HP 3585A Service Manual page 191

Model 3585A
crystal stages, and then back to a voltage signal again. The bandwidths of the crystal filter
stages are determined by the loop resistance. For example, in the first crystal stage different
combinations of resistance are switched into the circuit to determine the bandwidth. Since
the resistance switched into the circuit for a narrow RBW is small, an appreciable amount of
signal current is lost through that resistor and does not flow through the crystal (the series R
the crystal is about 1 500). This insertion loss is compensated for by selecting different emit­
ter resistors in the V to I converter in such a way that the correct amount of current is always
transferred through the crystal. The signal, after passing through the first two filter stages, is
buffered and continues onto the A 1 8, IF Gain, board.
The LC and crystal filter stages on the A 1 8 board are essentially the same as those on the
A 1 7 board. After the signal is filtered on the A 1 8 board, it passes through the 8 dB fixed
gain amplifier, the step attenuator (0, 4, 8, or 1 2dB of attenuation) and a series of three 1 6dB
amplifiers (0, 1 6, 32, or 48dB of amplification). This set of amplifiers and attenuators deter­
mine the reference level relative to the range.
To help in understanding the attenuator and amplifier operation on the A 1 8 board, consider
the following:
This condition will result in the signal appearing at the top of the display screen. Looking at
the graph in Figure 6-2, you can see that there is 8 dB of attenuation switched into the cir­
cuit. This nullifies the gain provided by the 8 dB amplifier. The net result is no gain or at­
tenuation of the signal as it passes through the A 1 8 board. Now change the input signal to
-45 dBm. As you can see in the graph, 1 6 dB of gain and -4 dB of attenuation are switched
into the signal path. If you recall the 8 dB fixed gain amplifier, you can see that 20 dB of gain
has been provided. Thus the input signal continues to appear at the top of the screen.
Finally, the input signal is buffered and is sent onto the A 1 9, IF FIlter No.2, board for final
IF filtering and processing.
IF
GAIN
(dBI
I F
ATTENUATOR
(dBI
OVERDRIVE
ATTENUATOR
( d B I
de
O F F S E T
RANGE REF L VL
=
41
3Z
- 11
- 4
- 1 9
-I
-3
-lZ
-11 -15 -27 -31
-lZ
1
-I
-zo
-10 -30
.10 0
t
MN;r:.REFEMNCE LEVEL
REFERENCE L E V E L
Figure 6·2. IF Gain and Attenuation Graph
input signal -25 dBm
= =
-43
-3 9 -••
-43
-'H
-
4 0 -'0 -OD -7 0 -ID
(dBml BELOW RANGE
Circuit Functional Descriptions
'.0
.4 0
.30
·zo
O F F S E T
( d B I
.10
0
-90
-100
6-7