Specifications

To make accurate measurements, the IF must be settled before
the Digital Signal Processing (DSP) is triggered. When operat
-
ing in common offset mode, the IF settles faster. In fact, even
though the offset VCO (and therefore the main VCOs) may still
be slewing in phase, the IF itself may already be settled because
the Source and LO are tracking. The two main VCOs are able to
track the offset VCO settling because of their higher
bandwidths. Therefore, measurements can be made sooner in
the common offset mode and overall system speed is thus faster.
However, the common offset mode can only be used when the
source and LO frequencies are relatively close, because the
range of the DDS limits the allowable difference. Additionally,
the source and LO must be operating in the same frequency
band. High-IF frequency measurements, such as Wide-Band
Noise Figure, must operate in the independent offset mode.
However, common offset mode is beneficial for typical S-parame
-
ter measurements where speed is an important parameter.
Harmonic
Generator
Mode
In most measurements, the presence of harmonics on the RF is
undesirable. However, for measurements of phase in the har-
monic measurements application, a reference is needed. The sec-
ond and third harmonics of the source are used, and therefore
must be of a significant enough level to be measured accurately.
A harmonic generator diode can be switched into the circuit to
pump the harmonics to levels above –45 dBc. Without the har-
monic generator, the harmonics may be lower than –45 dBc.
THEORY OF OPERATION SOURCE MODULE
MS462XX MM 2-9