions:
•
Interfaces
with the
front panel keys
and the
IEEE-488
Interface to
provide
access to the
Generator
functions.
3-3