[Elecraft] K3-P3 SSB spectrum

[Fred Jensen]

Exactly what I intended to say!  And Joe did it without waving his arms. 
:-) Thanks.

73,

Fred K6DGW
- Northern California Contest Club
- CU in the 50th Running of the Cal QSO Party 3-4 Oct 2015
- www.cqp.org

On 6/8/2015 11:44 AM, Joe Subich, W4TV wrote:
>
>  > He said my signal was very clean and asked me to crank up my Mic gain
>  > to the max which I reluctantly did for a short transmission. He said
>  > it stayed clean with no splatter.  Can that be?
>
> The K3 ALC is *very* effective.  It does not rely on over driving the
> final amplifier to produce a feedback voltage to reduce gain; instead
> it keeps the drive from the DSP modulation process to the level needed
> for the selected power output based on the results of the TXG (transmit
> gain) calibration.  That means the K3 does not overdrive the transmit
> IF stages, the low power (driver) amplifier or the KPA3 no matter how
> high you turn the mic gain or the compression level.
>
> Yes, the excess compression/mic gain can cause distortion in the DSP
> modulator but *will not* generate excess bandwidth.  Further, since
> the DSP generated transmit signal goes through the 2.7 or 2.8 KHz
> filter before passing through the transmit IF, any spatter if further
> reduced and - more importantly - no more is generated in the rest of
> the transmit chain because it is run strictly below clipping level at
> all stages (unless there is a *device failure* in the LPA or KPA3).
>
> Every other transceiver (except, perhaps the Flex 6000 series and
> ANAN) uses a closed loop ALC which is susceptible to distortion from
> both overshoot and misalignment of individual stage gains.  In the
> days of tube type transmitters this was not the issue it is now since
> tubes had a relatively "soft" clipping point - the transition from
> gain compression to hard clipping occurred over a large change in
> input levels - such that ALC feedback (typically grid current in the
> final amplifiers) started well before the tubes saturated.  However,
> with most transistors and FETs the change in input drive necessary
> to move from the 1 dB compression point to saturation is very small
> (as little as 1 dB or 20% increase in drive) and that can occur in
> any of several places (IF, driver, final).
>
> As soon as any single stage is driven even close to saturation the
> signal goes to crap - one need only look at some of the "big signals"
> on the band with a good SDR or the P3 to see many that look like
> AM signals with no carrier due to the amount of IMD that appears in
> the "other sideband".
>
>  > I thought any radio would splatter if over-driven.
>
> To review ... the K3's ALC circuits assure that none of the "post
> filter" stages are ever over-driven.  While the in-band audio can
> become badly distorted through a combination of excess drive to the
> DSP (high mic gain) and excess clipping/compression, once the DSP
> and IF filter have set the transmit bandwidth, the ALC *prevents*
> overdrive and assures that the frequency conversion and amplification
> processes do not allow the signal to become excessively wide.
>
> 73,
>
>     ... Joe, W4TV
>
>
> On 2015-06-08 1:56 PM, Richard Fjeld wrote:
>> Just a comment somewhat related to this thread.  Not long ago, a ham
>> with an SDR set-up was comparing signals in a round-table for proper
>> bandwidth.
>>
>> He said my signal was very clean and asked me to crank up my Mic gain
>> to the max which I reluctantly did for a short transmission. He said
>> it stayed clean with no splatter.  Can that be? I thought any radio
>> would splatter if over-driven.
>>
>> For me, seeing is believing.    Dick, n0ce
>>
>>
>>
>>
>>
>>
>>
>>> Date: Sun, 7 Jun 2015 16:58:26 -0400 From: lists at subich.com To:
>>> elecraft at mailman.qth.net Subject: Re: [Elecraft] K3-P3 SSB
>>> spectrum
>>>
>>>
>>> The TX Monitor is a time domain display - not a frequency domain
>>> display.  If you want to see the frequency domain display of your
>>> own signal, temporarily disconnect the RS-232 cable between the two
>>> (or connect another SDR to a directional coupler in the antenna
>>> line).
>>>
>>> 73,
>>>
>>> ... Joe, W4TV
>>>
>>
>>
>>
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