Frequency Offset Mode Operation; Lo Frequency Accuracy And Stability; Differences Between Internal And External R Channel Inputs - Agilent Technologies 8753ET User Manual

Making Mixer Measurements
Measurement Considerations

Frequency Offset Mode Operation

This mode of operation allows you to offset the analyzer's source by a fixed value, above or
below the analyzer's receiver. That is, this allows you to use a device input frequency range
that is different from the receiver input frequency range.
Mixers or frequency converters, by definition, exhibit the characteristic of having different
input and output frequencies. Mixer tests can be performed using the frequency offset
operation of the analyzer (with an external LO source) or using the tuned receiver
operation of the analyzer (with both an external RF and an external LO source). The most
common and convenient method used is frequency offset.
Frequency offset measurements do not begin until all of the frequency offset mode
parameters are set. These include the following:
• Start and Stop IF Frequencies
• LO frequency
• Up Converter / Down Converter
• RF > LO / RF < LO
The LO frequency for frequency offset mode must be set to the same value as the external
LO source. The offset frequency between the analyzer source and receiver will be set to
this value.
For a single-sideband mixer measurement, the RF source can be offset in frequency from
the input receiver frequency, allowing for a swept RF stimulus over one frequency range
and measurement of the IF response over another (in this case the output IF).
When frequency offset mode operation begins, the receiver is tuned to the entered IF
signal frequencies and then offsets the source frequency required to produce the IF.
Therefore, since it is the analyzer receiver that controls the source, it is only necessary to
set the start and stop frequencies from the receiver.

LO Frequency Accuracy and Stability

The analyzer source is phaselocked to its receiver through a reference loop. In the
frequency offset mode, the mixer under test is inserted in this loop. To ensure that the
analyzer phaselocks correctly, it is important that you use an LO source that has frequency
accuracy better than ±1 MHz and residual FM < 20 kHz RMS.

Differences Between Internal and External R Channel Inputs

Due to internal losses in the analyzer's test set, the power measured internally at the R
channel is 16 dB lower than that of the source. To compensate for these losses, the traces
associated with the R channel have been offset 16 dB higher. As a result, power measured
directly at the R channel via the R CHANNEL IN port will appear to be 16 dB higher than
its actual value. If power meter calibration is not used, this offset in power must be
accounted for with a receiver calibration before performing measurements.
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