Series 12000A Microwave Synthesizers
2-42
Publication 31231, Rev. J, October 2003
2.3.17
External Frequency Modulation
(Series 12500A/12700A Only)
Wideband FM/Narrowband FM
Press
[FM]
to select the modulation parameter. Refer to the modulation parameters and specifications
in Chapter 1 for detailed information.
Using Frequency Modulation
There are four modes of operation for Frequency Modulation.
In the Narrow mode, the input signal bandwidth is DC to 8 MHz. Applying an input of
±
1 volt will allow the
frequency to be shifted by up to
±
1 MHz (4-8 GHz), determined by the sensitivity setting, at any rate from DC
to 8 MHz.
•
The instrument frequency accuracy is a function of both the timebase accuracy and the accuracy of
the factory DCFM offset calibration. Typical frequency errors due to the DCFM offset are under
400 Hz (in the 4-8 GHz band)
•
Phase noise performance is the same as in CW mode. Some noise generated by the ADC used to
digitize the “inside the loop” portion of the modulation. This noise is relatively insignificant compared
to the deviation as long as the input voltage is set to 1 volt peak. If, for example, the deviation were
set to 1 MHz and no modulating signal were applied, the noise would be noticeable
In the Wide mode, the input signal bandwidth is 100 Hz to 8 MHz. The maximum deviation for a
±
1 volt input
is
±
20 MHz (4-8 GHz).
•
The Loop bandwidth is reduced to below 50 Hz allowing all modulation to be ’outside the loop’. For
this reason, the close-in phase noise is degraded
•
If the modulation index is greater than about 10,000 there will be some additional distortion due to
excessive phase shift at the phase detector. This is equivalent to a deviation of 10 MHz with a 1 kHz
rate
The two Phase Modulation modes (Wide and Narrow) are provided to allow generation with a constant
modulation index. These modes are created simply differentiating the modulation signal. Therefore all other
characteristics of each mode remain the same. The allowable range of modulation frequencies is 10 Hz to
100 kHz for Narrow mode and 100 Hz to 100 kHz for Wide mode.
Figure 2-25: Frequency Modulation Selection
NOTE:
Modulation inputs which are asymmetrical or that have a DC component will
shift the carrier frequency.
☛
Содержание Series 12000A
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