Instructions

Page 51
7.6 Radiated and Injected Signals
7.6.1 Self-radiated Signals
In common with most handheld spectrum analyzers, low level signals generated from the
internal circuitry can be picked up with an antenna.
Indicative measurements made with a 30cm (12”) whip antenna, mounted directly onto the
signal connector and extending in line with the instrument, showed self-radiated peaks typically
around -90dBm. Connecting the same antenna at the end of a 1m (3ft) extension cable and
moving it away from the instrument reduced these to below the noise floor at maximum
sensitivity.
7.6.2 External Signal Injection
The RF circuitry of the instrument is extensively screened in order to avoid signals being picked
up via any route other than through the signal connector. Nevertheless very high field
strengths, as might be created by a transmitter in close proximity, could cause measurement
errors through direct radiated signal injection. The vulnerability to this increases with increasing
frequency.
If such measurement errors are suspected, a measurement should be done with a N-Type
shorting connector in place, in order to quantify the effect.
7.7 Detector Types
The detector converts the signals captured during the sweep into the set of DC levels that are
shown on the display. A full explanation of detector types is outside the scope of this manual,
but some basic explanation is given here.
Each sample point on the display is a representation of the signal levels found within the range
of frequencies defined by each sample point. For example, a span of 27MHz results in each
display point representing signals within a “bucket” of frequencies 100kHz wide.
The default detector type of Positive Peak displays the highest signal value found within the
bucket. This is the most commonly used detector type and gives the most accurate result for
the majority of measurement situations whilst guaranteeing not to miss any signals.
Certain types of measurement, however, benefit from different detector types, and the PSA
Series 5 offers six other detector options. The degree to which the results differ from the
default detector depend not only on the signal but on the relationship between the RBW and the
bucket width.
The Negative Peak detector uses the lowest value found within the bucket. This is occasionally
used for comparative purposes in noise and EMC measurements.
The Sample detector uses the value found at the end of the bucket. It can have advantages
over the default detector when measuring CW levels close to the noise floor but risks missing
signals if the RBW is narrow relative to the bucket width.
The Lin and Log Average and RMS detectors use a level related to the average or root mean
square value of signals within the bucket. They are used for signal accumulation measurements
such as channel power.
The Alternate Peak detector distinguishes between signal and noise found within the bucket by
detecting whether the level both rises and falls within the bucket. It gives a more accurate
indication of noise than a peak or sample detector. However, under some circumstances signal
peaks can be displaced one sample point to the right.
Selection of detector type is explained in section 4.3.6.