Theory of Operation
3-4 2715 Spectrum Analyzer Service Manual
The RF Mother board assembly contains a2 GHz 2nd LO, a 2nd Converter,
several gain stages, a 5 MHzresolution bandwidth filter, and a 3rd Converter.
The 2nd Converter down converts the2110 MHz 1st IF to 110 MHz. The 3rd
Converter down converts the 110MHz 2nd IF to a 10 MHz 3rd IF. This signal is
routed to the VariableResolution module.
The VariableResolution contains several selectable gain stages prior to the filters
and a compensation amplifier.One of the nine filters is selected. Each filter has
an attenuator pad associatedwith it to compensate for losses in the filter. The
system selects the appropriate ampli fication factor as each filter is selected.
The 10 MHz IF signal is processed throughone of several Bandpass filters,
amplified once more, and thenrouted to the Log Amplifier board.
The Log Amplifier performs the logarithmic conversion,linear detection and
logarithmic detection of the incoming signal, and amplitude calibration. The log
display has scale factorsof 10 dB/div, 5 dB/div, and 1 dB/div. The module also
contains an FM detector,an audio amplifier, an amplitude limited output for the
period counter,and an out-of-band signal clamp.
The detector produces a voltage that correspondsto the input signal strength in
decibels. The detector output is thenvertically scaled and sent to the Display
Storage and Sweep boards.
The control processor usesthree 8 bit shift registers to control the Log Amplifier.
The Display Storage board containsthe circuitry for putting text and waveforms
onto the CRT display.
This board contains the following programmable functions:
HWaveformStorage —four 512 point waveforms
HDot Markers —up to two intensified markers
HText Storage—four 32 character by 16 lines of text
HAccumulator data —direct access to the output of the A to D converter
HNonvolatile memory —32 Kbytes total; 2 Kbytes used forwaveforms;
2 Kbytes used for text; the restis available for general use
When enabling the analog display,waveforms A, B, C, and D are turned on, but
not displayed. This results in a chopped blanking effectbetween the readout and
the analog display.The scanning alternates between the readout and the analog
display.
RF Mother Board
Assembly
VariableResolutionModuleLog AmplifierDisplay Storage