HP 8642A Operation And Calibration Manual

Model 8642A/B
Operation
Modulation, FM (cont'd)
212. Internal + ExtemalLowRateFMOff.This special function turns off the FM
modulation source selected with Special Function 112. When Special Function 112 is
turned off this way, the Signal Generator selects internal and external, ac-coupled FM
summed together.
6. FM Pre-emphasis On. Special Punction 6 can be selected to pre-emphasize
internal or external FM modulating signals with a 7 SO us time constant. (Pre-emphasis
boosts high frequencies in the modulating sIgnal, prior to modulating the carrier.)
The Signal Generator's PM pre-emphasis creates some restrictions on the allowed
combinations of modulation frequency and maximum deviation. See Figure 3-14 on
page
3-114 for FM deviation limits that apply when pre-emphasis is selected with the
internal modulation source. Table 3-21 (on page 3-115) is to be used in conjunction
wi t h Figure 3 -14 to calculate the maximum deviation possible. The maxim lim deviation
depends on the RF frequency band and the modulation frequency selected.
See
Figure 3 -14. The FM deviation is on the Y-axis in the graph. The values of
deviation are shown for Band 9 and the heterodyne band only. To calculate the
deviation values for the band you are using, use the "Divide Number" column in Table
3-21 and divide that number into the Y-axis deviation value.
For example, the 1.08 MHz FM deviation limit, with a 1 kHz modulation
frequency, would be changed to 2.16 MHz for band 10 (divide by 0.5); that same limit
for band 8 would be changed to 540 kHz (divide by 2); and for band 7, the limit would
be changed to 270 kHz (divide by 4)
The actual calculations used to determine deviation limits with FM pre-emphasis
on are as follows (N is the divide number from Table 3-21 on page 3-115, MF is the
modulation frequency):
For modulation frequencies greater than or equal to 1389 Hz, the maximum FM
deviation is equal to:
0.5 x lOG) {l022)
-----~~~~-.--
N vMF2 + 45030
For modulation frequencies less than 1389 Hz, the maximum FM deviation is equal to:
(1080
x MF) (1022)
N yMF2 + 45030
Table 3-20 (page 3-112) lists FM deviation limits with an external source.
lIP-Ill. When using FM pre-emphasis, HP-IB cocles may need to be entered in a
particular sequence Refer to the discussion about Sequence Dependency, which begins
on page 3 -12 in the HP- IE portion of this Section III.
206. FM Pre-emphasis Off. This special function turns off the pre-emphasis that is
selected
with Special Function 6
3-11 3

HP 8642A Specifications

General

Brand	HP
Model	8642A
Category	Inverter
Language	English