Effect of "mixer bounce" or
"sampler bounce" on high dynamic range measurements.
Nearly all VNAs suffer to some degree from a measurement
problem known as "mixer bounce" or "sampler
bounce". This manifests itself as a false response in
the measurement of a very good filter, making the filter's
stopband rejection appear worse than it really is. The problem is
caused by leakage of the VNA's LO signal and its harmonics
out of the test ports, and IF frequency modulation sidebands
on that LO leakage.
If you set a VNA to a CW frequency
and an S12 measurement, you can connect a spectrum
analyzer to port 1 and find the LO leakage. If you connect
a coupler between ports 1 and 2 so that there is a large
signal going into port 1 and you can observe the port 1 LO
leakage through the coupled arm, you may be able to see
that the LO leakage now has sidebands on it at the VNA's IF
frequency. Better VNAs have high LO to RF isolation in their
receivers to minimize this leakage.
So why is this modulated LO leakage a problem? Suppose
you are measuring a highpass or bandpass filter with excellent
stopband rejection (there is seldom a problem with lowpass
filters). While the VNA is measuring S21 at the stopband
frequencies, there is a large RF signal going into port 1,
assuming the filter is reflective. Harmonics of the port 1
LO leakage land in the filter's passband, and are passed
through to port 2. The IF sidebands on these LO harmonics
are down converted in the port 2 receiver channel, and cause
an IF response, which appears on the screen.
What can be done about this?
First, look for an operating mode which "turns off"
the unused receiver channels. If the port 1 receiver is not
working, it will not leak the modulated LO signal out of the
test port. In some HP network analyzers, this mode is
available and is called "alternate A and B" (A and
B refer to the receiver channels for ports 1 and 2 respectively).
Second, if your VNA has direct receiver access jumpers, you
can remove these and place pads in front of the R channel
and A channel receivers, probably 12 to 20 dB is a safe bet.
These will both reduce the RF signal going into these
receiver channels, and reduce the LO leakage coming out of
them. You will need to re-calibrate the VNA with the pads in
Finally, try reducing the VNA power level a little bit below
maximum. Although you usually want to use maximum power for
best dynamic range, you may find for some VNAs and some DUTs,
that reducing the power a few dB actually gives you
better dynamic range by decreasing problems like mixer bounce.