[Elecraft] [P3] How to measure bandwidth of RX signal

[Jim Brown]

On 4/28/2014 8:37 AM, Vic Rosenthal K2VCO wrote:
> What should work is to use the spectrum display of the P3. If you want 
> to know, for example, the bandwidth of a signal at 30 dB down, you 
> just find the points where the 'skirts' of the signal are 30 dB below 
> the peak. This is easy to do on the P3 which can display the signal 
> strength in dBm. 

Yes. And as N1AL recommends, use peak hold. I've done spectrum plots of 
various signals and conditions with the P3. I usually set the SCALE for 
a pretty wide range (like 72 or the full dB), and set the SPAN to about 
10 kHz. I use both the Peak Hold Mode and the Average Mode. In both 
modes, but especially with Peak Hold, we must take care to differentiate 
sidebands from other stations. I try to set the reference level so that 
the peak of the signal is at one of the level lines and the noise level 
is near the bottom of the display.

With almost any rig, you'll see small signals down the sides of the 
slope that are intermod products, and they are usually symmetrical on a 
CW or RTTY signal. A clean SSB signal should be well confined by the 
bandpass filter limits of the audio and TX filter skirts. Anything that 
extends beyond that is IMD. When a rig is badly overdriven, it's common 
to see splatter as horizontal streaks extending above and below the 
signal frequency.

Last week, I was on 20M SSB trying to work a weak DX station, but what I 
heard was massive splatter from a QSO about 5 kHz up the band. One guy 
was relatively clean, but the other was not. I broke the QSO to tell him 
he was clobbering a weak DX station. Turns out he was an aeronautical 
mobile using whatever radio was onboard. He didn't know whether he was 
transmitting AM or SSB, but thought it was AM. I assured him that it 
was, indeed, USB, and that the problem was that he was likely 
overdriving it badly. He went QRT to figure it out.

As a percentage of transmitted signals, I don't find contesters any 
better or worse than non-contesters -- we contesters stand out because 
during a contest, there are a LOT more of us. :)  I'm retired, so I have 
access to the ham bands during the day (when I'm not working on 
something else). It's not at all uncommon to tune around 30, 40, and 80 
during a weekday and hear fewer than a half dozen signals on these bands 
(if you hear any at all), and no more than a few dozen on most of the 
higher bands. Compare that to a major contest, when there are several 
hundred workable signals on every band from any given location for the 
duration of the contest, and for stations with better antennas, perhaps 
2-4X that number. The winners of contests like the ARRL SSB Sweepstakes 
make more than 2,000 QSOs, and you can work a station only once.

73, Jim K9YC