Tektronix 067-0532-10 Calibration Fixture Manual page 13

CIRCUIT DESCRIPTION
The instrument consists of several major circuit blocks. There are
two oscillators; one, a fixed, 3 MHz reference oscillator, and the
other a 65 to 500 MHz variable frequency oscillator. A series of
low-pass filters are used to limit the harmonic content to maintain
a low distortion sinewave. The detector head contains the detector
circuitry for maintaining the amplitude constant at the 50 ohm output
resistor. A precision DC divider provides a reference for the detector.
An error amplifier drives the series regulator circuit, which limits the
oscillator supply voltage and sets the oscillator signal ampitude.
The 3 MHz oscillator is a conventional Colpitts circuit. Power is from
a negative supply to the cathode of V20. The oscillator output is
filtered, then coupled to the low-pass filter network.
65-500 MHz Oscillator
The 65-500 MHz oscillator is manufactured by the General Radio Company.
It is powered from a negative supply via the regulator. The oscillator
output is capacitively coupled, C26, to the low-pass filter network.
Section 8 describes this oscillator.
Low-Pass Filter Network
The low-pass filter network consists of four filters, three switched,
and one always in series with the oscillator output. All four filters
are normally in series. As the frequency is increased past 108 MHz,
the first filter is switched out by relays K171A and K199A. The relays
are energized by cam actuated switches on the oscillator dial. As the
frequency is increased past 180 MHz, K141A and K169A switch the second
filter out. As the frequency is increased past 300 MHz, K111A and K129A
switch the third filter out. The fourth filter remains in the circuit.
Detector Head
The detector consists of peak-to-peak detector CR38, C38, CR36, and C36.
C38 is charged to a negative value by the DC reference. This establishes
the peak-to-peak voltage necessary to turn on CR36 and charge C36. The
output of the detector is applied to the error amplifier U66. CR34 and
CR35 provide temperature compensation for the peak-to-peak detector.
U66 is the error amplifier. It amplifies the signal difference between
the temperature compensating diodes and the peak-to-peak detector. Its
output drives Q70, the regulator circuit input amplifier.
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