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bandwidth is less than 100 kHz, BW5 is low (about -0.6 V). CR30 is reverse biased, and
the LC shunt network is driven through R62, setting the filter bandwidth at about 1 MHz.
The signal is buffered by Q14 and Q15. The output of buffer amplifier Q14/Q15 provides
a low impedance drive for the Low-Pass Filter. The Buffer Amplifier Q14/Q15 also drives
the center tap of L14 through the resistance of PIN diodes CR28 and CR29. This feedback
results in an apparent negative input resistance in parallel with L14. The negative input
resistance compensates for the parallel resistance present in the tank circuit because of the Q
of L14. The gain of the filter in narrow mode is set equal to that in wide mode by setting
the resistance CR28 and CR29. The resistance of CR28 and CR29 is set by adjusting the
diode bias current with FILTER AMPTD adjustment R67. L13 and C39 are frequency
compensation for Q14 and Q15.
BW5 Input Section
TP7 and TP8 provide a means to adjust the bandpass filter for equal gain in wide and
narrow modes. This can be done without changing IF bandwidth, which might in itself
introduce an amplitude change. With the analyzer in narrow resolution bandwidths (30 kHz
through 10 Hz), BW5 is approximately -0.6 V, which reverse biases CR30. This places the
with CTR adjustment C55. To place the Bandpass Filter in wide mode, TP7 and TP8 are
shorted together with a jumper wire. This forward biases CR30 through R89 and R63. Filter
amplitude can then be set with FILTER AMPTD adjustment R67 so that amplitudes are
equal in wide (test points jumpered) and narrow (jumper removed) modes.
Filter Temperature Compensation @
The Filter Temperature Compensation circuit stabilizes the gain of the bandpass filter with
temperature changes.
Op Amp Ul and associated circuitry provide temperature dependent bias for CR28 and CR29
to compensate for temperature-dependent change in the Q of L14 and the resistance of CR28
and CR29. As temperature increases, diode bias current increases as the voltage at Ul pin 6
increases. CR5 and CR6 operate as temperature-sensing elements to control the voltage at Ul
pin 6.
Low-Pass Filter @
The Low-Pass Filter consists of L15, L16, C56 through C59, R67, and R68. It has a
bandwidth of about 30 MHz, which removes harmonics of the 21.4 HMz IF signal to reduce
log fidelity variations between wide and narrow settings of the Bandpass Filter. The Low-Pass
Filter has a loss of 6 dB, which is compensated for by buffer amplifier QlS/Ql9, which has a
gain of 6 dB. R74* is factory-selected to adjust the gain of QlS/Ql9 for proper log fidelity.
4 MA3
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