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20.3. Performing Sweeps
Frequency sweeps are started and stopped using the hard keys Run and Stop. The Sweep menu
permits four different sweep types to be selected, and allows the resolution bandwidth to be changed,
see section 19.12.
The Stop key halts the sweep at any time and holds the trace. The Run key initiates a new sweep and
also resets Averaging and Peak Hold sweep modes. (Note that the averaging value is selected from the
System menu rather than the Sweep menu).
When the sweep is in Stop, any change to the sweep frequency settings or to the attenuator setting will
initiate a new single sweep. This is done to ensure that the live trace remains correct relative to the
graticule markings.
Sweep Rate and Update Mode
There is no user control of the sweep rate. The speed of sweeping is determined by the frequency span
and the resolution bandwidth; the analyzer sweeps at the fastest rate possible for the Span and RBW
selected, subject to a maximum rate of approximately 5Hz.
The approximate sweep time can be calculated from the following formulae (span in MHz):
For RBW = 1MHz (PSA2701 only), sweep time = 200ms + (span x 2ms)
For RBW = 280kHz, sweep time = 200ms + (span x 7ms)
For RBW = 15kHz, sweep time = 200ms + (span x 75ms)
Thus, for example, a sweep of 10MHz span at 15kHz RBW will take about 1 second; whilst a sweep of
1GHz span will take around 8 seconds at 280kHz RBW, or 2 seconds at 1MHz RBW.
Note that for fast sweeps (less than 0.67 seconds) the display is updated at the end of the sweep with all
points re-written simultaneously. For slower sweeps, the display is updated synchronously with the
sweep and the old sweep values are sequentially over-written with the new values. In this mode the
progress of the sweep is indicated by a moving yellow line at the bottom of the graticule.
Video Filter
The video filter has only two settings, on or off. The setting has no effect on the sweep rate.
By default the video filter is turned On. At 280kHz/1MHz RBW this reduces the effects of system noise
within the PSA1301/2701 and improves the identification of low level signals just above the noise floor.
The filter attenuates noise signals above about 10kHz. It has a negligible effect on peak amplitude
measurements for signals at -60dB below the reference level or higher.
Where users wish to evaluate the noise performance of the system under test, they may wish to turn the
video filter Off.
At 15kHz RBW the effect of the video filter is small.
20.4. Level Measurements
The normal display has a range of 80 dBs down from the top of the graticule (the reference level). The
reference level is set from the Level menu, see section 19.10. With the attenuator off, the reference
level is -20dBm, with the attenuator on it is 0dBm.
Although RF signal measurements are most commonly made in dBm (power relative to 1 milliwatt into
50 Ohms), EMC measurements are normally made in dBµV (power relative to 1 microvolt into 50 Ohms).
The dBm/dBµV key toggles between these two measurement modes.
Note that, in dBµV mode, the top-of graticule reference becomes 87dBµV (-20dBm) with the attenuator
off, and 107dBµV (0dBm) with the attenuator on. The graticule lines are shifted so that subsequent lines
are in multiples of 10dBµV.
Note that any signal levels above the reference level can cause non-linearity in the detectors which could
affect the amplitude accuracy of other signals which are below the reference level.