Keysight Technologies N9040B UXA Manual page 55

Measurement Related Tasks and Concepts
Setting the Measurement System
Figure 3-3
measurement, while during calibration, when the DUT was not connected, only
one band (at the IF frequency) was used.
If the assumptions about the parameters being flat over the frequency between
the two sidebands are valid, your results will show a doubling in power (3 dB
increase) in the noise level during the measurement of any downconverting
DUT. There is also a doubling of measured power when using the system
downconverter, but compensation is not required because the calibration
power is also doubled.
This 3 dB increase in measured power with downconverting DUTs can be
corrected using the Loss Compensation Setup screen (MEAS SETUP, Loss
Comp tab). Set Loss Comp Mode to Fixed, enter a Fixed Value of –3 dB, and
set Temperature to the noise source's cold temperature. The DSB power
addition occurs for both the Hot and Cold noise from the noise source, and the
noise created in the input of the DUT. A temperature value can be assigned to
this loss using the Before DUT Temperature. Using the Cold temperature of the
noise source (often assumed to be 290 Kelvin) corrects for this, and the
analyzer will give calibrated results comparable to those that would have been
given by an SSB measurement.
DSB measurements are not appropriate for making measurements where DUT
performance, or noise source ENR, have significant variation over the
frequency range
DSB measurements need care to determine their filtering needs.
When making a Double Side Band (DSB) measurement with RF Frequency
Context, the frequencies you specify as the RF Start and Stop frequencies
refer to the Lower Side Band (LSB) only.
LO Leakage (with specific DSB information)
LO leakage is a problem when working in the 200 kHz to 3.6 GHz range. It can
be avoided by tuning the LO to frequencies greater than 3.6 GHz. Above
3.6 GHz, the analyzer's input filter progressively attenuates the LO signal. For a
DSB downconverter measurement with the LO frequency below about 3.6 GHz,
a lowpass filter will be needed. The cutoff frequency must be chosen to pass
the IF frequency of the measurement. The amount of attenuation over the LO
frequency range has to be sufficient to reduce the LO leakage down to the
broad-band (10.0 MHz - 3 GHz) noise level presented to the analyzer input.
With most DSB Downconverter measurements, the IF is made low, with
respect to, the RF and LO frequencies, so filter needs are not complex.
Low pass filters with cutoffs at low frequencies, may exhibit spurious
resonances and leakage at low microwave frequencies. It may be
necessary to use a pair of lowpass filters, one microwave, one RF, to
assure a stopband attenuation over a wide frequency range.
N9069C Noise Figure Measurement Guide
shows that noise from two bands are combined during the
.
[ F ± F ]
LO IF
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