[hpsdr] ALC and power control

[Jim Sanford]

Warren:

Thank you very much for a complete answer.

My vacuum-state amplifiers will go to full power with less than the full 
output of HPSDR/Munin.  I do have a monitor scope.  So, I think I will 
proceed as follows:

     Initiate two-tone test.

     Increase drive until the trapezoid pattern shows beginnings of 
non-linearity.

     Back off drive until linearity is restored.

If I correctly understand what you explained, given this, I can never 
exceed that amount of drive, and therefore never over-drive my 
amplifiers.  That's what I was after.

Thank you so much!

Thanks and very 73,

Jim

wb4gcs at amsat.org


On 11/22/2018 11:31 AM, Warren Pratt wrote:
> Hi Jim,
>
> There should be no output power spikes on our openHPSDR implementations.  In addition to being designed NOT to generate such spikes, Adam, VA7OJ, has tested ANAN radios for this several times.
>
> As far as power control, I can offer the following information:
> * Except for the original Penelope board, the maximum output power level is set by controlling the current supply in the DAC.  Based upon the setting of the Drive control (and the "PA Settings"), software just specifies a constant value for this.  The value changes only when the Drive control or PA Settings are changed.
> * Within the transmitter channel in the WDSP library, I ramp up the signal amplitude, along a raised cosine curve, at the beginning of a transmission and ramp it down at the end.  This avoids any "click" that might be heard as the result of a discontinuity in the waveform.
> * The ALC block in the WDSP transmitter absolutely does not allow the signal to exceed the maximum level.  This is done without distorting by looking ahead at the coming waveform and gradually adjusting gain before any peak (caused by, for example, a filter overshoot) gets to the output.
>
> HPSDR does not offer any kind of ALC input for the transceiver board.  I'm definitely not a fan of ALC feedback and view it as closing the barn door after the horse has already left.  In other words, it decreases the power based upon feedback that the maximum power has already been exceeded.  That said, there are a couple solid-state amps out there that apparently do not have sufficient protection mechanisms built-in to protect themselves, for example against a high-SWR condition.  According to operation manuals, they need to have an ALC line that they can pull to protect themselves against NON-SPIKE conditions such as this.  Because of this, sometime ago, Clyde, K2UE, designed an external board that accepts the ALC line from an amplifier and converts its level such that it can be used to drive an unused low-speed ADC input on many of our transceiver boards.  Doug and I supported this in software.  I don't know the current status on that.
>
> As an aside, due to an email change, I don't believe my post will make it through to the reflector.  Feel free to re-post it there if you'd like.
>
> 73,
> Warren  NR0V
>

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