HP 8642A Operation And Calibration Manual page 209

Model 8642A/B
Operation
Special Functions (cont'd)
DETAILED DESCRIPTIONS OF ALL SPECIAL FUNCTIONS
kUI1I'd)
112. Internal
+
External Low Rate 1']\1 On
5. External Low Rate FM On
These special functions typically provide 0.4 Hz. to 200 kHz external ac-coupled j:M with the Signal
Generator remaining in a phase locked state Input coupling required for these functions makes the
Signal Generator especially sensitive to de voltage fluctuations Theref0re. rein lively slllall dc voltage
changes can cause the Sigll;}1 GellClator to lose phase lock, requiring one 10 two seconds te> regain the
locked cDIldiliun Special functions 112 or 5 is turned off when another nvl modulatlon source is selected
113. IVloclulation Fr(,'quency Correction Off. This special fUIlction 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
modul~
tion frequency
~\vitching
time is decreased by appr()ximately 200 ms Refer 10 Modulation
Frequency, page
.~-l
33, for a discusslon of modulation frequency accuracy with Speci::ll Function 113
selected
114. Negative <PM Polarity. This special function causes a positive voltage applied to the external
FM/<pM INPUT to cause a phiise lag and a negative voltage applied to cause a phase lead.
115. Negative FM Polarity. This spe,;ial fUIlction l..allses
~
positive voltage applied to the external
F1'VI/<pM INPUT to cause
;J
negative frequency shift and a negative voltage applied to calise a positive
frequency shift.
116.
DC 1'1\'1 Correction Off. This special funet iOIl preve III s 1 he initial fl'eq uency correct ion 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 all internal counteI' 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 approxilllately 200
ms.
117. DC FIVI lJpdate 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
veo
that L'ccurs over time and
temperature. In the DC FM Update Mode, the Signal Generator measures and dispLlYs these RF
frequency
I~hanges
RefeJ to Modulation, FM, page 3-116
H 8. AC-Coupled DC FM On. When the Signal Generator is in the norIllal ac-col.lpled 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 Fl'vl (DC PM), the phase lock loop is unlocked and the
external signal is applied directly to the unlocked
yeo.
With Special Function 1 I 8 selected, the input
signal is ac-coupled to ,he unlocked FM VCO blocking any
a~pl1ed
dc component (TYPIcal
low-frequency corner frequency is I liz) Using DC FM avoids the phase shifts associated with
ac-coupJed FM circuitry and allows lllaximuII1 stereo separation An external dc-coupled FM source must
be in lise with Special Function I I 8 t~) get these benefits
119. Disable S('ttling. Tllis special function allows the Signal Gcuerator's microprocessor to
resp~)lld
to
!lew commands wlth(lut waiting for tlte internal circuitry t~) ,ettIe, illlpr0ving re,p~)nse tiI1le by up t~) 40
ms. When using this special
f\lncti~)Jl,
it's possible to select a fUllction before the previolls fUllctioll has
finished settlrng. Also Spccwl FUnctIon 119 disables
()\It-~)f
-lOCK messages
3-173