Posted by
Elecraft mailing list on
May 30, 2015; 10:58pm
URL: http://elecraft.85.s1.nabble.com/K3-FSK-power-transients-tp3712691p7603632.html
Today I put the old Bird wattmeter in line and found that the average power out of the K3 does *indeed* drop down and become somewhat erratic in fsk441 mode. As noted previously the PEP output stays the same although my watt meter shows some erratic behavior.
So my wattmeter is telling the truth. And my K3 has a problem. (I tried different computers and sound cards - all the same). The power fold back is in FSK441 and a similar mode called JTMS. Modes that vary the tone more slowly - JT65 and JT6M - do not cause the same large power drop (minor fluctuations in power only).
To use an separate receiver I had to switch from 50 MHz to 28 MHz and there I eventually found that if I run <12 watts (final amp not switched in) then the PEP/Average is quite near 1:1 (or 10:10). Its only when I go to a power level where the final is required, that I get this strange effect.
I get quite a high PEP/Average ratio on voice too, but it does not sound bad ...
The final still produces plenty of power, so it is not fried. I went through and repeated the transmitter gain calibration - to no effect.
So I guess I am going to contact Elecraft on Monday.
73
Ken
-----Original Message-----
From: Joe Subich, W4TV [mailto:
[hidden email]]
Sent: Friday, May 29, 2015 3:12 PM
To: Ken;
[hidden email]
Subject: Re: [Elecraft] K3 FSK power transients
> But I stand corrected on the average. I was computing an average of > the crest, not the effective value of the wave. Since these are sine > waves, the effective value is 0.707 of the crest. Of course, when the > crest value is varying over the period under consideration, you might > want to average the crests and then multiply by 0.707, which is kind > of what I was doing.
That is not the definition of peak to average in the RF domain.
For a CW (sine wave modulation) signal, the peak to average ratio is
1:1 or 0 dB. Since FSK (specifically, in this case FSK-441 - which is
4FSK) is a sequence of sine waves, the theoretical peak to average ratio is 0 dB. Now, if you introduce AM into the FSK modulation, e.g., some tone or tones have different levels, the crest factor departs from
0 dB.
In this case, Peak Power (or PEP) is the maximum average peak (CW) case and "average power" is the longer term average of the peak power. In a linear system, PEP is the CW (single tone) power while average power - per FCC definition is the value to which a capacitor would charge based on multiple peaks (or the average of the peaks of the modulation). For 4FSK where three of the 4 tones were 1.4 V Peak (1.0 V RMS) while the fourth tone was 1.12V (0.8 V RMS), the average voltage would be:
(3 * 1.4 + 1.12)/4 or 1.33 V Peak. The Peak voltage is the highest of the four tones or 1.4 V. Thus the Peak to average ration for a 4FSK signal with one tone at 80% of the other three would 1.4/1.33 - or
0.45 dB.
I'd strongly suggest you get an audio oscillator with a known, stable output level, pick one tone - either 1323 or 1764 Hz - and run it into you K3, set the mic gain to "4 bars of ALC" and measure the CW output power at that frequency. Then, without changing the audio level or mic gain, measure the CW level of your K3 at 882, 1323, 1764 and 2205 Hz. My guess is that you will find that one tone or another produces significantly less power than the others - probably because of a filter alignment or ripple issue.
73,
... Joe, W4TV
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