
WSA5000 Functional Overview
Frequency Domain Triggering
Frequency domain triggering relies on the embedded real-time FFT mechanism to
transform the sampled signal from the time domain to the frequency domain. The
WSA5000 uses a 1024 point real-time FFT core embedded within the FPGA to transform
1024 time domain IQ samples to 1024 frequency domain FFT bins. Each bin is an
average of the spectral activity over a range of 125MHz divided by the DDC decimation
rate divided by the 1024 FFT points.
The frequency domain triggering supported by WSA5000 is a level trigger type, used to
capture any signal above the noise floor within a specified frequency range. The user
defines a single amplitude level within a frequency range. The frequency range
encompasses all FFT bins with center frequencies within the range defined by START
and STOP. If the sampled signal amplitude exceeds the defined trigger level at any
single sample within the defined frequency range, the trigger will occur and the
subsequent IQ data capture will proceed.
illustrates the association of the time domain and the frequency domain. The
internal frequency domain data lags the time domain data by 1024 samples at the rate of
125 MSa/s. After a trigger event is detected, the subsequent time domain IQ data is then
stored to memory.
1024 samples
at 125 MSPs
IQ data associated with
the trigger is buffered
trigger
100 MHz IBW
real-time
FFT
real-time
FFT
Figure 6:
Association between Time and Frequency Domain
The measurable range of the input signal, and the corresponding allowable trigger level
range, varies depending on the selected center frequency, the calibrated reference level
and the attenuation setting. When the attenuation is in the circuit (
ON), the maximum trigger level to use is -10dBm; and when the attenuation is out
(
OFF), the maximum is -30dBm. The threshold error is approximately
-3dBm or less when the trigger level is 20dBm above the noisefloor. When the level is
within 20dBm of the noisefloor, the threshold error increases as the signal gets closer to
the noisefloor.
Note:
The threshold error is relative to the measured input signal level, which is
dependent on the calibrated reference level. The reference levels could be custom
ThinkRF WSA5000 Wireless Signal Analyzer Programmer's Guide
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