I think there may be some semantic confusion over the term "averaging" and how averaging affects noise when making spectral measurements, so let me clarify what I mean. My comments are specific to the P3 but are fairly general.
The P3 has an AVERAGE function. It allows you to perform averaging of video traces over time intervals between about 50 milliseconds and 1 second. If I am trying to measure the dBm value of random noise, the trace looks somewhat ragged at the lowest averaging times. The trace on the display will bounce up and down several dB. I want the average value of the trace because that's what gives me the noise spectral level. I can do some visual averaging of the ragged trace to obtain the average. However, enabling longer averaging times in the P3 makes this easier because it reduces the jaggedness and the trace converges to a pretty smooth one. However, the smooth trace has exactly the same *average* value as the jagged trace. Anyone who has a P3 can demonstrate this to themselves. So, when I say the averaging hasn't reduced the noise level, it's the average level of the noise that hasn't changed. On the other hand, if I really want to make a weak narrowband signal stick out of the noise, then I will reduce the noise bandwidth of the spectrum measurement. The narrower bandwidth will filter out more noise in the RF (not video) domain. In the P3 you do this by reducing the frequency span. With the P3 you can vary the frequency span between 200 kHz and 2 kHz. Because the noise bandwidth is approximately span/450 in the P3, a 2 kHz span, for example, should give a factor of 10 (or 10 dB) reduction in average noise compared to a span of 20 kHz. When dealing with narrowband coherent signals, this can really make very weak signals become visible on the display when they are virtually invisible in a larger measurement bandwidth. I hope this clears up any confusion. 73, John W1FV -----Original Message----- From: Topband [mailto:[email protected]] On Behalf Of Jim Brown Sent: Thursday, February 25, 2021 10:39 PM To: [email protected] Subject: Re: Topband: FW: The WD8DSB mini-flag antenna (LONG!) On 2/25/2021 5:16 PM, John Kaufmann via Topband wrote: > The P3 averages power, not amplitude, so using longer averaging times just > smooths the display and doesn't reduce random noise. It has nothing to do with power. Last I looked, the P3 is reading and displaying the instantaneous voltage in the IF, and can be calibrated to voltage at the input. I've been doing swept measurements of complex quantities for nearly 40 years, first at audio frequencies and now at RF. Averaging DOES cause random contents of a bin to approach zero (or the noise floor), making correlated signals stand out. This has long been well understood. I the principle to measure the dynamic response of broadcast signal processing in a peer-reviewed paper to the Audio Engineering Society in 1986. The test signal was a swept sine embedded deep in musical program material to the point that it was barely audible to a trained listener, and detected by a synchronized swept narrowband detector. Because the swept excitation and swept detector are synchronized, the measurement produces the complex response of the system, and program material, being non-coherent, averages out. http://k9yc.com/AESPaper-TDS.pdf 73, Jim K9YC _________________ Searchable Archives: http://www.contesting.com/_topband - Topband Reflector _________________ Searchable Archives: http://www.contesting.com/_topband - Topband Reflector
