[Elecraft] K3 pwr out on digimodes
Joe Subich, W4TV
lists at subich.com
Fri Jan 16 12:30:44 EST 2009
Julian,
> I did my test for output over a range of frequencies with 4-5
> bars of "ALC" and as I stated, the power varied by about 2dB.
> If ALC is being applied at the DSP that is just plain wrong
> in my opinion, as it cannot compensate for the variations in
> gain at different frequencies due to the shape of the filter.
This is incorrect ... the purpose of ALC is not to control the
shape of the passband across the SSB filter it is to prevent a
SSB signal in total from overdriving the power amplifiers. The
purpose of equalization is to flatten the filter response. ALC
and EQ are not the same ... you need to use the proper tool for
the job.
No ALC system can compensate for the filter ripple. ALC is
nothing more than a system of compression (or clipping). In
general, ALC does not act until the drive reaches the threshold.
In a "traditional" transceiver ALC is derived from grid (or
base/gate) current in the final amplifier and there is no ALC
action until the grid/base/gate current begins to rise. To
obtain a constant power output, the user must adjust the drive
level as the drive to the final amplifier changes based on
modulating frequency. If the user does not adjust for changes
in gain across the transmit passband, operating just before
the onset of ALC with one frequency will result in significant
ALC on another modulating frequency ... conversely, operating
with (audio) drive at the threshold of ALC where the filter
losses are least will result in significant lowering of power
where the filter ripple is the greatest.
The effect is NO DIFFERENT than the variation of output power
with modulating frequency in the K3. If the DSP based ALC
in the K3 keeps the level of the 15 KHz RF constant, the
output power will vary with frequency based on the response
of the IF filter.
The purpose of ALC is not power level control. It's purpose
is to keep the drive level below the point at which any stage
in the RF chain begins to saturate (compress or clip).
The responsibility of output power control with any radio is
the power level control (PWR in the K3). However, the K3
PWR control is not a closed loop control. The power control
simply sets the 15 KHz drive based on the system gain (with
filter loss) at the single frequency used when performing the
power calibration.
If you want to tune by clicking around in the waterfall
instead of using the tuning knob you need to do one of two
things ... use EQ to compensate for the filter response or
design your software to compensate by adjusting the audio
level with frequency.
In short, you are asking both the ALC and PWR controls to
do something that they are not designed to do. The ONLY
WAY to maintain a constant output level on any transceiver
is to use a closed loop power control. Closed loop power
controls are not appropriate to SSB systems because time
constants sufficient to maintain constant power across the
IF passband for digital operation would result in frequency
dependent clipping and/or compression in voice operation.
The only ways to maintain constant levels across the
passband are by using an IF filter with very low ripple
or by generating RF "on frequency"
73,
... Joe, W4TV
> -----Original Message-----
> From: elecraft-bounces at mailman.qth.net
> [mailto:elecraft-bounces at mailman.qth.net] On Behalf Of Julian, G4ILO
> Sent: Friday, January 16, 2009 8:53 AM
> To: elecraft at mailman.qth.net
> Subject: Re: [Elecraft] K3 pwr out on digimodes
>
>
>
>
>
> Don Wilhelm-4 wrote:
> >
> > The K3 suggested ALC level for digital modes is *4* bars. The 6 bar
> > indication is for SSB (actually listed as 5-7 bars).
> > According to the K3 FAQ page - when the indication is at 5
> bars, the DSP
> > ALC is just being "tickled". For data modes, ALC action is not
> > desirable, thus the 4 bar recommendation.
> >
> > Up to (but not including) the 5th bar on the ALC meter, the meter is
> > indicating something similar to VU units rather than ALC
> action - the
> > 5th bar is the onset of ALC.
> >
> >
>
> I did my test for output over a range of frequencies with 4-5
> bars of "ALC" and as I stated, the power varied by about 2dB.
> If ALC is being applied at the DSP that is just plain wrong
> in my opinion, as it cannot compensate for the variations in
> gain at different frequencies due to the shape of the filter.
>
> When ALC is applied at the output, as in SSB mode, the effect
> is a flat response. As long as only a little ALC is used
> there is no adverse effect on transmitted IMD when using PSK31.
>
> I think the dangers of using ALC when running PSK31 have been
> wildly overstated in some quarters. I have always operated
> PSK31 using just enough audio drive to give a bar or two of
> ALC and now having the KK7UQ meter I have been able to
> measure the effect of this when using an FT-817, K2 and K3.
> On the FT-817 some worsening of the IMD is noticeable but the
> signal is still clean, typically -27 or -28dB. On the K2 one
> bar of ALC makes no difference at all. Incidentally the K2
> has the cleanest PSK31 signal of all, typically -33 or -34dB
> at 5W output. The K3 at 5W on 80m measured -28 or -29dB
> whether using DATA A mode (with no 'real' ALC being applied)
> or SSB mode with one or two bars of real ALC.
>
> -----
> Julian, G4ILO. K2 #392 K3 #222.
> http://www.g4ilo.com/ G4ILO's Shack
> http://www.ham-directory.com/ Ham
> Directory http://www.g4ilo.com/kcomm.html KComm for
> Elecraft K2 and K3
> --
> View this message in context:
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>
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