Theory of Operation
1705A Spectrum Monitor 4-5
by a 7-pole, 120 MHz, low-passfilter that rejects out-of-band frequencies. The
Mixer,U190, with input frequencies of 45 -- 100 MHz, andthe Local Oscillator
operating at[Fi+ 130 MHz]provide a 130 MHz 1st IF. The signal output from
the Mixer is terminatedby a75Ω, 3 dB pad made up of R291, R292, and R293.
The IF signal is accoupled into the amplifier, U296, with a gain of 20 dB.
R396, R391, and R392 form a 6 dB pad that outputs the 136 MHz 1st IF signal
that approximatesthe L--Band output level for inputs 10 dB higher.
L393, L391, L389, C292, and C290 form alow-pass filter that is peaked at the
1st IF frequency(136 MHz) and approximately 20 dB down at the Local
Oscillator frequency. Harmonics of the Local Oscillator frequency are at least
30 dB down.
The Local Oscillator(LO) is a Voltage Controlled Oscillator (VCO) whose
output frequencyrange is 175 MHz to 230 MHz. The oscillator is a differential
pair (pins 7--8 and 5--11)contained in U283. The oscillatoremploys positive
feedbackthrough L284 to the base tank circuit (L280 and C281). CR280 is a
varactorwhose capacitance is determined by the instantaneous level of the
pre-corrected ramp from the Sweep Generator (Diagram 3). The center tap of
T187 is the LO output providing an amplitudeof approximately +7 dBm.
Q20 and Q21 form aswitching circuit that turns off the VCO when the L--Band
input is selected. When/LBAND goes low, Q21 is shut off, causing Q20 to
unsaturateand disconnect the --11.8V from the VCO.
The 70 MHz tunerassembly (A5) is contained in a rigidly mounted shield.
Control signals and the --11.8 V supply arebrought into this shielded tuner
through feedthroughcapacitors.
IF AmplifierDiagram 2This diagram shows theL --Band and 70 MHz inputs to the various conversion
stages. Italso shows the Local Oscillator/Tripler circuitry,Mixers, Amplifiers,
IF filters, and Log Detectoroutput.
Both Local Oscillator (LO) signals arederived from a single crystal, whose
operating frequencyis 119.8 MHz. The crystal is a 5th overtone type in a Butler
oscillator configurationusing Q13. Q11 buffers the oscillator and drives a 3 dB
isolation pad on the LO input ofDouble Balanced Mixer (DBM) U25. Q12 is a
cascode stage driving a tank circuit at t he 3rd harmonic(359.4 MHz). The tank
circuit is tappeddown to drive a three-section helical resonator, FL1, tuned to
359.4 MHz. The output fromFL1 is coupled into DBM U28.