Theory of Operation
3-10 2715 Spectrum Analyzer Service Manual
A relay switch at the input of theS tepAttenuator selects the RF input signal or
the internal calibrator signal. See Figure 3--3. Since the calibrator signal comes
into the Step Attenuator assembly aheadof the attenuators, it can be used to
verify the accuracy of the Step Attenuator and ot her parameters. Selection is
made through the INPUT MENU.
LO
IF
RF
PREAMP
Calibrator
100MHz/−22 dBm
RF IN
75 Ω
StepAttenuator
1.8GHz32110 MHz4.5GHz
1stConverter
Attenuator
2.11–3.91GHz
4
From1st LO Buffer 2110 MHz to 2nd Converter
32.2GHz for Option 50 and Option 75
42.11–4.26 GHz fo r Option 50 and Option 75
Figure 3- 3:At tenuator and 1st Converter Diagram
Also, an amplifier with 18 to 20 dB of gain may be insertedin the signal path at
the output of the Step Attenuator. This is the Preamp mode, selectablethrough
the INPUT MENU. It is used to enhance sensitivity,when small signals are
applied to the input, by maintaining the same signalto 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 thegain of the preamp will then
attenuate both the signal and thenoise. Thus, signal to noise ratio remains nearly
constant. In the meantime, the instrument makes correctionsin the IF to maintain
a calibrated reference level.
The Low Pass Filter attenuates all out-of-bandfrequencies. It rejects all
frequencies above1. 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 frominternally generated frequencies above 1.8 GHz
(above 2.2 GHz for Option 50 and Option 75).
Low Pass Filter