[Elecraft] KX3 - UHF emissions

[Darren Long]

Hi,

The #61 ferrite torroids and snap-ons arrived today. I've confirmed that the
sproggies I'm receiving in my other radios aren't being picked up in the near
field of my KX3 by relocating the entirely disconnected KX3 to the other end of
(my admittedly small, terraced) house, and the strength of the spurii remain
roughly the same.  I've also taken the KX3 upstairs to within 8ft of the discone
in the loft and have seen the strength of the same specific sproggy increase by
about 20dB.

My I/Q cable assembly includes a short adapter with a 2.5mm plug and in-line
3.5mm socket, followed by an inline stereo line-isolator and then on to to the
extension cable to the PC (all other connections being with 3.5mm TRS other than
the one on the rig).

The torroids I bought are Fair-Rite 5961001201, I got 5. I am able to get 2
turns around the torroid with the short 2.5/3.5mm adapter, which when straight
is about 6 inches long.

Without nothing connected to the KX3, I'm seeing the spur on 389.4MHz at about
-68dB, as observed on my USRP using my loft discone. The noise floor is about -80dB.

Plugging in the adapter without the 2 turn choke boosts the receive signal to
about -46dB.

Plugging in the adapter with the 2 turn choke puts the spur at about -65dB,
about 20dB improvement than without the choke (how much the bunching of the
cable into a small bundle contributes to this I don't know).

After connecting up the full compliment of cables and liberally scattering the
remaining 4 #61 torroids and 6 snap-ons around the rest of the KX3's own
personal rats-nest, the spur is at about -61dB, around 20dB over the noise floor
in the USRP (sampling at 125kHz).

This is a decent improvement, over the same setup without the benefit of the #61
ferrite, but I think there's still some way to go before satisfaction is achieved.

Also, it should be remembered that this is just a spot check on a single
frequency. I have good reason to suspect that there are other sproggies related
to this fundamental, and then a vast range of other fundamentals to consider,
all equally likely to generate some cruft in undesirable places. I have no idea
yet how effective the ferrite is at taming the full spectrum of unwanted
emissions.  I will have time to investigate further over the coming weekend.

If there was a way to nail this inside the KX3 box, I would be very grateful.

I've often thought that an optical SPDIF interface for the I/Q output would have
made sense.

Cheers, 73

Darren, G0HWW

On 26/06/14 19:07, Wayne Burdick wrote:
> "Dave Lankshear" <dave at lanks.plus.com> wrote:
> 
>> I'd very much like to see a response from Elecraft on this one....
> 
> Hi Dave,
> 
> It may be possible to further reduce emissions, and we'll post if we have any new information. Some external cable treatments were suggested to the original poster by the world's leading authority on such issues. We added a 60-MHz low-pass filter right at the KX3's output early on, though the current issue isn't related to the antenna jack.
> 
> A word on design compromises: The KX3 is a very small all-band/all-mode radio with a high degree of integration, intended for lightweight field use. To hit our size/weight/cost targets, we chose to use miniature PCB-mount connectors where practical, and did not go overboard on internal or external shielding. We did use state-of-the-art multi-layer PCB design techniques, which provides a significant shielding effect for RF paths. As a result, the KX3 passed all required tests, and very few of the thousands of KX3 owners have had any issue with emissions. 
> 
> That said, if solutions to specific problems become practical, we will test and document them and make components available.
> 
> 73,
> Wayne
> N6KR
> 
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