Specifications

9
Figure 6-1
DSB measurements are easier to perform since they don’t
require the additional burden of image rejection filter
design and matching. When a mixer with a DSB specified
noise figure is going to be used in an SSB application,
careful correction is needed. (See reference 5 for more
information)
b) For double sideband measurements, select
a LO frequency as close as possible to the
RF band of interest.
The choice of LO frequency, and the resulting IF, can make
a dramatic difference in the results of DSB measurements.
Instruments typically display the average of the LSB and
USB noise figures. They measure the power in the IF band
(which is the sum of both sidebands after conversion),
diminish that power by half (3 dB), and display the result.
The closer the USB and LSB are together, the more likely
they are to be equal (as shown in Figure 6-1) and the more
likely the default 3 dB correction will be accurate. Since
noise power versus frequency for a mixer is rarely flat, if
too wide an IF is used the error and its correction will be
unknown. To minimize this error, choose the LO frequency
as close as possible to the RF band of interest, within the
limitation that the resulting IF cannot be below the lower
frequency limit of the instrument being used (often
10 MHz).
● HINT 6:
Account for mixer characteristics
If the device under test is a mixer:
• Measure the same sideband(s) that will be used in
the application of the mixer.
• For double sideband measurements, select a LO
frequency close to the RF band of interest.
• For single sideband measurements, select a LO far
from the RF band of interest, if possible.
• Choose the LO to suit the mixer.
• Filter the RF signal (i.e. the noise source) if necessary
to remove unwanted signals that would mix with the
LO’s harmonics or spurious signals.
• Always document a frequency plan to identify which
of the above precautions are necessary.
a) Select a double sideband or a single
sideband measurement.
A mixer will translate input signals and noise from the
upper sideband (USB) and lower sideband (LSB) as
Figure 6-1 shows. (Note that the LSB and USB are
separated by twice the IF.) A double sideband (DSB)
measurement, shown in Figure 6-2, measures the noise
powers for both the USB and LSB. Some receiver systems,
like those in radio astronomy, intentionally use both
sidebands. DSB noise figure measurement is appropriate in
these cases. In many applications the desired signal
will be seen in only one sideband. A single sideband (SSB)
measurement is appropriate in these cases. In SSB
measurement setups, the noise power in the unwanted
sideband is suppressed by appropriate “image rejection”
filtering at the input of the mixer.
Noise Power
Freq
LSB
USB
F
LO
F
IF
average