Specifications

12
Noise figure measurements
using a mixer as the DUT
When a down conversion is included
in the noise figure measurement, for
example measuring the noise figure
of a mixer, there are some additional
setups to consider. For this example
let us use a mixer as a down-converter
with an IF at 70MHz, LO at 1GHZ
and both RF sidebands are used,
930 and 1070 MHz (DSB):
• The measurement, as well as
calibration, is made at the
IF frequency.
• When an IF frequency is chosen,
it is a good idea to keep the
frequency as low as possible in
order to avoid large differences in
ENR values between the upper
and lower sidebands when using
DSB mode. This is because it is
the ENR value at the LO that is
used in the measurement
(compromise since it is centered
between the two sidebands)
• Since this device has some loss, it
is recommended that the internal
preamp be used.
• Compensate for two sidebands by
selecting double side band. Any
broadband noise in the LO will
directly affect results. This can
be solved by either a high pass or
low pass filter at the LO port that
removes the noise at the IF
frequency. Place an IF filter at the
input of the spectrum analyzer to
remove LO feed through. Usually
mixers have around 20 dB of
isolation between the LO-IF port
so the high powered LO will
seriously affect results.
Figure 13.
Setup for
measuring a mixer
Instructions Keystrokes
Set up the PSA for down conversion {Meas Setup} {Int Preamp On
}
measurements. It is recommended that the [Mode Setup] {DUT Setup…} {Down Conv}
internal preamp be used when measuring
devices that have low gain.
Set up the source for +7 dBm at 1 GHz. On E4438C press [Frequency] [1] {GHz}
[Amplitude] [7] {dBm} [RF On].
Set up the LO frequency (Figure 13). Scroll to “Ext LO frequency” using tab keys
then enter [1] {GHz}. Tab to “sideband” and
choose “DBS”.
Set up the fixed IF frequency. [Frequency] {Freq Mode} {Fixed}
{Fixed Frequency} [70] {MHz}
Calibration: connect the noise source to [Meas Setup] {Calibrate} {Calibrate}
the input of the PSA.
Measure the DUT: Connect the mixer IF (I) To add more averaging, press [Meas Setup]
port to the PSA, the LO (L) port to the signal then {Avg Number On
).
source and the RF (R) to the noise source.
Perform measurements on mixers.