Test Procedures R&S SMU200A
1007.9845.82 1.18
E-12
SSB Phase Noise
Test assembly
See section "
Test Assembly for SSB Phase Noise", page 1.4.
Test method The two generators are set to the test frequency and synchronized with a
phase offset of 90° (phase quadrature). Mixing to 0 Hz suppresses the RF
carrier, and due to the phase quadrature the mixer supplies a voltage
corresponding to the phase difference between the input signals. This is
measured by the LF spectrum analyzer and can be converted into SSB
phase noise.
Measurement
Set the levels of the two generators in accordance with t
he
s
pecifications of the mixer used (unmodulated or I/Q modulation wit
h
m
ax. level)
.
F
or calibration, reduce the level of the DUT by 40 dB and det
une one
s
ignal generator by 20 kHz. Check the signal for harmonics
; the 2nd
and
3rd harmonic should be more than 30 dB below the fundamental.
Measure the reference value at 20 kHz at the analyzer and note i
t
down.
S
et the detuned signal generator to the previous frequency and s
et
t
he signal generators for phase quadrature. To this end, raise th
e
level of the DUT to the previous level and call Delta Phase in the
F
requency/Phase menu. Observe the output voltage of the mixer
on
t
he oscilloscope and vary the phase until the voltage becomes zer
o.
R
ead the noise voltage, normalized to a bandwidth of 1 Hz
(noise level) from the analyzer
.
Evaluation
Determine the difference relative to the reference level
and add to the
v
alue found 6 dB for the second (correlated) sideband and 40 dB for
level switchover.
If the S/N ratio of the reference source is not at least 10 dB better
than that of the DUT, the noise component of the reference signal
generator must be determined and subtracted as well. This will apply
,
i
f one is using a second SMU as reference source instead of item
2.
T
he corrected S/N ratio is the wanted measured value.
Example: The measured reference level is assumed to be 12 dBm. A
noise level of -78 dBm (1 Hz) is determined at 20 kHz. The
difference is 90 dB, plus the correction for the second sideband
(6 dB) and level switchover (40 dB), yielding an S/N ratio of -136
dB or a noise level of -136 dBc (dB referred to the carrier
power). If two identical signal generators have been used, the
result must be reduced by 3 dB for the (uncorrelated) noise
power of the reference signal generator.
The final result is then -139 dBc.
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