R&S SMU200A Instrument Design and Function Description
1007.9845.82 3.3 E-6
Output Unit
Frequency Doubler Module
By using frequency doubling, the Frequency Doubler module expands the frequency range of the
synthesizer upward to 3025 MHz. Tunable highpasses and lowpasses connected in series ensure
sufficient suppression of subharmonic spurious.
Divider Unit
The Divider Unit divides the spectrum of the base octave from 750 MHz to 1500 MHz into the frequency
range from 200 MHz (500 kHz) to 750 MHz by using a programmable 2^N divider. The division factor
range is from 2 to 4 (1024). Harmonic spurious caused by frequency division are not filtered out.
Frequencies above 750 MHz are routed via a bypass. The frequency spectrum at the output of the
Divider Unit ranges from 200 MHz (500 kHz) to 3025 MHz.
Output amplifiers with level preset ensure a constant output level at the module output.
Synthesis Extension Module SYNEX
The Synthesis Extension (SYNEX) module adds frequency and phase modulation capability (option
B20) and can be equipped with low phase noise option (B22).
This module works only in combination with the module standard synthesis.
The source of the modulation signal is chosen by a modulation switch. FM/PhiM is generated by analog
to digital conversion of the modulation signal and feeding it into a digital synthesis device in the module
DDS-Modulator. The output of the DDS is fed as reference signal into a phase locked loop (PLL), the
FM-loop. At higher modulation rates the VCOs of this PLL are modulated directly via D/A-converter and
a level setting device to adjust to the steepness of the VCOs. The input signal of the standard synthesis
is fed into the FM-loop to convert the VCO frequency down to the reference frequency supplied by the
DDS-Modulator. The option B22 provides two high quality quartz reference oscillators. From these
references several signal are derived by multiplying, dividing and filtering.
The individual function modules are described below.
Modulation Switch
This module consists of an input switch matrix, aliasing filter and a A/D-converter to feed modulation
signals into the DDS-Modulator from internal or external sources.
DDS-Modulator
This module consists mainly of the DDS modulator implemented as a FPGA. The 14 bit output of this
FPGA is D/A converted and filtered. The frequency range is about 17 to 19 MHz, with FM 25 to 29 MHz.
Analog FM
This module provides the modulation signal to directly modulate the FM-loop VCOs. The fine scaled
signal from the DDS is D/A converted, filtered and coarse scaled by a level setting device.
FM-Loop
The FM-Loop consists of 6 switched VCOs, providing 750 to 1512.5 MHz output frequency. A mixer
down converts the VCO frequency to the DDS output frequency. Phase detector, switchable loop filter
and pretune facility complete this loop. A divider in the IF branch allowes to handle great FM deviations
without overdriving the phase detector.
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