HP 85662A, Troubleshooting And Repair Manual

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20 Linear Gain Step
Stages 6 and 7 are used as a switchable 20 gain step in LINEAR mode at low reference
levels. When the LG20 control line is high (more positive than -5 V), the collector
saturates to the -12 VTV supply, turning on PIN diodes CR6 and CR9 in Log Amplifier
Stages 6 and 7. Stage 6 and 7 each provide approximately 10 of non-limiting gain when
LG20 is high. The combined gain of stages 6 and 7 is adjusted to 20 by varying the
resistance of CR6. R14 sets the bias current and thus the resistance of CR6.
Pin Diode Attenuator
In LOG mode, attenuation of the PIN Diode Attenuator is varied with temperature to
compensate for bias variations with temperature in the Log Amplifier Stages. The attenuation
is removed in the LINEAR mode.
LOG Mode
In LOG mode, Q7 is turned off because is off. CR30 is forward biased by the LOG/LIN
control line through R63. PIN diodes CR28 and CR29 are forward biased by through
R62 and R61. PIN diodes CR26 and CR27 operate with fixed bias current through R56 and
R57. CR26 through CR29 form a resistive voltage divider with CR28 and CR29 operating as
a variable resistance. Op amp which biases CR28 and CR29, is a temperature dependent
voltage source. The voltage at pin 7 becomes more negative as temperature increases,
causing the attenuation to increase. The attenuation of the PIN Diode Attenuator is set by
R61 (refer to
and -12 VTV Adjustments).
LINEAR Mode
In LINEAR mode, is off and and Q7 are on. Q7 supplies additional drive for CR26
and CR27 through R58. This biases CR26 and CR27 so that their resistance is small
total). CR28 and CR29 are reverse biased through R63, since LOG/LIN is pulled to -12
VTV in linear mode. There is no attenuation, and the signal from Q14 is coupled directly to
emitter follower Q5.
Detector Driver
The signal output from the PIN Diode attenuator is applied to emitter follower Q5, which
provides low-impedance drive for the Detector. Bias for Q5 is supplied by R65, which with
R64 forms a voltage divider for the signal from the Q5 emitter. The signal at is linearly
related to the displayed vertical signal in both LINEAR and LOG modes. The signal is sent
by coax cable to the 21.4 MHz IF OUTPUT connector on the rear panel.
Temperature-Dependent Regulators
The -12 VTV and PIN Diode Regulators provide temperature compensation for the Log
Amplifier Stages. CR37 and CR38 operate as the temperature sensing elements to control
the two voltages. controls to regulate the -12 VTV supply, which provides a
nominal -12 V to a network for distribution throughout the Log Amplifier assemblies.
The -12 VTV voltage is adjusted to provide proper log fidelity; it becomes more negative
as temperature increases to correct for variations in log diode resistance. produces a
nominal -7 V and forms the current source for CR28 and CR29 in the PIN Diode Attenuator.
The voltage of
pin 7 becomes more negative as temperature increases to control the
attenuation of the PIN Diode Attenuator.
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