Theory of Operation
4-6 1705A Spectrum Monitor
DBM U28 is driven by the L--Band Tuner output at489. 9 MHzthrough a
LPF/Notch filter consisting ofL11, L12, and series resonant circuit C69 and
W13. W13 is a shortedlength of 50Ωcoax forming a high-Q inductor. This
provides a narrownotch at 590 MHzto eliminate a spurious output from the
L--BandTuner.
The output from DBM U28 drivesU31 which provides about 8 dB gain to
compensatefor the mixer loss and optimize the system dynamic range. This
signal is then combinedwith the 70 MHz tuner output with 3 dB combiner T1.
The band-passfilter, FL2, is a three-section helical resonator with about 8 MHz
bandwidth.
U25 is a double balanced mixer that convertsthe 130.5 MHz2nd IF frequency to
the 10.7 MHz 3rd IF frequency. The LO frequency (119.8 MHz) is below the
2nd IF frequencyby the 3rd IF frequency. J6 is a test jumper that can be lifted to
evaluateprior circuitry or to insert a signal at the 3rd IF frequency to trouble-
shoot the circuitsthat follow.
The 2nd IF Amplifier,U22, has a gain of 20 dB. U22 requires a 6 V Vcc; VR3
and R136 are usedto derive the voltage from the +11.8 V supply. C31 and C32
are decouplingcapacitors. The output signal level is --20dBm maximum. J5 is
another test jumper that can be used to access prior circuits or insert a signal at
the 2nd IF frequency.
The 10.7 MHz 2nd IF signal passes through the 300 kHzR esolutionfilter or a
combination of the10 kHz and the 300 kHz Resolution filters. The switching of
filters is accomplished by a combination of a control signal (/1 0KHZ) from the
microprocessor(Diagram 5) and a switching network consisting of U23D, E, and
F,Q9, and Q10, and switching diodes (CR22, 23, 25, 26, 27, 28, 29, and 30).
When the control line is low,the signal path is through the 10 kHz filter. The
300 kHz filteris always in the circuit. Q10, CR22, and CR30 turn on to enable
the 10 kHz filter;Q9, C R23, and CR28 turn offto switch out the bypass line.
When the controlline goes high, Q10 turns off to turn off CR22 and CR30; Q9,
CR23, and CR28 turn on to shunt the 10 kHz filter and provide300 kHz
resolution. R175, the 300 kHz Gain adjustment, is setto match the loss of the
10 kHz filter. J10 is another test jumper provided to access prior circuits or
insert a signaldirectly into the Log Detector.
The input to the Log Detector,U32, is amplified by U29 and pre-shaped by FL3
and FL4, a 300 kHz Resolution filter,and then ac coupled to U32. When the
10 kHz filteris selected, the signal is filtered by the 10 kHz filter and then
applied to the Log Detector. The output current from the Log Detector is
processedby Q24 and Q25 to the 15 kHz low-pass filter at U27 pin 3 (Dia-
gram 4). The filteroutput then drives the Vertical DeflectionAmplifier.
Input Filter,Mixer, and IFAmplifier
Resolution Filter and LogDetector