Frequency Modulation; Software Operation - Fluke 6060B Instruction Manual

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