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Making External Sweeps with Noisy Signals on the 2700 Series and ATS-2

Created 17 Apr 2009

Introduction

Many real world signals are noisy, which can cause problems when trying to use them to make measurements. For example, when performing an external 20 Hz to 20 kHz sweep with a noisy stimulus signal, the frequency meter may fail to stabilize and read properly. This can cause external sweep measurements to fail, because they depend on accurate frequency readings to identify the tones and plot data points on the graph.

AP2700 sweep panel AP2700 external sweep panel

Figure 1 AP2700 Sweep and External Sweep panel configuration.

To better understand this, we’ll review how an external sweep measurement is conducted. The sweep panel configurations we are using for the SYS-2722 and ATS-2 are shown in Figures 1 and 2 respectively.

ATS-2 sweep panel

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 can be anywhere between 19 and 21 Hz, according to the 5% tolerance setting on the AP2700 external sweep 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 for settled 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 that the value (Frequency) has not changed. If the 20 Hz signal is stable, and the settling parameters for the meters are satisfied, then the 20 Hz measurement is recorded and the sweep continues by waiting for the next frequency as allowed by the panel settings.

Now, if excessive noisy signals from the DUT (device under test) cause us to have unstable readings that interfere with proper triggering of the sweep, what can we do?

Harmonic Distortion Analyzer Solution

Both the AP2700 and ATS-2 control software have a DSP program that can make real-time frequency measurements which are virtually immune to noise. The Digital Analyzer “Harmonic Distortion Analyzer” shown in Figure 3 uses FFT technology to make its measurements. The frequency meters display the frequency of the highest-amplitude component of the applied signal. Because noise is usually at a much lower level than the level of the highest-amplitude component of the stimulus signal, the readings are stable and the settling algorithms produce a settled reading quickly.

ATS-2 distort panel

Figure 3 ATS-2 Harmonic Distortion Analyzer

To set this up, on the sweep panel change Source 1 to the Distortion meter’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 control software can continue to use the analog analyzer level and function meters for measurements. The ATS-2 control software only allows one analyzer to be selected at a time, so use the Amplitude and Harmonic Sum meters to make Level and Distortion (THD) measurements, as shown in Figure 4 for the Data 1.Instrument.

ATS-2 distort panel

Figure 4 ATS-2 Sweep panel, using harmonic distortion analyzer

External Sweep Triggering Improvement

There’s a number of ways to improve external sweep triggering, so let’s briefly review them to make sure we are choosing them correctly:

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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