Created on 2009-04-17 20:21:00
Introduction
Many real world signals are noisy, which can cause problemswhen trying to use them to make measurements. For example, when performing anexternal 20 Hz to 20 kHz sweep with a noisy stimulus signal, the frequencymeter may fail to stabilize and read properly. This can cause external sweepmeasurements to fail, because they depend on accurate frequency readings to identifythe tones and plot data points on the graph.
Figure 1 AP2700 Sweep and External Sweep panel configuration.
To better understand this, we'll review how an external sweepmeasurement is conducted. The sweep panel configurations we are using for theSYS-2722 and ATS-2 are shown in Figures 1 and 2 respectively.
Figure 2 ATS-2 Sweep panel configuration.
The sweep starts by looking for the first measurement point,as defined by the sweep start parameter. For this example, this frequency canbe anywhere between 19 and 21 Hz, according to the 5% tolerance setting on the AP2700 externalsweep panel, and the spacing setting on the ATS-2 sweep panel. The external sweep algorithm monitors the Source 1 instrument parameter(Frequency A meter) for the required value.When it gets a settled reading for the frequency, it then looks forsettled level readings from the meters defined by the Data 1-6 instrument parameters(Level A). When it gets those, it checks the Source 1 meter again to see thatthe value (Frequency) has not changed. If the 20 Hz signal is stable, and the settlingparameters for the meters are satisfied, then the 20 Hz measurement is recordedand the sweep continues by waiting for the next frequency as allowed by thepanel settings.
Now, if excessive noisy signals from the DUT (device undertest) cause us to have unstable readings that interfere with proper triggeringof the sweep, what can we do?
Harmonic Distortion Analyzer Solution
Both the AP2700 and ATS-2 control software have a DSPprogram that can make real-time frequency measurements which are virtuallyimmune to noise. The Digital Analyzer “Harmonic Distortion Analyzer” shown inFigure 3 uses FFT technology to make its measurements. The frequency metersdisplay the frequency of the highest-amplitude component of the applied signal.Because noise is usually at a much lower level than the level of thehighest-amplitude component of the stimulus signal, the readings are stable andthe settling algorithms produce a settled reading quickly.
Figure 3 ATS-2 Harmonic Distortion Analyzer
To set this up, on the sweep panel change Source 1 to the Distortionmeter's Channel 1/A Fundamental Frequency (Distortion.Ch.1 Fund Freq in AP2700,or Harmonic.ChA Fund Freq in ATS-2), as shown in Figure 4. The AP2700 controlsoftware can continue to use the analog analyzer level and function meters formeasurements. The ATS-2 control software only allows one analyzer to beselected at a time, so use the Amplitude and Harmonic Sum meters to make Leveland Distortion (THD) measurements, as shown in Figure 4 for the Data1.Instrument.
Figure 4 ATS-2 Sweep panel, using harmonic distortion analyzer
External Sweep Triggering Improvement
There's a number of ways to improve external sweeptriggering, so let's briefly review them to make sure we are choosing themcorrectly:
- Use the distortion meter, as described above, when the instrument is having trouble measuring the frequency of a tone with a high level of noise or distortion. The FFT based frequency meter picks out the strongest tone present and ignores everything else.
- Adjust the threshold (AP2700) or Min. Lvl (ATS-2) sweep parameter to gate out any noise, tones, or program material that might falsely trigger a reading to take place. Note that once the threshold is exceeded, no suppression of noise occurs.
- Loosen the settling parameters in the settling panel when the frequency or level of the tone is unsteady and won't trigger. This may compromise the accuracy of the measurements, so don't alter the settings more than necessary.
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