Specifications

10
Balanced and double-balanced mixers have more than
one diode to perform the frequency conversion. This
improves the LO to IF isolation (20 dB and better), but
usually requires more LO power or biasing. This higher
power may raise the LO’s noise output, produce spurs
away from the set frequency, or produce LO harmonics
inside the mixer. Any of these can mix additional, yet
unwanted, RF signal into the IF, or leak through to the IF.
This may raise the measured noise and possibly overdrive
the analyzer’s first stage to a non-linear state. Experiment
with the LO to see what improves performance and
produces the lowest noise figure.
e) Filter the RF signal (i.e. the noise source)
if necessary.
It may be necessary to filter the RF noise source to remove
frequencies that are outside the band of interest as shown
in Figure 6-4. This can remove unwanted signals that
would mix with the LO or its harmonics or spurious signals
and raise the measured noise figure.
To determine if sideband-averaging error is a problem, set
up the noise figure measurement for a mixer with swept
LO and fixed IF (modern noise figure analyzers allow this).
Monitor the noise figure reading. If the noise figure values
change dramatically as the LO sweeps, then SSB
measurement is recommended (See reference 5 for
further information.)
Figure 6-2
c) For single sideband measurements, select
a LO far from the RF band of interest,
if possible.
The farther the LO is from the RF, the higher will be the
IF, and the less stringent will be the rolloff requirement
for the filter that suppresses the unwanted sideband as
shown in Figure 6-3.
d) Choose the LO to suit the mixer.
Choose the LO to avoid spurious output where the mixer is
sensitive and to avoid high broadband noise floor.
Filter the LO if necessary to diminish spurious signals
and broadband noise, since the IF pass band will include
noise power one IF away from the LO frequency and
one IF away from any spurious signals or harmonics of
the LO.
Noise
Figure
Analyzer
Noise source
IF Amplifier
Mixer
f
LO
IF
f
RF
LO
Measurement System
Noise
Source
Drive
(DUT)
f