HP 8642A Operating Manual page 193

Model 8642A/B
Operation
Special Functions (cont'd)
DETAILED DESCRIPTIONS OF ALL SPECIAL FUNCTIONS (cont'd)
112. Internal + External Low Rate FM On
5. External Low Rate FM On
These special functions typically provide 0.4 Hz to 200 kHz external ac-coupled FM with the Signal
Generator remaining in a phase locked state. Input coupling required for these functions makes the
Signal Generator especially sensitive to dc voltage fluctuations. Therefore, relatively small dc voltage
changes can cause the Signal Generator to lose phase lock, requiring one to two seconds to regain the
locked condition. Special functions 1 1 2 or 5 is turned off when another FM modulation source is selected.
113. Modulation Frequency Correction Off. This special function disables the single-band calibration of
the modulation frequency oscillator that normally occurs whenever the modulation oscillator frequency is
set or the internal modulation oscillator is put into use. When this calibration is disabled, the typical
modulation frequency switching time is decreased by approximately 200 ms. Refer to Modulation
Frequency, page 3-133, for a discussion of modulation frequency accuracy with Special Function 113
selected.
114. Negative OM Polarity. This special function causes a positive voltage applied to the external
F M / $ M INPUT to cause a phase lag and a negative voltage applied to cause a phase lead.
115. Negative FM Polarity. This special function causes a positive voltage applied to the external
F M / $ M INPUT to cause a negative frequency shift and a negative voltage applied to cause a positive
frequency shift.
116. DC FM Correction Off. This special function prevents the initial frequency correction that occurs
when dc-coupled FM is selected. Normally, 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 then 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.
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 and displays these RF
frequency changes Refei to Modulation, FM, page 3-116.
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 118 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..) LIsing DC FM avoids the phase shifts associated with
ac-coupled FM circuitry and allows maximum stereo separation. An external dc-coupled FM source must
be in use with Special Function 1 1 8 to get these benefits.
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, improving response time by up to 40
ms. When using this special function, it's possible to select a function before the previous function has
finished settling. Also Special Function 1 19 disables out -of-lock messages.
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