Bandpass Filters - HP 85662A Troubleshooting And Repair Manual

10 dB Linear Gain Step
Stage 5 is used as a switchable 10 dB gain step in linear mode at low reference levels. When
control line LG10 is low (-12 V typical), Q17 is off and CR27 is reverse biased, preventing
CR27 from being a signal path. When control line LG10 is high (more positive than -3 V),
the Q17 collector is saturated to the -12 VTV supply, and PIN diode CR27 is forward biased.
LG10 adjustment R83 sets the bias current (and thus resistance) of PIN diode CR27. The
resistance of CR27 plus R54 is set so that the gain of Stage 5 will increase by 10 dB when
CR27 is turned on. (Refer to the foregoing gain formula.) Since PIN diode resistance is
unaffected by signal level, Stage 5 operates as a non-limiting amplifier. This 10 dB gain step,
in conjunction with a similar 20 dB gain step in A4A2, is used for the last 30 dB of step gain
(lowest 30 dB of reference level) in linear mode. See the schematic note for the reference levels
at which this gain step is checked.
Bandpass Filters ©
In narrow resolution bandwidths, noise generated in the log amplifiers is reduced by the
Bandpass Filter following Log Amplifier Stage 5. The bandwidth of this filter is switchable
between wide (>20 MHz) and narrow (about 1 MHz). In resolution bandwidths of 30 kHz
or less, the filter bandwidth is narrow. This reduces the log amplifier noise contribution 90
dB down from the reference level. In resolution bandwidths of 100 kHz and greater, the
bandwidth of the filter is wide enough so as not to affect the shape of the wide resolution
bandwidth filters. (See Figure 2.)
LOG STAGE 5
BW5
INPUT
SECT I ON
R63
-A/W-
« — • V A ■—< *
C47
-15V
CR26 CR29
@ ®
Figure 2. Bandpass Filter, Simplified Schematic
Bandpass Filter Switching
Emitter follower Q13 buffers the output of Log Amplifier Stage 5 and drives the Bandpass
Filter. The filter bandwidth is determined by the resistance through which the LC tank
network (L14 and C52 through C55) is driven. In resolution bandwidths of 3 MHz to 100 kHz,
bandwidth control line BW5 is high (+15 V) and CR30 is forward biased through R63,
resulting in a low filter drive impedance and wide bandwidth. When the analyzer resolution
A4A3 3