Operation
Model 8642A/B
Modulation, FM (cont'd)
116.
DC FM Correction Off. This special function prevents the initial frequency
correction that occurs when dc-coupled FM is selected. When dc-coupled FM is selected,
the FM phase lock loop is open, causing an offset in the selected RF output frequency.
To correct this initial offset, the dc-coupled signal is momentarily switched off, while an
internal counter is used to determine the amount of RF frequency offset. The frequency
offset is corrected, and the dc-coupled signal is switched back on (DC FM Correction).
Disabling this correction causes a decrease in switching time by approximately 200 ms.
During Phase Continuous Frequency Sweep (Special Function 123), dc-coupled FM is not
possible when DC FM Correction is on.
216.
DC FM Correction On. This special function re-enables the correction disabled
with Special Function
1
16.
117.
DC FM Update Mode On. With the FM loop unlocked, RF output frequency
changes with a change in applied dc signal and with the normal drift of an unlocked
VCO that occurs over time and temperature. In the DC FM Update Mode, the Signal
Generator measures these changes in RF frequency and displays the changing RF output
frequency on the display.
DC FM Update Mode requires continuous microprocessor and counter action
resulting in a slight increase in noise, spurious signals and radio frequency interference
(RFI).
(These increases do not degrade specifications.)
The time required to turn on the internal modulation oscillator or change
modulation frequency is increased by approximately 200 ms for all internal modulation
types when the DC FM Update Mode is selected.
With Special Function 11 7 selected during RF frequency sweep, changes in RF
frequency are not reflected on the display. During Phase Continuous Frequency Sweep
(Special Function 123), dc-coupled FM is not possible when DC FM Update Mode is on.
217.
DC FM Update Mode Off. This special function disables the DC FM update that
occurs with Special Function
1 1
7.
118.
AC-Coupled DC FM On. When the Signal Generator is in the normal ac-coupled
FM mode, the external signal is applied to the FM loop through circuitry that uses a
phase lock loop to ensure the accuracy of the output signal. In dc-coupled FM (DC FM),
the phase lock loop is unlocked and the external signal is applied directly to the unlocked
VCO.
With Special Function 11 8 selected, the input signal is ac-coupled to the unlocked
FM VCO blocking any applied dc component. (Typical low-frequency corner frequency
is 1 Hz.) Using DC FM avoids the phase shifts associated with ac-coupled FM circuitry
and allows maximum stereo separation. An external FM source must be in use with
Special Function 11 8 to get these benefits.
218.
AC~Coupled DC FM Off. This special function re-selects the dc coupling to the
FM circuitry disabled with Special Function 118.
119.
Disable Settling. This special function allows the Signal Generator's
microprocessor to respond to new commands without waiting for the internal circuitry to
settle. When using this special function, it's possible to select a function before the
previous function has finished settling. When settling is disabled, response time is
improved by up to 40 ms. Special Function 119 disables out-of-lock messages.
219.
Re-enable Settling. This special function re-enables the settling that is disabled
when Special Function 119 is selected.
248.
Calibrate the Modulation Frequency Bands Again. This special function enables
you to initiate the calibration sequence that normally only occurs during instrument
power-up. This function can be used, for example, to recalibrate the modulation
oscillator frequency bands if normal modulation frequency correction has been disabled
with Special Function
1 1
3. Calibration typically takes one to two seconds.
3-116
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