Theory of Operation
2715 Spectrum Analyzer Service Manual 3- 11
The 1st Converter converts theincoming RF signals to the 1st IF. Input signals
are applied through the Step Attenuatorand Low Pass Filter and through the 1st
LO Buffer Amplifier.
The 1st Converter receives the RF signal through the Low Pass Filter and a 1st
LO signal from the 1st LO Buffer Amplifier.These signals combine to produce
mixing products that are filteredto yield the 2110 MHz IF signal.
The mixer output is coupled to the input of a balanced amplifier where the signal
is split into two paths. The signals in the two paths are 90 degreesout of phase.
The signals are recombined at the output, yielding a gain of approximately
10 dB. Any reflections to the input are dissipatedin a 50 Ωtermination. The
output is also terminated in 50 Ωto assurea match at the output port.
The Bandpass Filter (a four cavity filter) is a low loss, narrow band filter that
only passes the 2110 MHzIF signal to the 2nd Converter. Any other frequencies
are reflectedback to the 1st Converter and dissipated in the termination. In
addition, the filter prevents the 2nd Converter LO and mixer products from
feeding back into the 1st Converter.
Each end resonator is capacitivelycoupled to external circuits through a coupling
hat plugged into a 3 mm connector.Inter cavity coupling is provided by coupling
loops that protrude from the machined filter top. The resonant frequencyof each
cavity is determined primarilyby the depth of a gap in the underside of the filter
top and is fine tuned with a tuning screw o n the side of each cavity. When
properly tuned, the filter returnloss is >25 dB from either end (in a 50 Ω
system). Figure 3--4 shows the equivalent electrical circuit.
CavityL and C
Cavity1
CouplingLoops
TuningScrew
RF Input
Cavity2 Cavity 3 Cavity 4
Figure 3-4: Equivalent Circuit for the Bandpass (4 Cavity) Filter
1st ConverterBandpass Filter