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Model 35601A
Service
The 14MHz VCO is an LC Colpitts oscillator; it operates at either 14MHz or 14.8MHz,
depending on the state of L14 (which selects either divide by 35 or 37). The output of the
divider is 400kHz which is compared with the 400kHz derived from the 10MHz reference or
the 10MHz VCXO.
The Mixer Driver divides the 14 or 14.8MHz signal by 10 and then by 4, utilizing U201 and
U200, respectively. U200 also provides a local oscillator signal for the AM mixer that is 90
degrees out of phase with the LO for the PM mixer.
6-44. X5 Amplifiers
At the output of the X20 amplifier in channel B and at the output of the 0.16Hz High Pass
Filter of channel A is a bypass path and a X5 Amplifier path. Either the bypass path or the
X5 amplifier may be selected. The purpose of the amplification is to provide more gain when
measuring phase noise. The amplifier path has a frequency response compensation for the
roll-off of the 350kHz IF of the -hp- 3585. This compensation corrects the amplitude
response when measuring sideband noise. The bypass path and the output of the X5
Amplifier join at the input of the Low Pass Filters. The output of the Adaptive Coupler also
joins Channel B at this point. L6 connects Channel A and Channel B.
6-45. Lowpass Filters
Each Lowpass Filter is programmable to provide either a 100kHz, a 1kHz or a 10Hz cutoff
frequency. The output of each filter is fed into a unity gain buffer. These are the Output
stages which drive the channel A and channel B inputs of the -hp- 3582.
REMOVING THE A3 BOARD FOR SERVICING
When servicing is required, the A3 board may be removed by pulling up all of the black re
tainer pins. After this, the board may be placed vertically (for ease of testing) by rotating the
board toward the back of the instrument and inserting the bottom edge and the side edge in
to the plastic holders provided. You may desire to remove a few cables to facilitate the
rotating of the board.
6-51
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