Making Phase Noise Measurements With Voltage Control/The 12Db Amplifier - HP 35601A Operating And Service Manual

Service
Model 35601A
6-59. Making Phase Noise Measurements with Voltage Control/the 12dB Amplifier
In order to make Phase Noise measurements on oscillators from 5MHz to 18GHz, a phase
locked loop must be established between the reference oscillator and the oscillator under
test. Either of these oscillators may be voltage-controlled. In this case, a frequency correc­
tion signal (from the output of a Mixer on the A4 Board) must be provided for the voltage-
controlled oscillator. The output for this phase locked-loop control is taken from the output
of K9, the 2MHz Lowpass Filter, or the 2MHz Lowpass Filter bypass. The signal at this
point passes through an inductor and fuse Fl (which protects U4 when K10 is closed). The
clamp diodes CR7-10 also protect U4. With K10 closed, L15 and the stray circuit
capacitance form a 1MHZ low pass filter. U4 amplifies the phase error signal by 12dB (a fac­
tor of four) then passes the signal to the Phase Locked-Loop Control Board.
REMOVING THE A4 BOARD FOR SERVICING
When servicing is required, the A4 board may be accessed by following these procedures:
1. Remove the trim strips from the top and bottom of the front panel frame.
2. Remove the retaining screws for the front panel; these are normally hidden by the trim
strips. The front panel assembly should now be easily removable; the various cables will still
be attached, of course.
3. The A5 board should be removed by sliding it off the plastic retaining pins and the con­
nector that joins it to the A4 board.
4. Remove the screws securing the stainless steel cover to the back of the front panel
assembly.
5. Now that the A4 board is accessible, the A5 board may be plugged into the A4 board
using the right angle connector (-hp- part number 035601-66508) provided in the accessory
kit (-hp- part number 035601-84401).
The A4 board may be tested while in this configuration, although you may experience some
difficulty in identifying some of the components. If this is the case, or if a component on the
board needs to be replaced, the A4 board may be removed by continuing with the following
step:
6. The A4 board is removable from the front panel assembly by removing the nuts from
the front panel connectors. To do this, use the nut drivers with the protective plastic inserts
that are available as accessories (9/16" nut driver and protector 8720-0008 and 8710-0560;
5/16" nut driver and protector 8720-0003 and 8710-0559).
Make sure to use the nut drivers with protective plastic inserts to
remove the nuts from the front panel connectors. Failure to do this
will probably result in cosmetic damage to the front panel finish.
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