K3: Output power on PSK31 and AFSK

[Jerry T. Dowell]

I am running beta 2.63/1.95 firmware. Although power output is normal in CW,
USB and LSB modes (setting SSB to operate with several bars of ALC), I can
only get about 10-20W out on PSK31 and 50-60W out on AFSK before ALC kicks
in. This is the case whether I use LINE IN or FP/RP MIC connections for the
input. It seems to me that I was able to comfortably get at least 50W out in
these modes when I first tried them several months ago, although I am not
sure of that. The low power also occurs if I use USB instead of DATA A for
the digital signals.

Has anyone else run into this problem? Am I forgetting something?

Jerry  AI6L

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[Richard Ferch]

Hi Jerry,

The setup for the K3 in PSK31 is quite different from the way you set up
most other radios. See
<http://n2.nabble.com/K3-Transmit-IMD-td466326.html#a466329> for an
explanation. There was a discussion about this just over a month ago on this
reflector, as a result of which I personally changed my practices
considerably from what I had been doing before with another radio.

As I understand it, the first 5 bars on the K3's ALC meter are not really
ALC readings, at least not in the same sense they are on other radios.
Provided your power control is set properly, you can have 5 bars reading on
the ALC meter without any significant increase in IMD (as measured at my
station with a PSKmeter in passive measurement mode). It appears as if the
first 5 bars on the ALC meter are really just audio level readings (like a
VU meter), and it's only after that point that the ALC actually kicks in.
Even after the K3's ALC kicks in at 5 bars, it does not seem to introduce
non-linearity, which is a drastic change from how many other radios behave.

The appropriate way to set up for PSK31 appears to be the following:

First, set the power control to the desired power setting. For PSK modes,
I'd suggest making sure it is no higher than 50 watts. I believe this may
actually be the most critical setting. If your power is set higher than 50
watts average, then if your Line In level is high enough so that your
average power can exceed 50 watts, you could have instantaneous peaks that
exceed 100 watts. If these instantaneous peaks go into clipping, you will
have IMD regardless of the ALC meter reading. By setting the requested power
to 50 watts or less (5 watts if you don't have a KPA3), you prevent the
signal from going into clipping on instantaneous peaks.

Next, adjust the Line In level with the Mic gain control until you get four
to five solid bars on the ALC meter. This should allow you to get your
average power on PSK31 up close to the desired power setting without IMD.

Actually, in my experiments I could push the Line In setting higher without
any apparent ill effects, but without increasing power either. There doesn't
seem to be any point to doing this, though.

As for AFSK RTTY, that is not sensitive to non-linearity - it's like CW. You
can quite happily run AFSK RTTY at 100 watts power with 5 bars showing on
the ALC meter and still have a clean RTTY signal.

I have my digital mode software set up to switch the K3 automatically into
DATA A mode with a power setting of 50 watts for PSK31, and to AFSK A mode
with a power setting of 100 watts for RTTY. My sound card volume control is
set low enough to avoid non-linearity in the sound card, and the Line In
setting is set just high enough to reach 5 bars on the K3's ALC meter (LINE
8 in my case). So far that seems to be working just fine.

The biggest limitation may well be heat dissipation. I often operate with
the PA temperature display showing in the VFO B readout. For short
contest-style overs in RTTY, the finals hardly even get warm, but for longer
overs such as ragchews in PSK31, they heat up quite a bit even at 50 watts,
and the fans seem to work a lot harder than they do in CW.

One peculiarity I notice is that while I am transmitting, the temperature
reading seems to be lower than during receive. I suspect that this may be
due to the drop in supply voltage at higher current draws (even with a
stiffly regulated power supply, resistance in the supply cable will result
in a drop in voltage at the radio). Anyway, the net result is that in RTTY
the fans may be on while the radio is receiving, then turn off when the
radio goes into transmit, only to come back on again as soon as the
transmission ends(!)

My 2c worth.

73,
Rich VE3KI
K3 #1595



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[Don Wilhelm-4]

Jerry,

I am confused about your saying "before ALC kicks in", and it makes me
wonder if you are using enough audio gain.

The latest recommendation from Lyle is to increase the line-in gain (or
the soundcard gain) until you have 4 to 5 bars indicated on the 'ALC'
bargraph.  At that level, Lyle assures us that we are not driving the K3
into ALC compression and that is the designed operating point for the
K3. See below.

If you do not adjust the gain to give 4 to 5 bars of indication on the
"ALC" bargraph, you will not have full power output.  But please, use
the lowest power to maintain your QSO on PSK31 - other operators will
appreciate it because you will not be wiping out their QSO on a nearby
frequency.

73,
Don W3FPR

If I may quote part of his reflector post dated 4/06/2008:
--------------------------------------------------------------------
In this case, the ALC meter display is acting as a "VU Meter" and is
being used to help you set the Tx audio path gain correctly.  It is not
showing you how much you are over-driving the PA.  This is why you can
set it up in Tx test mode, when you are not transmitting any RF at all.

If you have a little too much drive, the DSP will scale things back at
the output of the mic amp.  Thus at the 5th ALC bar, the DSP is at the
threshold of gain compression.  At the 6th bar, it is cutting back gain
by 6 dB or so.
--------------------------------------------------------------------

Jerry T. Dowell wrote:

> I am running beta 2.63/1.95 firmware. Although power output is normal in CW,
> USB and LSB modes (setting SSB to operate with several bars of ALC), I can
> only get about 10-20W out on PSK31 and 50-60W out on AFSK before ALC kicks
> in. This is the case whether I use LINE IN or FP/RP MIC connections for the
> input. It seems to me that I was able to comfortably get at least 50W out in
> these modes when I first tried them several months ago, although I am not
> sure of that. The low power also occurs if I use USB instead of DATA A for
> the digital signals.
>
> Has anyone else run into this problem? Am I forgetting something?
>
> Jerry  AI6L
>  
>

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[Jerry T. Dowell]

Thanks to everyone for the information on setting up the K3 for digital
modes. My low power problem was solved by running TX gain calibration and by
setting the LINE IN gain for a couple of bars on the ALC indicator. Also, I
finally realized that most wattmeters will indicate half the actual power on
PSK since the spectrum is two-tone. This will be the case for all wattmeters
that actually measure the RF voltage, as contrasted with the bolometer type.

Jerry   AI6L



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[Don Wilhelm-4]

Jerry,

The Behringer 802 mixer is inexpensive and will do that for you (plus a
lot of other stuff)
Feed each receiver's audio into the Line IN jack below the microphone
jacks.  The PAN control will be used to adjust the amount of blend you
want.  In addition you will have level controls for each channel as well
as a single control for the headphone level and another for the main out
- lots of flexibility.

The downside of the Behringer 802 is that it does not contain a power
amplifier, but has more than adequate output for headphones - add a
stereo power amplifier if you need or want to drive speakers.

73,
Don W3FPR.

Jerry T. Dowell wrote:

> Thanks to everyone for the information on setting up the K3 for digital
> modes. My low power problem was solved by running TX gain calibration and by
> setting the LINE IN gain for a couple of bars on the ALC indicator. Also, I
> finally realized that most wattmeters will indicate half the actual power on
> PSK since the spectrum is two-tone. This will be the case for all wattmeters
> that actually measure the RF voltage, as contrasted with the bolometer type.
>
> Jerry   AI6L
>  
>

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