428 Chapter8
IF Section
Log Amplifier Assembly (P/O A4)
10.6952 MHz VCXO
Refer to function block E of A4 log amplifier schematic diagram (sheet 2
of 4) in the
HP 8560 E-Series Spectrum Analyzer Component Level
Information.
The purpose of the 10.6952 MHz voltage-controlled crystal oscillator
(VCXO) is to provide an LO for down-converting the peak of the
10.7 MHz IF filter passband to 4.8 kHz. Since the peak of the passband
of the 10.7 MHz IF filters is 10.7 MHz ±300 Hz, the frequency of the
VCXO is between 10.6949 MHz and 10.6955 MHz. This frequency can
best be measured at the collector of A4Q202.
The center frequency of the 300 Hz resolution-bandwidth filters and the
1 Hz to 100 Hz filters should differ no more than 10 Hz. If the center
frequency is different by more than this, or if no signal is present in the
1 Hz to 100 Hz resolution-bandwidth settings, troubleshoot the 10.6952
MHz VCXO.
Error message ERR 539 may occur if the VCXO is not oscillating. If
problems exist with the VCXO control voltage, error messages
ERR 536 or ERR 530 may occur.
Between sweeps the VCXO, at times, is turned off. To prevent the
oscillator from turning off, press
PRESET, FREQUENCY, 0.3 GHz, SPAN,
1
kHz, SGL SWP, CAL, and IF ADJ OFF.
Input Switch
Refer to function block D of A4 Log Amplifier Schematic Diagram (sheet
2 of 4) in the
HP 8560 E-Series Spectrum Analyzer Component Level
Information.
The input switch switches between log and linear modes. In addition it
contains a 20 dB attenuator which is used only in digital resolution
bandwidth settings. CR207, CR208, and CR209 form the input switch.
CR205 and CR206 switch in R234 when in linear mode to maintain a
constant impedance at J3. CR210, CR211, CR212, and CR221 switch
the 20 dB attenuator in and out.
LO Switch
Refer to function block F of A4 Log Amplifier Schematic Diagram (sheet
2 of 4) in the
HP 8560 E-Series Spectrum Analyzer Component Level
Information.
The LO switch switches the limiter input between the 10.7 MHz path or
the 10.6952 MHz VCXO path.
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