Tektronix RSA3408, Руководство по обслуживанию

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RSA3408B Analyzer Service Manual
2- 1
Theory of Operation
This section describes the electrical operation of the RSA3408B analyzer using
the major circuit blocks or modules shown in Figure 6--1 on page 6--3.
RF Input
The RF input circuit block consists of a relay for signal switching (Input Relay),
Step Attenuator, Programmable Bandpass Filter, and a control circuit for these
components. This block adjusts the level of input signal as appropriate in
accordance with reference level settings, and sends the signal to the 1
st
converter.
Input Relay located just after the RF INPUT connector is for calibration signal
switching. It switches to pass the internal calibration signal when self gain-cal-
ibration or IF flatness calibration is selected. The step attenuator that follows is a
programmable attenuator of 5 dB/step. It attenuates an input signal up to 55 dB.
Band Relay switches signal paths between the RF1 band and the RF2/RF3 band.
In the RF2 and RF3 bands, bandwidth of the signal is limited by the Program-
mable Bandpass Filter (BPF) to avoid occurrence of images and/or spurious
signals. This Programmable BPF is able to adjust its center frequency in the
range of 3.5 GHz to 8 GHz, and adjustment to the center frequency is achieved
with the current output from the A100 Cal Generator board. In a same way, the
attenuator and relays are controlled with currents from the A100 board.
On the A100 Cal Generator board, various components are installed, such as: a
group of registers to save the setup data from Hardware Controller, driver
circuits for attenuator and relays, D/A converter circuit for generation of tuning
current to Programmable BPF, ALC circuit to maintain the amplitude of the
calibration signal at a constant level, IQ modulator for IF flatness calibration,
and other components.
Down Converter
The RF3 module contains the 1
st
Converter, 1
st
Local Oscillator, 1
st
IF Amplifi-
er, and other components. After level adjustment in the RF input circuit, the
input signal is applied to the 1
st
Converter. Mixed with 1
st
LO signal, the input
signal is frequency-converted into 1
st
IF signal of approximately 4231 MHz.
Oscillation frequency of 1
st
Local Oscillator is controlled with a synthesizer
(RF4 module) in this case, so that 1
st
IF signal is always maintained at approxi-
mately 4231 MHz. After this, the 1
st
IF signal passes through the 1
st
IF
Amplifier that has a gain of approximately 17 dB, and is sent to the 2
nd
Converter (RF2 module).
RF3 Module