[Elecraft] IMD and supply voltage

[Wayne Burdick]

Greg,

Even a rig with predistortion capability will run out of steam and exhibit greater IMD when running from low supply voltages. Predistortion attempts to compensate for the compression by increasing drive, but there must be enough headroom to allow the algorithm to work. Note that the rear panel of the ANAN 100D specifies "13.8V DC". I doubt that the ARRL did any testing at lower voltages. There are other situations where inadequate headroom might come into play, too, such as when driving a somewhat reactive load that causes the transmitter to hit a current limit.

While this particular radio emphasizes predistortion, there's more than that involved in keeping the bands clean. Figure 3 of the QST review for the rig (page 47) shows transmit phase noise that is about 15 dB higher than a K3+KSYN3A or a K3S at a 100-kHz offset. This 15 dB of additional wideband noise would in many situations (such as FD or multi-transmitter stations) cancel out the value obtained with predistortion.

73,
Wayne
N6KR

On Oct 29, 2015, at 2:07 PM, Gregory Schippers <kc8hxo2 at gmail.com> wrote:

> I personally found the recent review in QST of the ANAN 100D using
> pre-distortion to lower TX distortion products to be the most exciting
> thing I have read in that rag in a LONG time. I DO hope they are
> setting a standard that others will strive for. It was so much better
> than any other rig they have ever tested, that is was literally "off
> the chart".  Not sure I want a rig controlled ONLY with a computer
> interface, but this might sway my decision to at least try one!
> 
> Greg in West Mitten, KC8HXO
> 
> On Thu, Oct 29, 2015 at 2:58 PM, Jim Brown <jim at audiosystemsgroup.com> wrote:
>> On Thu,10/29/2015 12:06 PM, frank wrote:
>>> 
>>> Also, you did not answer my questions.  Maybe I should ask them one at a
>>> time.
>> 
>> 
>> I thought Wayne answered them quite well, but I'll try to rephrase his
>> answers in terms that are easier to understand.
>> 
>> 1) Contemporary ham rigs are designed to operate from automotive DC voltages
>> as a matter of convenience.
>> 
>> 2) The output transistors used in these rigs produce less distortion at
>> higher supply voltages.
>> 
>> 3) Most rigs are optimized for 13.8 - 14 VDC, and are tested at that level
>> by ARRL.
>> 
>> 4) Most commercial power supplies marketed to hams are regulated to 13.8 -
>> 14.5 VDC, and that voltage is internally adjustable.
>> 
>> 5) Most rigs will work at lower DC power supply voltages.
>> 
>> 6) When operated at lower DC power supply voltage, their distortion will
>> increase.
>> 
>> 7) When operated above their design output power, distortion will increase.
>> 
>> 8) Doing both 6 and 7 will increase distortion even more.
>> 
>> 9) Distortion in the K3 and K3S output stage is specified for full power
>> output at 13.8 - 14 VDC, and distortion is greatly reduced when run at lower
>> power levels.
>> 
>> My own comments:
>> 
>> Distortion causes the most QRM when running high power. Many of the best
>> power amps reach full power with drive power in the range of 50-60 W -- the
>> KPA500 needs about 28W, my Titans (I have three) need between 40-60W,
>> depending on condition of the tubes and the operating frequency. An ACOM
>> 1010 needs between 50 and 70 watts, again depending on the operating
>> frequency.
>> 
>>> This is state of the art?
>> 
>> How much do you want to pay for "state of the art?" How about the infamous
>> pentagon $800 hammer?
>> 
>> According to ARRL measurements, the K3 produces the cleanest CW signal of
>> any current rig. See my summary of their published data in the following
>> link
>> 
>> k9yc.com/TXNoise.pdf
>> 
>> See my measurements of my K3 and several other rigs in this link
>> 
>> http://k9yc.com/P3_Spectrum_Measurements.pdf
>> 
>> 73, Jim K9YC
>> 
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