Display (Sg-D) - HP 3585A Service Manual

Circuit Functional Descriptions
6·3. RF/IF (8G·A)
The RF IIF section of the -hp- 3585A allows the instrument to accommodate a wide variety
of signal sources and a wide dynamic range of input signals. Input impedances of 500, 750
and 1 MO are available. The input signal is processed by a series of attenuators and
amplifiers so that it comes within the level range required by the instrument.
After the input signal is at the proper level, it is mixed down to a frequency that is at the
cent er of the intermediate frequency filter. This is accomplished through a series of mixers
that mix the input signal with a sweeping signal from the Local Oscillator such that the entire
frequency range of interest will, piece by piece, be allowed to pass through the intermediate
frequency filters and subsequently be detected.
Once the signal is mixed down it goes through the intermediate frequency filters. This series
of multiple selective filters allow only a relatively small band of frequencies through, allow­
ing individual frequency components to be measured.
The filtered signal is then compressed by the Log Amplifier so that the wide analog
amplitude range of the filtered signal is converted to a more usable logarithmic scale.
To aid in the display of the frequency domain information, a Video Filter is added. This
filter basically smooths out the display.
Finally, the completely processed input signal is analog-to-digital (AID) converted and sent
to the processor for processing and subsequent display.
6·4. Local Oscillator (8G·8)
The Local Oscillator section is centered around a very stable 10 MHz temperature controlled
crystal oscillator reference and a Fractional N Synthesizer. The synthesizer loop operation is
a function of resolution bandwidth as selected from the instrument front panel. Full loop
operation will be discussed later in this manual section.
Essentially, the Local Oscillator section produces the sweeping LO signal, and two fixed LO
signals used in the mixing process of the RF IIF section of the instrument. Also generated is a
reference signal and a sweeping signal used by the Tracking Generator section of the instru­
ment.
6·5. Processor, I/O and Keyboard (8G·C)
This section of the instrument performs all calculations and controlling functions necessary
for the operation of the instrument. Included within this section is ROM, RAM, input! out­
put (I/O) control, keyboard, and the central processor.
The heart of the central processor is a hybrid device used in many desktop computers and
larger computer systems. ROM provides a complete operating system to the processor­
controlled instrument. RAM is used by the processor as needed, and can also be used to
store up to three front panel configurations.
The I/O portion of this section controls all interfacing between the processor and other por­
tions of the instrument. All display functions, keyboard monitoring, RF IIF controlling,
Model 3585A