HP 3585A Service Manual page 192

Circuit Functional Descriptions
As the input signal enters the A 1 9 IF Filter No. 2 board, it passes through the Overdrive At­
tenuator. During normal operation this circuitry acts as a unity gain amplifier. An overdrive
condition exists when the reference level (REF LVL) is greater than the Range. Since gain
was needed when the reference level was greater than the Range, attenuation is needed in the
overdrive condition. This attenuation is provided by the Overdrive Attenuator (0, 4, 8, or 1 2
d B o f attenuation is available). After overdrive attenuation the signal enters the Overdrive
Limiter so that the amount of overdrive is limited to a fixed limit. The remainder of the A 1 9
board functions like the filter stages o f the A 1 7 board.
6·1 6. Log Amplifier lA 1 4)
The A14, Log Amplifier, board's primary function is to convert the linear IF signal to a log
signal. The signal input level to the log amplifier is critical; therefore, as the IF signal enters
the A 1 4 board, it passes through a variable gain amplifier. This amplifier is adjusted to pro­
vide the signal level required by the log amplifier. After amplification, the signal passes
through a 350 kHz bandpass filter (30 kHz RBW filter) before entering the log amplifier.
Once the linear signal is converted to a log signal, it passes through a
(LPF) to rid it of any harmonic content. Finally, there is an averaging detector which actual­
ly rectifies the log signal and capacitively stores it. The output of the log amplifier is a rec­
tified log version of the input signal.
The linear IF signal after pass band filtering goes to the IF Output on the rear panel of the in­
strument, and the ac log signal goes to the A46, Counter, board.
6· 1 7. Video Filter lA 1 5)
The A 1 5 , Video Filter, board performs three functions: adds required dc offset, provides
video filtering, and generates dB/DIV amplification as needed.
Before discussing the circuitry of this board, an explanation of dc offset is needed. If you
will recall, the gain and attenuation stages in the IF Section provided only a 4 dB resolution
in input signal processing. The instrument, however, is capable of reference level resolution
to tenths of a dB. The dc offset gives this additional resolution. For example, consider a
reference level of -25 dBm and an input of -26 dBm. Now change the reference level to -26
dBm. Somehow the signal has now got to appear at the top of the screen. What happens is
that 1 dB of gain is programmed into the dc offset D/ A converter and is presented to the
summing amplifier to be added to the signal coming from the A 1 4 board. The various
amounts of dc offset are illustrated by the bottom plot of the graph in Figure 6-2.
The input signal comes into the Video Amplifier board from the A14 Log Amplifier. This
signal is then summed with the required dc offset. The output of the summing amplifier is 5
volts for a full scale input and varies 50 mV /dB. Thus a signal that is 1 00 dB down from full
scale would yield 0 volts at TP3 using a 1 0 dB/DIV front panel setting.
The signal then goes through the video filter. This filter is a single pole RC network with dif­
ferent resistance and capacitance switched into the circuit for the different video bandwidths
(VBW). Finally the signal goes through the dB/DIV amplifier that provides the gain cor­
responding to the display scale selected . A Video Output to the back panel of the instrument
is also made available.
6-8
Model 3585A
600
kHz low pass filter