GENERAL RADIO COMPANY 1263-8 Operating Instructions Manual page 19

GENERAL RADIO COMPANY
Section
5
OUTPUT
RECTIFIER ERRORS
5.1 GENERAL.
The ability of the Type 1263-B Amplitude-Regulating Power Supply
to maintain a constant oscillator output necessarily depends on the char-
acteristics of the output rectifier. The power supply maintains within
close limits the rectified de produced by the output rectifier, but oscil-
lator harmonics and frequency characteristics of the rectifier can cause
errors.
5.2 OSCILLATOR HARMONICS.
The Type 874-VR Voltmeter Rectifier operates almost as a peak
detector at the high rf voltages, as do most rf rectifiers covering a wide
frequency range. This means that the rectifier can recognize only the
peak value of the rf waveform and can produce a de output proportional
to
it.
The amount that the radio-frequency fundamental can vary depends
on the percentage of harmonic voltage in the waveform. "In addition, if
the phase of the harmonic changes with frequency with respect to the
fundamental, the variation can be twice the amplitude of the harmonic,
even if the amount of harmonic does not change.
Unfortunately, this condition often exists in wide-range high-fre-
quency oscillators.
If
the harmonic frequency approaches the resonant
frequency of the voltmeter rectifier (refer to paragraph 5. 3), the error is
magnified.
Above about 300 Me, a low-pass filter should be used between the
oscillator output and the voltmeter rectifier to reduce errors due to har-
monics. The frequency range over which such a filter is effective is
limited to somewhat more than one-half the cutoff frequency up to the
cutoff frequency. Thus, the 500-Mc low-pass filter can be used effec-
tively between 300 and 500 Me, the 1000-Mc low-pass filter between 600
and 1000 Me, and the 2000-Mc low-pass filter between 1200 and 2000 Me.
To cover the gaps between these ranges, other filters with suitable cut-
off frequencies are necessary. Usually there is no need to do anything
below 300 Me, but in the 500-to-600 Me range, harmonics usually become
a problem.
Another way of
minimizing
the effect of harmonics is to operate
the output rectifier at a very low level where the characteristics approach
square law. The minimum reliable control level
is
approximately 0.2
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