[Elecraft] IMD and supply voltage

[Wayne Burdick]

Jim,

This appears to *not* be true, at least for one radio lauded by those concerned about TX IMD.

Earlier this year there was a long forum thread on Eham.net lamenting the "fact" that 12-V ham rigs were not as clean as 12-V marine rigs. One radio in particular, the Icom M802, was touted by forum participants as having far better IMD specs than any 12-V ham transceiver.

We downloaded the manual for we this radio and found that Icom was using the same devices and essentially the same amplifier circuit that we were. Still, we gave them the benefit of the doubt and purchased a new Icom M802 (from Amazon, $1813) to test in our own lab.

We very carefully measured transmit IMD at 100 watts on several channels, covering the full range of the HF marine band. Here is an example plot from about 12.28 MHz:

   http://www.elecraft.com/Icom%20M802%20100-W%20IMD.jpeg

This plot shows the two 3rd-order products being down by about 27 and 30 dBc, respectively. A plot for the Elecraft K3 posted earlier on this same forum showed these tones down 33 and 36 dBc--about a 6-dB improvement over this particular marine radio. 

A bit on our test setup: We used a very clean 14.0 V DC power supply with short cables, a high-performance analog 2-tone generator, and a very hefty 50-ohm nonreactive dummy load. We set the top of the spectral plot at 100 W and equalized the amplitude of the tones at -6 dBc, consistent with the usual ARRL method. The tests were done by our senior RF engineer, and I'm sure he was not overdriving the spectrum analyzer :)

Chances are that virtually all marine HF SSB radios being made today are using the same 100-W MOSFET PAs that we and other ham manufacturers do. As far as we can tell, there is no magic in the marine radios that is making them better. On the other hand, their marketing department may be better funded.

If you have another marine radio in mind that you think is using different/better circuitry, please let me know so we can repeat this exercise if necessary.

73,
Wayne
N6KR



On Oct 29, 2015, at 1:04 PM, jim <jbollit at outlook.com> wrote:

> Jim,
> 
> The marine divisions of Kenwood, et.al. already have low cost, low voltage,
> PA solutions with better IMD specs, primarily due to channel spacing for the
> marine band.
> 
> Jim
> 
> W6AIM