[Elecraft] K3 FSK power transients

Fri May 29 11:46:42 EDT 2015

I was going by this definition:

http://en.wikipedia.org/wiki/Crest_factor

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 means the PAPR (peak to average power ratio) for a sine wave is, by definition,  3 dB.  If you have more than one tone present at any moment the PAPR must be higher than 3 dB (see the URL above).

But that is not what our fancy power meters measure.

These power meters are supposed to measure Peak Envelope Power and average power. With complex waveforms like speech there is no analytical relationship between PEP and average. It depends on the user's voice and the amount of speech processing used. The peaks can easily be 10-20 times the average in an uncompressed voice signal.

For FSK441, the modulating wave is mostly pure tones (of a constant amplitude if all goes well) and some minimal "keying sidebands" due to the 441 baud modulation. So I was thinking there must be some way to calculate PEP/average.

The result of a pure tone, in SSB, is a steady carrier and the filtered envelope of that RF is the same as the average power - so RF PEP/average is 1.0. Mind you, the *instantaneous* value of the RF voltage wave will be 1.414 times the RMS value, but the usual definition of PEP says that a steady carrier has PEP equal to average power. And, when I send one of the steady tones, "73" for example, the Wave Node meter reads PEP equal to average  (within some tolerance).  What happens is that I see the average power reading rise to the PEP.  The PEP reading stays the same. I imagine the K3 is riding herd on the peak and keeping it constant (even with no bars showing).

Time for lunch here ;-)  But I will send a note to the Wave Node guy and see what he says about those peaks and dips.

Ken

-----Original Message-----
From: Joe Subich, W4TV [mailto:lists at subich.com] 
Sent: Friday, May 29, 2015 10:50 AM
To: Ken; elecraft at mailman.qth.net
Subject: Re: [Elecraft] K3 FSK power transients

A quick reply since I'm on my way out ... I'll look more in depth later.

I am concerned about the audio waveform generated by your WaveNode device.  I see peaks to 128 +/- and dips to 64 +/- with a significant amount of asymmetry.  The asymmetry would make me wonder if there is not some clipping (bad transistor) in the LPA, KPA3 or in the WaveNode device itself.

Perhaps the only way to know for sure would be to use a second receiver and look at the *recovered* audio.

 > Mine is using the more steep-sided 8 pole filter and has the new  > synthesizer board and the latest firmware updates.

I'm also using the 2.8 KHz filter, new synthesizer and most recent production firmware for the KSYN3A.  Note there was no change in DSP code to support the KSYN3A so I would not expect the firmware to be an issue.

 > PEP/average (power) ratio is the (voltage) crest factor squared.  It  > is hard to get the effective(average) value of the wave by eyeball.
 > But you could take the min/max and divide by 2.

No - Crest factor is 10*log(Peak/Average) in terms of power and
20*log(Peak/Average) in terms of voltage.  Since the modulation is single sine waves (not a complex waveform) the average (RMS) is 0.7 x peak.  The lowest voltage I see is 0.8V, the highest is slightly less than 1.0 (call it 0.98).  Using the average of 0.98 and 0.80 (there should be about as many of each tone) we get 20Log(.98/.89) or 0.84 dB which is a reasonable match to the Peak to Average ratio reported by the LP-100.

> I tried backing down the line level to where no ALC action was visible 
> but there are still "dips".

Can't do that.  With less than 4 bars of ALC the DSP code will try to "follow level" and may make things worse depending on the time constant.

73,

    ... Joe, W4TV

On 2015-05-29 10:04 AM, Ken wrote:
> Good job Joe.
>
> My understanding is that the monitor output of the K3 should be a fair 
> representation of what another receiver would see - minus the effects 
> of propagation and any external power amplifier. But it is just really 
> something convenient to use (as evidenced by the quickness of your 
> response).
>
>> From a subjective/eyeball view of the waveform - we can see your K3 
>> is doing a better job than mine. I wonder what the differences are?
> Mine is using the more steep-sided 8 pole filter and has the new 
> synthesizer board and the latest firmware updates.
>
> PEP/average (power) ratio is the (voltage) crest factor squared.  It 
> is hard to get the effective(average) value of the wave by eyeball.
> But you could take the min/max and divide by 2.  On your graph, the 
> max is 1.0 and I see two points where it dips to about 0.75 (at 0.5 ms 
> and 11.5 ms).  That gives a crest factor of 1.14 and a PEP/average 
> power ratio of 1.3. In decibels that is 2.28 dB which is similar to 
> what a filter passband ripple might be.  Anyway, it's better that what 
> I am seeing.
>
> Some other artifacts are visible on the 2 kHz segments. For example, 
> at 2 ms there is a cycle that is shorter than the cycles on either 
> side;  at about 11.5 ms there is a cycle that is taller than the ones 
> on either side. Some of this is probably in the actual signal as 
> generated. Looking back at my "no-radio" graph I can see a small 
> fluctuation in the level of a 2 kHz segment where there are 9 cycles 
> in a row...
>
> In terms of RF sample, my power meter provides a scope function right 
> off the RF detector. I have included two pictures of this. These are 
> several overlaid scans snatched from a CQ message. The power readings 
> of my wattmeter must come from squaring this voltage measurement. It 
> has pretty high frequency response (0.1 ms features can easily be seen 
> - this is a bit much if used for the power meter readings imho).
>
> One of the things I tried yesterday was some "pre-emphasis" using a 
> software sound equalizer on my audio.  The tone levels at the monitor 
> output are closer in level, to each other, with the equalizer 
> inserted.  But there are still ups and downs in the envelope of 
> monitored waveform.  In terms of power, the graph with the 
> equalization "on" shows fewer upward peaks in the power level (second 
> file attached).  I have not tried with the linear amplifier on yet, 
> but it looks like the PEP/Average ratio has been reduced a bit with 
> the equalizer on (from 1.8 to 1.6) so I guess I will keep it.
>
> The "dips" in power are present in both graphs. Perhaps that is 
> inherent in the modulation? I don't know how to measure audio power at 
> the input to the radio.... if the signal from the sound card does not 
> have these dips, then the radio must be doing it somehow. I tried 
> backing down the line level to where no ALC action was visible but 
> there are still "dips".
>
> Later Ken
>
>
>
> -----Original Message----- From: Joe Subich, W4TV 
> [mailto:lists at subich.com] Sent: Friday, May 29, 2015 8:35 AM To:
> Ken_ke2n; elecraft at mailman.qth.net Subject: Re: [Elecraft] K3 FSK 
> power transients
>
>
> I don't know what a *monitor* output proves since it is *not* an RF 
> sample.  However, I have attached a Spectrum Lab Time Domain capture 
> as requested.  Notice the crest factor is well less than 1 dB.
>
> 73,
>
> ... Joe, W4TV
>
>
> On 2015-05-29 7:45 AM, Ken_ke2n via Elecraft wrote:
>> Joe -  I am using data mode with "DATA A" selected.  (This mode uses 
>> the SSB filter and turns off equalization and compression. But of 
>> course the ALC is "on").
>>
>>
>>
>> What would be really helpful is if you would download a copy of 
>> Spectrum Lab and run the monitor output of your K3 through it while 
>> transmitting FSK441. Then we would both have a common point of 
>> comparison (eliminating the vagaries of power meter PEP algorithms).
>>
>>
>>
>> http://www.qsl.net/dl4yhf/spectra1.html
>>
>>
>>
>> The time domain scope is under the "components" tab.  The default 
>> settings with the initial install will work fine, although you will 
>> have to select your input device (audio card) if it is not the system 
>> default. That is under the Options/Audio settings tab.
>>
>>
>>
>> Regards
>>
>> Ken
>>
>>
>>
>> From: Joe Subich, W4TV-4 [via Elecraft] 
>> [mailto:ml-node+s365791n7603599h9 at n2.nabble.com] Sent: Thursday, May 
>> 28, 2015 10:27 PM To: Ken_ke2n Subject: Re: K3 FSK power transients
>>
>>
>>
>>
>> Are you using DATA A or trying to force FSK-442 through the K3 in USB 
>> with the TX_EQ enabled?  Again, I see *NO* frequency effects (crest 
>> factor < 0.1 dB at 30, 60 and 100 W) with my K3 when using DATA A and 
>> the LINE In.
>>
>> Also make sure your sound card does not have any "enhancements"
>> enabled.
>>
>> 73,
>>
>> ... Joe, W4TV
>>
>>
>> On 2015-05-28 9:53 PM, Ken_ke2n via Elecraft wrote:
>>
>>
>>> 2015-05-27_20_48_16-Time_domain_scope_no_radio.gif
>>>
>> <http://elecraft.365791.n2.nabble.com/file/n7603596/2015-05-27_20_48_
>> 1
>>
>>
6-Time
>> _domain_scope_no_radio.gif>
>>> 2015-05-27_19_07_20-Time_domain_scope.gif
>>>
>> <http://elecraft.365791.n2.nabble.com/file/n7603596/2015-05-27_19_07_
>> 2
>>
>>
0-Time
>> _domain_scope.gif>
>>>
>>> I hope this works.
>>>
>>> I have shown, in two attached images, the tones being produced by 
>>> WSJT FSK441 and then the monitor output of the K3.  You can see a 
>>> substantial drop off of the low frequency tones and some kind of 
>>> additional modulation
>>
>>> of the overall envelope (most noticeable with the higher frequency 
>>> tone,
>> but
>>> I think it affects all). Not sure if this is ALC action or something 
>>> else.
>>
>>>
>>> Anyway, the K3 makes a bit of a mess out of some otherwise 
>>> nice-looking tones.
>>>
>>> Ken
>>>
>>>
>>>
>>> ----- 73 Ken
>>>
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>>
>>
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