[Elecraft] K3 pwr out on digimodes

[Joe Subich, W4TV]

> No, I do not misunderstand what ALC does. Perhaps I am just 
> not very good at explaining what I am trying to say. I 
> realize that ALC can only reduce the gain. That is why it is 
> necessary to set the audio drive level so that you get a 
> little bit of ALC across the whole passband range that you 
> want to use. Then, the ALC will be able to reduce the gain by 
> the necessary amount so that the RF output is constant across 
> that passband range.

Absolutely not!  When conventional rigs are generating ALC 
their final amplifiers are ALREADY in compression and they 
are generating IMD in the PSK31 signal.  PSK31 can not have 
ANY compression or clipping without causing increased IMD. 
Increasing the drive to the point that a comventional rig 
generates some level of ALC at all points virtually assures 
the generation of unacceptable IMD at most frequencies in 
the IF passband. 

> The FT-817 is not capable of 15W. But that is beside the 
> point. 

OK, do it at 5 watts ... 

> What would this experiment prove? I know as well as you do 
> that it would not be identical. That's why I am saying that 
> some form of level control needs to be in operation across 
> the whole of the passband you wish to use. 

That's my point exactly.  The K3 does not provide level control 
across the passband it provides a constant output from the DSP 
modulator (15 KHz IF).  

> A small amount of ALC with a slow time constant will not harm 
> the IMD in PSK31 mode.

Wrong! ANY COMPRESSION OR CLIPPING will effect the INTENTIONAL 
amplitude shaping in the PSK31 signal designed to prevent IMD 
and control the banwidth of the transmitted signal.  So called 
IMD is the result of a non-linear amplitued response in the 
audio/IF/RF chain.  PSK31 drive can be thought of as a signal 
that has been "predistorted" in such a way to cancel the 
modulation sidebands.  If that (amplitude) predistortion is 
lost due to compression or clipping at any point in the 
transmitter chain, the result is the appearance of the normal 
modulation sidebands (known as IMD). 
 




> -----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 4:40 PM
> To: elecraft at mailman.qth.net
> Subject: Re: [Elecraft] K3 pwr out on digimodes
> 
> 
> 
> 
> Joe Subich, W4TV-4 wrote:
> > 
> > 
> > This is your fundamental misunderstanding of ALC.  Traditional
> > ALC does NOT (and can not) increase gain - it can only DECREASE 
> > gain to bring the grid/base/gate current down to the level at 
> > which the final amplifier is not being overdriven.  DSP based 
> > ALC is similar, it can only adjust the modulator (DSP) output 
> > to the level determined to be appropriate at the single point 
> > in the IF filter at which the power calibration (5W/50W) was 
> > made.  
> > 
> > ALC (and power control) can not compensate for differences in
> > gain across the passband - in other words, non-flat response. 
> > If you wanted to compensate for that non-flat response, it 
> > would be necessary to calibrate the power (5W/50W) at regular 
> > intervals across the passband and build a gain vs. frequency 
> > table.  The DSP would then need to measure the frequency of 
> > any narrow band modulation and adjust the assumed gain of the 
> > power control circuit based on the gain of the IF chain at 
> > that frequency.
> > 
> > 
> 
> No, I do not misunderstand what ALC does. Perhaps I am just 
> not very good at explaining what I am trying to say. I 
> realize that ALC can only reduce the gain. That is why it is 
> necessary to set the audio drive level so that you get a 
> little bit of ALC across the whole passband range that you 
> want to use. Then, the ALC will be able to reduce the gain by 
> the necessary amount so that the RF output is constant across 
> that passband range.
> 
> 
> 
> > The PSK31 operator must adjust his drive (mic gain or power
> > control) with ANY transmitter if he changes his "subcarrier" 
> > frequency.  Even with conventional ALC if the drive is set 
> > to the ALC threshold when operating at the point of maximum 
> > gain, the output will be reduced when operating at other places 
> > in the passband (particularly when operating at the edges of 
> > the passand). 
> > 
> 
> No. This misses the whole point of the convenience of "click and call"
> operating with programs like Digipan and HRD. I never 
> suggested that the
> drive must be set to the ALC threshold when operating at the point of
> maximum gain. It must be set so that you get some ALC across 
> the whole of
> the passband you wish to use (probably corresponding to no 
> more than the
> -3dB bandwidth points, beyond that and the steepness of the 
> filter slopes
> would cause unacceptable distortion of even a narrowband 
> signal such as
> PSK31.) Then you will achieve constant power across that 
> range, because the
> ALC will be able to apply varying amounts of gain reduction 
> to achieve that
> constant power level. The K2 can do this. Why can't the K3?
> 
> 
> 
> > Try this experiment with your FT-817 ... set the power level 
> > to the maximum, turn off any compression, set your soundcard 
> > to 1700 Hz and the level for 15 Watts out of the FT-817 with 
> > NO ALC.  Now, without making any other changes measure the 
> > power output with a 200 Hz tone and a 3200 Hz tone.  Is it 
> > identical AT ALL THREE points? 
> > 
> 
> The FT-817 is not capable of 15W. But that is beside the 
> point. What would
> this experiment prove? I know as well as you do that it would not be
> identical. That's why I am saying that some form of level 
> control needs to
> be in operation across the whole of the passband you wish to 
> use. A small
> amount of ALC with a slow time constant will not harm the IMD 
> in PSK31 mode.
> The K2 demonstrates this.
> 
> In SSB mode the K3 ALC provides the level control I am 
> describing. If I turn
> CMP off and switch the audio source to LINE IN it works 
> exactly as I expect
> it to work in data modes (apart from not disabling EQ, which 
> is a different
> issue.) In DATA A mode it does not do this, so you must 
> constantly tweak the
> drive level whenever you change frequency if you wish to 
> transmit a specific
> power level.
> 
> If this is how the K3 is supposed to operate then I shall put 
> it up for sale
> right away, because it is not something that people who use 
> data modes a lot
> want to do. (Well, perhaps there are some, because if I have 
> learned nothing
> else from my participation in ham forums like this, it is 
> that some people
> actually like doing things the hard way.)
> 
> -----
> Julian, G4ILO. K2 #392  K3 #222.
> http://www.g4ilo.com/ G4ILO's Shack   
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