Jackson Labs PHASE STATION 53100A, User Manual

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PhaseStation 53100A User’s Manual V1.04 25-Apr-2022 51
undergoing residual noise testing was affected by a WiFi access point, with interference
appearing as an intermittent train of low-level impulses near 10 Hz. Try to keep all RF
radiation sources – intentional and otherwise! – well away from your testing area.
Crosstalk
Finally, don’t confuse power-line spurs or other sources of low-frequency interference
with
crosstalk
. This is an insidious artifact that may be observed in high-performance
measurements – or worse, may go unnoticed.
When working with direct-digital analyzers such as the PhaseStation and TimePod,
crosstalk tends to appear when the signal frequencies at the
input ports are nearly the
same, but not precisely. As the two signals’ phases slowly approach, coincide, and
separate, any coupling between them creates a spurious response at the difference
frequency. Appearing at a similar magnitude in both AM and phase noise plots, these
spurs also cause peaks and nulls in ADEV and related measurements.
Crosstalk can also occur when a
third
signal, incidental to the measurement
environment, causes beatnote effects that are asynchronous to both test inputs. This
case is illustrated in the plot above, where a TimePod 5330A was used to measure a
high-stability OCXO against another one of similar quality.
In this case, while the OCXO driving REF IN was very close to 10 MHz, the OCXO driving
the TimePod’s
INPUT
port was set approximately 3 Hz higher for demonstration
purposes. Also present in the room (but not participating in the measurement) was a
Figure 16: Interference from external 10 MHz source