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:topband-bounces+john.kaufmann=verizon.net@contesting.com] On Behalf
Of Jim Brown
Sent: Thursday, February 25, 2021 10:39 PM
To: topband@contesting.com
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
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