THEORY OF OPERATION
Frequency Modulation 2-8.
Frequency modulation (FM) is programmable with three digits of resolution in six
ranges. The deviation is programmed using the 12-bit FM DAC and three FM range
bits. The FM DAC and range settings are dependent on the programmed deviation
and the RF output frequency. The FM DAC and FM Range settings for each
frequency band and FM deviation range are shown in Table 6E-2 in Section 6E. The
FM/øM modes are selected by the control bit PMODL.
The maximum programmable FM deviation is dependent on the RF output
frequency. FM deviations up to 4 MHz may be entered regardless of the output
frequency. However, the STATUS indicator is flashed and the FM DAC is clamped at
full scale if the entry is beyond the allowed upper limit for that frequency band. The
maximum programmable deviation in each frequency band is depicted in Section 4C,
"Modulation" in the Operators Manual.
The FM oscillator loop runs at 80 MHz with several modes of operation. In the low
deviation, low noise mode, the oscillator runs with the highest Q. As deviation is
increased, a linearizer is added to maintain low distortion, which somewhat reduces
spectral purity. At higher deviations, the tuning sensitivity of the oscillator is
increased, again causing a somewhat higher phase noise. At this deviation, the
linearizer is used to maintain low distortion.
The phase lock circuit runs off of various reference frequencies depending on the
deviation selected. To provide a large amount of deviation at low rates, a very wide
range phase detector is used in the wide deviation ACFM mode. Full deviation can be
used down to an FM rate of 100 Hz. An alternate mode of operation that uses the
lowest reference phase detector frequency and the wide range phase detector for all
deviations will allow very low modulation rates for less than maximum deviation.
In DCFM mode, full deviation can be used down to DC levels. The generator is not,
however locked to the main timebase in this mode. When DCFM is enabled, the FM
oscillator's center frequency is set to the previous locked center frequency ±1 kHz by
automatic zeroing circuitry in conjunction with the software routine.
Phase Modulation 2-9.
Phase modulation (øM) is programmable with three digits of resolution in six ranges.
Phase modulation is internally normalized to 10 kHz, then programmed as FM
deviation. The øM index is multiplied by 10 kHz (regardless of the modulation
frequency) to get the "equivalent" FM deviation. Refer to Table 6E-2 (Section 6E) to
determine the FM DAC and range settings from this "equivalent" FM deviation.
The maximum programmable phase modulation deviation is dependent on the RF
output frequency. Phase modulation deviations up to 400 radians may be entered
regardless of the output frequency. However, the STATUS indicator is flashed and the
FM DAC is clamped at full scale if the entry is beyond the allowed upper limit for that
frequency band. The maximum programmable phase modulation deviation in each
frequency band is depicted in Section 4C, "Modulation", of the Operators Manual.
Phase modulation is achieved by reconfiguring the modulation circuits to cause a true
phase modulation response for both internal and external modulation inputs. The
display is correspondingly changed to indicate deviation in radians. Two modes are
available: large deviation at a limited bandwidth and limited deviation for higher rate
bandwidth.
2-4
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