Re: Harmonics and Relay Replacement

Jim Sheldon
 

I've just finished the tests up through 20 meters and it's only 40 that I have any issues with.

Test parameters - since the power output gets lower the higher in frequency you go, on each band I set the SA so that the carrier was on 0 dB each time I changed bands. My rf pickup is from a 50 ohm dummy load through a voltage divider to the SA to keep input below +30 dBM.

80 Meters 3.500 MHz
7.000 (2nd harmonic) - 65.7 dBC
14.000 (3rd harmonic) -76.4 dBC

40 Meters 7.000 MHz
14.000 (2nd) -58.0 dBC
21.000 (3rd) - 47.2 dBC

30 meters 10.000 MHz
20.000 (2nd) -76.9 dBC
30.000 (3rd) -75.8 dBC

20 meters 14.000 MHz
28.000 (2nd) -65.2 dBC
42.000 (3rd) -64.8 dBC

I didn't test 17, 12 and 10 meters at this time but I suspect their harmonic suppression is in the same boat as the rest.

I did do a quick test on 20 meters with 1KHz tone into the mic jack (I'm a CW op and don't keep a microphone in the shack), power out about 7 watts, SA set so carrier was 0 (top) of screen and got basically the same results - 2nd harmonic @ 28 MHz was -65.0 dBC and 3rd @ 42 MHz was -63.9 dBC, very close to the same as I got for CW.  There were a few spurs that showed up on the 20 meter test, but they were all -80 dBC or better (I do have the 2nd 45 MHz filter in place).  

It appears that the Axicom relays DO provide a significant harmonic reduction over the originals used. ( Except for 40 meters on MY uBITX board.  I suspect there is something else in play there that I need to track down).  I still need to swap relays on the 2nd board and check it but as I said earlier that will be most likely tomorrow.

Jim Sheldon, W0EB

From: "Gordon Gibby" <ggibby@...>
To: "BITX20@groups.io" <BITX20@groups.io>
Sent: 10/10/2018 2:37:30 PM
Subject: Re: [BITX20] Harmonics and Relay Replacement

Thanks,  Jim for doing those tests!!


Sorry to hear the 3rd wasn't reduced any further.    In my tests of the external daughterboard idea, the 3rd harmonic on that particular band was reduced by 1.5 S units, and calibrations with inline attenuators suggested 1 S unit = 12 dB in that region.   Due to the granularity of the S-unit display on the ICOM 718 it might have really only been just over 1 S Unit...but still it appears the daughterboard (which deals more with the wiring) achieved a higher degree of harmonic reduction.


It is great there is such a huge group of people working on various solutions!!!


Gordon..


Ref:   http://arrl-nfl.org/wp-content/uploads/2018/08/uBitx-External-Daughter-Board-Construction-Document.pdf



From: BITX20@groups.io <BITX20@groups.io> on behalf of Jim Sheldon <w0eb@...>
Sent: Wednesday, October 10, 2018 3:17 PM
To: BITX20@groups.io
Subject: Re: [BITX20] Harmonics and Relay Replacement
 
OK peeps,
I measured the harmonics with my spectrum analyzer.  I can only show the fundamental, 2nd and 3rd harmonic so it's  not a complete test, but the results though positive, are not all that encouraging.

First, with the original relays, Carrier @ 7.000 MHz set to 0 on the vertical scale.  2nd harmonic was -58 dB from the carrier and the third was only -45 dB from the carrier.  I didn't measure the 4th & 5th though they were farther down than the 3rd.

Changed out the relays and measured again without changing any settings.

7.000 MHz carrier @ 0 (top) of the vertical scale.  2nd harmonic was -67.6 dB down and 3rd was -47 dB down.  That was a 2 dB improvement on the 3rd harmonic but not enough to call it a reliable harmonic fix with the Axicom relays.  It did help and it IS in spec, but there are other things that most likely need to be addressed to make it better.  It IS a step in the right direction and did NOT require complete re-design of the filters.  At least for 40 meters.

I will run the tests on the other bands and see how they come out, but I'm not holding out for much if any improvement over the 40 meter spec.

Ashhar Farhan, the Axicom relays DID help some in my V4 board, but not sure the added expense is justified for production.  I think the filter redesign on the production boards would be a far better long term solution IMO.

I am going to change out the relays on my other V4 board and test it as well but that will be later this evening or tomorrow.  If I don't report on that one, it will mean I didn't find any significant difference between the two boards.

The Axicom part # for the relays I used is V23105A5403A201.

Jim Sheldon, W0EB


------ Original Message ------
From: "Ashhar Farhan" <farhanbox@...>
Sent: 10/10/2018 12:58:56 PM
Subject: Re: [BITX20] Harmonics and Relay Replacement

Guys,
If someone else too can confirm this fix. We will start ordering axion relays from now on. - f

On Wed, 10 Oct 2018, 23:15 Jim Sheldon, <w0eb@...> wrote:
Yup, and after opening up one of the original relays that I removed from a V4 board in prep for putting new ones on (they should be here within the hour) it was extremely obvious that those relays were not designed with RF in mind.  There is no shielding of the relay coil at all and all the contacts are on movable arms that run parallel to each other and the full length of the relay as well as close to the relay coil itself.  The armature is mostly made of plastic so doesn't provide much (if any) shielding between the coil and the movable contact arms.  The movable arms are each made up of 2 parallel, approximately 2mm wide strips of metal that are separated in the middle of the top of the relay by about 1 mm.  (Pictures not possible as the relays were pretty much destroyed in the dissection.)  I believe the parallel arm construction was for current carrying capability (not needed in small signal RF relays) as the contacts are rated to 125V @ 60 watts (their designation) in the data sheet.  The width & placement of these contact arms would allow a whole lot of coupling between both sets of contacts and I really believe it's one of the culprits though not necessarily the entire problem.  

Changing the relays to an RF rated set with proper isolation certainly can't hurt and may just wind up being the simplest fix for the problem.

The Postman just delivered the new relays from Digi-Key so off to install them and run some tests to see if I can prove or disprove Mike Doty's data.  

I do believe my results will corroborate his after chopping up one of the old relays to see what was in it.

Jim Sheldon


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