Rf/If Description (Service Group A); Input (Al); Conversion (A2 Thru A5); If (A17 Thru A19) - HP 3585A Service Manual

Model 3585A
6· 1 2. RF/lF DESCRIPTION (Service Group A)
The RFIIF section is the second largest section in the instrument. It performs signal condi­
tioning on the input signal from the time a signal enters the instrument until it is AID con­
verted for the processor. This section can be divided into six subsections. They are:
Each of these subsections will be discussed in more detail. For the discussions that follow,
you should refer to Figure 6-3. RFIIF Block Diagram in this manual section and the
schematic drawings found in Service Group A of Volume Two.
6· 1 3. Input (A1 )
The Input provides two major signal paths. One is the 5001750 terminated input and the
other is the 1 MO input. Signals entering at the terminated input are monitored by an
overload detector. If an overload is detected, an interrupt flag is set high and signals the pro­
cessor. When the flag line is set high, it also causes a "dummy load" to be switched into the
input path to prevent circuitry damage. The impedance switching determines the input ter­
mination impedance as selected on the front panel or via HP-IB programming.
Attenuator selection is made in accordance with the range setting. Attenuator control is
from the processor via a series of opto-isolators to the relay coils. For example: A Range set­
ting of -25 dBm removes all attenuators from the signal path. A Range setting of -20 dBm at­
tenuates the input signal by 5 dBm.
In the 1 MO signal path, the attenuators operate in an identical manor and simultaneously
with the 5001750 attenuators. The 1 MO Buffer provides impedance matching for the 1 MO
input. This buffer is also used to insure a
because of dc coupling used later on in the circuitry.
Relay K 1 4 selects which signal path is input to the 1 1 dBm amplifier. The amplifier provides
the necessary gain to improve the signal-to-noise characteristics of the instrument. The input
signal then passes through a 4 1 MHz low pass filter and on to the output buffer.
The Autorange Detector monitors the signal level out of the 1 1 dB amplifier. The detector is
a comparitor circuit that compares the input signal to a range up and a range down
threshold. The results of this comparison is monitored by the processor and the input at­
tenuators are set such that the largest frequency component is at or very near the top of the
display screen.
INPUT LOG AMPLIFIER
CONVERSION VIDEO FILTER
AID CONVERTER
IF
0
Vdc offset on the signal path. This is required
Circuit Functional Descriptions
6-5