Model 8642A/B
Operation
Modulation, FM (cont'd)
Spurious Signals (Spurs) That Occur with the Internal Source
Internally generated spurs (with levels specified less than -100 dBc) might be present
when the internal modulation oscillator is in use. These spurs will be eliminated when no
internal modulation source is modulating the carrier, and the modulation oscillator
output (MOD OUT) is off.
Modulation Switching Time
The time required to turn on the internal modulation oscillator or to switch between
modulation frequencies can be reduced by approximately 200 ms (depending on
modulation frequency), with a loss in modulation frequency accuracy, by selecting
Modulation Frequency Correction Off (Special Function
1
13). For more information,
refer to "Speeding Up Modulation Switching Time" on page 3-132.
The time required to turn on the internal modulation oscillator is increased by
approximately 200 ms if DC FM Update Mode (Special Function 117) is active. For
more information, refer to "117. DC FM Update Mode On", page 3-116. When
switching from a dc-coupled FM source (for example, EXT DC or INT + EXT DC) to a
non-de-coupled FM source, RF frequency can take up to 2 seconds to settle to within
100 Hz of the selected frequency.
Selecting Heterodyne Band Operation
Special Function 8 (Prefer Het Band) can be used to cause the Signal Generator to
operate in the heterodyne band, regardless of FM deviation, whenever RF frequency is
less than
1
32.1 87501 MHz. This function prevents the Signal Generator from switching
from the heterodyne band when it would be possible to use the divide bands. Refer to
"How to Select the Het Band" on page 3-41.
Mixed Modulation
The Signal Generator can simultaneously modulate the RF signal with more than one
modulation type. The Signal Generator does not allow simultaneous AM and pulse
modulation or FM and $M. FM is allowed in the following combinations:
Any one of these types:
• Internal FM,
• External ac- or dc-coupled FM,
• Internal + external ac- or dc- coupled FM,
• External low rate FM (Special Function 5), or
• Internal + external low rate FM (Special Function 11 2)
With any one of the following:
• Internal AM,
• External ac- or dc-coupled AM,
• Internal + external ac- or dc-coupled AM,
© Internal pulse modulation, or
• External dc-coupled pulse modulation.
NOTE
All internal modulation sources use the internal modulation oscillator,
therefore all internal modulation, as well as MOD OUTPUT, have the
same modulation frequency.
3-111
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