Stanford Research Systems CG635, User Manual

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Operation 20
CG635 Synthesized Clock Generator
the divider remains at 1024 and the RF VCO increases in corresponding steps to
1024×2.0 MHz = 2.048 GHz. At 2.1 MHz, however, the CG635 crosses an octave
boundary because the RF VCO does not have the range to provide 1024×2.1 MHz =
2.1504 GHz. Therefore, the CG635 changes the divider to 512 and slews the RF VCO
back to 512×2.1 MHz = 1.0752 GHz.
The CG635 has fifty-one, overlapping octave bands. They are summarized in Table 11.
Table 11: CG635's Fifty-one, Overlapping Octave Bands
Band Divider Min Frequency (Hz) Max Frequency (Hz)
0 1 960,000,000 2,050,000,000
1 2 480,000,000 1,024,000,000
2 4 240,000,000 512,000,000
3 8 120,000,000 256,000,000
4 16 60,000,000 128,000,000
5 32 30,000,000 64,000,000
6 64 15,000,000 32,000,000
7 128 7,500,000 16,000,000
8 256 3,750,000 8,000,000
9 512 1,875,000 4,000,000
10 1024 937,500 2,000,000
11 2048 468,750 1,000,000
… … … …
49 2
49
0.000,001,705,302 0.000,003,637,978
50 2
50
0.000,000,852,651 0.000,001,818,989
When the user changes the frequency, the CG635 attempts to stay within the same
octave. If an octave switch is required, however, and the frequency is in one of the first
eleven bands, the CG635 will disable the output, forcing it low before changing the
divider and slewing the RF VCO to the new frequency. When the VCO has settled at the
new frequency, the output will be re-enabled in a “runtless” fashion. This is done to
protect the user’s application from spurious clock pulses and frequencies as the dividers
are changed.
For frequencies in bands eleven to fifty, the CG635 uses DDS technology to seamlessly
change dividers. Since no spurious pulses are generated, the output is not disabled.
In addition to the normal methods of stepping frequency by the current step size, the user
can also step the frequency by factors of two by accessing the secondary functions
FREQ/2 and FREQx2.
Phase
The CG635 can adjust the phase of the output by up to 360° per step. The phase may be
set to arbitrary values, or stepped up and down by configurable step sizes by following
the instructions described in the Front-Panel User Interface section at the beginning of
this chapter. If the user enters a phase that requires the output to adjust by more than
360°, the CG635 will briefly display “Step Error” and leave the phase unchanged.