Specifications

22
This chapter presents the recommended proce-
dures and hardware configurations for making
absolute phase noise measurements on pulsed
carriers. For a complete description of how
pulsing the carrier affects phase noise measure-
ments, refer to Agilent Application Note 1309
Pulsed Carrier Phase Noise Measurements
(literature number 5968-2081E).
Absolute phase noise measurement
considerations
Measurement noise floor
The measurement noise floor represents the
lowest level of noise the system is capable of
measuring. For CW signals, it is set by the noise
of the test set (phase detector and LNA), the noise
of the comparison reference oscillator, the noise
of the microwave downconverter, or a combination
of all three—provided sufficient power is available
at the phase detector input ports. Additionally, for
pulsed carrier signals, the duty cycle of the pulsed
signal further degrades the CW test set noise floor
by a 20*log (duty cycle) factor. The recommended
power levels for the microwave phase detector are
+7 to +10 dBm for the reference input port and
0 to +5 dBm for the signal input port. The recom-
mended power levels (CW or peak) for the RF phase
detector are +15 to +23 dBm for the reference input
port and 0 to +15 dBm for the signal input port.
Measurement offset range
The offset range for the measurement is limited
to PRF/2, based on sampling theory.
PRF feedthrough
If the magnitude of the PRF feedthrough overloads
the phase noise test set LNA for more than 10 per-
cent of the pulse width, measurement accuracy
may be degraded. If the magnitude of the PRF
feedthrough causes the LNA to be set to a lower
than appropriate gain or the baseband signal ana-
lyzer to be set to a less sensitive range than needed,
overall system noise floor can be degraded. The
70420A phase noise test set provides a group of
internal low-pass filters that can reduce the magni-
tude of the PRF feedthrough signals. When measur-
ing pulsed carriers, it is recommended that the
operator place the test set LNA low-pass filters
in manual and select the filter that provides just
enough offset range for the pulsed measurement
(PRF/2). Refer to Table 3-1 for the proper selection
of the LNA low-pass filter. If these internal low-
pass filters are not sufficient for a particular PRF,
an external phase detector with external low-pass
filtering will be required.
Table 3-1. Selecting an Internal Low-Pass Filter
PRF LNA Low-Pass Filter
≤40 kHz 20 kHz
>40 kHz to ≤400 kHz 200 kHz
>400 kHz to ≤4 MHz 2 MHz
>4 MHz to ≤40 MHz 20 MHz
3. Making absolute phase noise measurements on pulsed carriers