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: li...@subich.com To:
elecraft@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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