Model 8642A/B
Operation
Modulation, FM (cont'd)
212.
Internal + External Low Rate FM Off. This special function turns off the FM
modulation source selected with Special Function 11 2. When Special Function 11 2 is
turned off this way, the Signal Generator selects internal and external, ac-coupled FM
summed together.
6. FM Pre-emphasis On. Special Function 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 FM 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
with Figure 3-14 to calculate the maximum deviation possible. The maximum 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:
(1.5 X 10
6
) (1022)
N V
MF2 + 45030
For modulation frequencies less than 1389 Hz, the maximum FM deviation is equal to:
(1080 X MF) (1022)
N V
MF2
+
45030
Table 3-20 (page 3-112) lists FM deviation limits with an external source.
HP-IB.
When using FM pre-emphasis, HP-IB codes 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-IB 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.
3-113
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