Theory of Operation
4-2 1705A Spectrum Monitor
driven with a pre-corrected sweep ramp. Pre-correction is required to make up
for the inherentVC O nonlinearity.
A +18 V supply is connectedto the L--BAND input connector so that it canbe
used to power aB lockDown C onverter(usually at the antenna). Thesupply can
be switched on and offby arecessed slide switch on the rear panel. Thevoltage
is generatedon a separate circuit board that is mounted inside the 1705A rear
panel.
The L--BandTuner output passes through a 1stIF filter which contains a notch at
590 MHz to eliminatea spurious mixing product. It is then mixed with an L.O.
of 359.4 MHz to produce a 2nd IF output at 130.5 MHz. Thisoutput is
amplified by an8 dB gainMMIC and is combined with the 70 MHz tuner
output.
The 70 MHz input consists ofa 7-pole, 120 MHz, bandwidth low-pass filter; a
VCO (which, like the L--Band VCO, is driven by a pre-corrected sweep ramp); a
mixer; and a20 dB amplifier. The70 MHz input circuits also output a 136 MHz
1st IF with a gain of 0 dB ±3dB.
The 136 MHz 2nd IF is converted a third time to produce a 3rd IF frequencyof
10.7 MHz. Thecrystal-controlled Local Oscillator operatesat 119.8 MHz to
provide the 10.7 conversion. Theoscillator’s output is tripled to 359.4 MHz to
provide the 130.5 MHz conversion for the L--Band Tuner output. Athree-sec-
tion helical resonatoris used for the 130.5 MHz IF filter.
An additional band-passcrystal filter, centered at 10.7 kHz, with a 10 kHz
bandwidth, can be addedby front-panel selection, to provide narrow resolution.
The 300 kHz bandwidth filteris always in the circuit regardless of the front-pa-
nel RESOLUTION selection. Maximumbandwidth of the 1705A is 300 kHz.
The resolution filtersdrive a FS K receiver IC. Onlythe m eter output of this IC
is used to provide avoltage proportional to the log of the input power. This
drives a selectablevideo filter and the VerticalDeflection Amplifier.
The output of the Ramp Generator drivesthe Horizontal Deflection Amplifier
(Diagram4), Gain Control (SPAN/DIV), and the Marker Generator. The Ramp
Generatorfree runs with its repetition rate controlled by the front-panel SWEEP
SPEED control. Theamplitude of the ramp remains constant.
The Gain Control, which providesthe ramp that is eventually used to drive the
VCOs, consists of an operationalamplifier with selectable input resistances. The
resistanceselected is dependent upon the SPAN/DIV setting selected fromthe
front panel. Theoutput ramp from the Gain Control circuitdrives the Sweep
Shapers.
The CENTER FREQUENCY control provides an offsetto the sweep ramps in
all SPAN/DIVsettings except FULL. In the FULL SPAN/DIVsetting, a
IF Amplifier Circuits(Diagram 2)Sweep Generator Circuits(Diagram 3)
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