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Theory of Operation
2715 Spectrum Analyzer Service Manual
3-11
The 1st Converter converts the incoming RF signals to the 1st IF. Input signals
are applied through the Step Attenuator and Low Pass Filter and through the 1st
LO Buffer Amplifier.
The 1st Converter receives the RF signal through the Low P ass Filter and a 1st
LO signal from the 1st LO Buffer Amplifier. These signals combine to produce
mixing products that are filtered to 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 degrees out of phase.
The signals are recombined at the output, yielding a gain of approximately
10 dB. Any reflections to the input are dissipated in a 50 Ω termination. The
output is also terminated in 50 Ω to assure a match at the output port.
The Bandpass Filter (a four cavity filter) is a low loss, narrow band filter that
only passes the 2110 MHz IF signal to the 2nd C onverter. Any other frequencies
are reflected back 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 capacitively coupled 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 frequency of each
cavity is determined primarily by the depth of a gap in the underside of the filter
top and is fine tuned with a tuning screw on the side of each cavity. When
properly tuned, the filter return loss is >25 dB from either end (in a 50 Ω
system). Figure 3--4 shows the equivalent electrical circuit.
Cavity L and C
Cavity 1
Coupling Loops
Tuning Screw
RF Input
Cavity 2 Cavity 3 Cavity 4
Figure 3- 4: Equivalent Circuit for the Bandpass (4 Cavity) Filter
1
st
Converter
Bandpass Filt er