Originally posted May 10, 2014
Improve SNR 10 to 20 dB? Absolutely!
Last time, I introduced the technique of time averaging, also known as vector or coherent averaging. When available in a signal analyzer and used on suitable signals, the SNR improvements are impressive, as shown below.
Compare the results of trace averaging (top left) and time averaging (top right) on the same signal with the same amplitude scale of 10 dB/div: Trace averaging reduces the variance of the results, while time averaging substantially reduces noise in the measurement. The pulsed signal’s RF envelope is shown in the lower trace, along with time-gate markers and the level of the magnitude trigger (dashed white line).
In this example a pulsed signal repeats with consistent phase and the IF magnitude trigger of the 89600 VSA software is used to align successive acquisitions of the signal for 100 time averages. The average noise level is reduced by about 20 dB, while the measured signal level is unaffected. Note, however, that the variance is higher in the time-averaged result.
The bottom trace in the figure shows the averaged time record from which the results are calculated. In this example, time gating is used to isolate the spectrum measurement on the pulse only.
Most measurement types can be calculated from the averaged time record, including spectrum, phase, delay and analog demodulation. All these measurements will benefit from the lower effective noise or better SNR of the time averaging.
As described last time, this technique has two big drawbacks: the need for a signal that repeats in a coherent fashion and some way to trigger the averaging process. Fortunately, repeating signals are common in wireless and aerospace/defense applications, especially those that use signals generated from arbitrary waveform generators or similar processes.
In addition, it is not necessary for the entire signal to repeat. Time averaging can be combined with gated or other time-selective measurements focused on the repeating portion of a signal that otherwise changes in some way from cycle to cycle. For example, the preambles of many digitally modulated signals repeat consistently even when payload data varies from frame to frame.
As for triggering requirements, Agilent signal analyzers and VSA software offer several solutions. The IF magnitude trigger, mentioned previously, is often a good choice and can be used with signal captures or recordings. External triggers are also available in many applications, especially when signal generators are used to generate framed signals or when the trigger associated with the repeat of an arbitrary waveform is available.
Another trigger source is the periodic timer or frame trigger function available in some Agilent signal analyzers. This function offers a high degree of precision and flexibility in generating periodic triggers from the analyzer itself rather than the input signal. More on this in a future post.
Lastly, I should mention that time averaging works on CW signals as well as those that are time-varying. Of course, with CW signals one can also reduce RBW to improve sensitivity. With time-varying signals you sometimes can’t do that because narrow RBWs can filter out some of the signal you’re trying to measure.