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Theory of Operation
3-10
2715 Spectrum Analyzer Service Manual
A relay switch at the input of the Step Attenuator selects the RF input signal or
the internal calibrator signal. See Figure 3--3. Since the calibrator signal comes
into the Step Attenuator assembly ahead of the attenuators, it can be used to
verify the accuracy of the Step Attenuator and other parameters. Selection is
made through the INPUT MENU.
LO
IF
RF
PREAMP
Calibrator
100 MHz/−22 dBm
RF IN
75 Ω
Step Attenuator
1.8 GHz
3
2110 MHz4.5 GHz
1st Converter
Attenuator
2.11–3.91GHz
4
From 1st LO Buffer
2110 MHz to 2nd Converter
3
2.2 GHz for Option 50 and Option 75
4
2.11 – 4.26 GHz for Option 50 and Option 75
Figure 3- 3: Attenuator and 1st Converter Diagram
Also, an amplifier with 18 to 20 dB of gain may be inserted in the signal path at
the output of the Step Attenuator. This is the Preamp mode, selectable through
the INPUT MENU. It is used to enhance sensitivity, when small signals are
applied to the input, by maintaining the same signal to noise ratio from the input
to the 2nd Converter. When a small signal is applied to the RF INPUT, and the
Preamp mode is activated, both the signal and the noise level are amplified an
equal amount. Any attenuation less than the gain of the preamp will then
attenuate both the signal and the noise. Thus, signal to noise ratio remains nearly
constant. In the meantime, the instrument makes corrections in the IF to maintain
a calibrated reference level.
The Low Pass Filter attenuates all out-of-band frequencies. It rejects all
frequencies above 1.8 GHz (above 2.2 GHz for Option 50 and Option 75),
preventing them from reaching the mixer input and creating unwanted images. It
also reduces emissions from internally generated frequencies above 1.8 GHz
(above 2.2 GHz for Option 50 and Option 75).
Low Pass Filter