Low Distortion Fm/$M On; Modulation Frequency Correction Off - HP 8642A Operating Manual

Model 8642A/B Operation
Modulation, FM (cont'd)
Table 3 - 2 1 . Factor for Y-Axis FM Deviation Quotient
Band
10
9
8
7
6
5
4
3
2
1
Het
RF Frequency Range
1057.500001 to 2115.000000
(8642B only)
528.750001 to 1057.500000
264.375001 to 528.750000
132.187501 to 264.375000
66.093751 to 132.187500
33.046876 to 66.093750
16.523438 to 33.046875
8.261719 to 16.523437
4.130860 to 8.261718
0.100000 to 4.130859
0.100000 to 132.187500
Divide Number
0.5
1
2
4
8
16
32
64
128
16
1
7. Low Distortion FM/<t>M On. This special function switches shaper circuitry into
the FM/'I'M loop, reducing total harmonic distortion. With Special Function 7, total
harmonic distortion is typically less than 0.03% for FM deviations less than 7% of
maximum deviation. For example, if operating at 98 MHz (divide band 6; refer to Figure
3-13 on page 3-106), you can calculate the maximum allowed deviation to obtain the
low-distortion performance by multiplying 7% times 187.5 kHz (0.07 X 187.5 kHz =
1 3.13 kHz). To obtain a higher maximum deviation level, you could select the Het band
(Special Function 8) which would have a maximum deviation of 7% times 1 500 kHz (0.07
X 1500 kHz = 100.5 kHz).
When using Special Function 7, external rates should be less than 50 kHz and
internal rates should be less than 15.8 kHz. If rates exceed these limits, distortion may
exceed 0.03%.
Total harmonic distortion is better than standard performance at all deviations
and rates with this special function selected. The phase noise performance is degraded by
approximately 20 dB at a 20 kHz offset, and by approximately 10 dB at a 200 kHz offset
when in the Low Distortion FM/<t>M mode.
207. Low Distortion FM/'I'M Off. This special function switches out the shaper
circuitry that is inserted with Special Function 7.
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. For more information, refer to
"Speeding Up the Modulation Switching Time", page 3-132.
213. Modulation Frequency Correction On. This special function re-enables the
modulation frequency correction disabled with Special Function 11 3.
115. Negative FM Polarity. This special function causes a positive voltage applied to
the external FM/<I>M INPUT to cause a negative frequency shift and a negative voltage
applied to cause a positive frequency shift.
215. Positive FM Polarity. This special function causes a positive voltage applied to
the external F M / $ M INPUT to cause a positive frequency shift and a negative voltage
applied to cause a negative frequency shift.
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