EMRFD Message Archive 8268
Message Date From Subject 8268 2013-02-18 20:20:53 Jerry Wolczanski KK7B MicroR2 problem I've been corresponding with a fellow ham who is computer shy, so I shall post his problem in the hopes that this group can lead him in the right direction. He's from Oklahoma.
He has a KangaUS KK7B MicroR2 40 meter kit that he built and thinks it is suffering from IMD from strong AM BC stations in/near the 40 meter band. He can reduce, but not eliminate, the problem by placing "about 10dB worth of attenuation" ahead of the receiver and adding some audio gain on the back-end with a LM-386.
The sound "....is as if the desired signal were being transmitted by an old boat-anchor transmitter with serious stability problems...."
He also has a "rock-bender, direct conversion receiver that he also built and he does NOT think this receiver has the IMD problem.
He's using a short coax fed vertical with loading coil and about 1000' worth of radials.
Using his commercial TS590 and a little probe antenna, he listed to the microR2 LO during this "IMD"....and sure enough, when the CW signals in the microR2 sounded sick, the microR2 LO also sounds sick. In the daytime, it's OK.
He thinks the microR2 might need to have the LO shielded. I think he needs some sharp front-end tuning - maybe both? He does not think a bandpass filter will do the trick as one potential offending signal is S0 +40DB on 6.875MHz.
He's using a relay to switch the antenna so a T/R switch can be out of the potential equation.
So there you have it!
8269 2013-02-18 22:00:49 victor Re: KK7B MicroR2 problem Checking the schematics, the VFO is a LC tuned oscillator directly connected to the mixers through a passive 90 degrees hybrid. It is quite possible that the VFO is being pulled by very strong broadcast stations signals that are in the 40 meter band. Using a filter would be very difficult because a very high-Q filter is needed to separate the close frequencies. The easier solution is indeed the use of an input attenuator. I recommend him to build a step attenuator with a few 10dB steps and adjust it to the maximal attenuati 8270 2013-02-18 23:29:55 Pete Burbank Re: KK7B MicroR2 problem Jerry,
I also am in Okla and have a Kanga Micro R2 and it also has osc. pulling
problems on 40M. It is well shielded with a 40M loop antenna. The station on
6.875 is WWCR out of Nashville (one of six "preacher" stations there). They
very, very strong here. I don't get any osc. pulling with converters ahead
of the KK7B DC 40M receiver so it must be getting in at the antenna. Victor
in that an attenuator is the best solution.
73 Pete NV4V/5
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8271 2013-02-19 06:54:40 Juanjo Pastor Re: KK7B MicroR2 problem How about a narrow trap (big L, small C) tuned on the offending station?
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8272 2013-02-20 07:22:55 Gian Re: KK7B MicroR2 problem 8274 2013-02-20 16:35:39 kb1gmx Re: KK7B MicroR2 problem Putting a 3125 based mixer into a uR2 would be a challenge as the
whole radio is very small. It would have to fit where two minicircuits TUF3 mixers are. Putting it off board would enter the realm of grounding issues.
First make sure the LO is stable, sounds like a maybe at best.
The LO uses a toroid so shielding is good but the tuning cap
wires and grounding can be suspect.
The radio must be in a metal box with care in grounding.
an attenuator before the input may help.
Tuned traps may or may not be effective depending on the source and
One thing that comes to mind is the VFO fet might just be a weak one and needs a few more turns
8275 2013-02-20 20:36:01 Jerry Wolczanski Re: KK7B MicroR2 problem Hmmm, I'm having trouble getting messages onto this group. I'll try again!
I like the idea of a trap, if it's narrow enough.
The front-end filter I thought would work is tunable. I used it
8276 2013-02-20 20:36:01 Jerry Wolczanski Re: KK7B MicroR2 problem I actually like the idea of a trap, and even if it knocked down the desired signal by a small amount, it probably wouldn't hurt a thing, in fact it would likely help matters.
My suggestion to him was a narrow, tunable filter. The one I used for years
8277 2013-02-21 03:24:07 Gian Re: KK7B MicroR2 problem Hi Allison,
I do not believe it is impossible to find space for the FST3125 I-Q mixer as I suggest. Using hugly style on a piece of PCB, the assembled unit should be around 20x30mm (just below 1" by just over 1") with the height limited by the trimmers (1/2"). The trimmers could be replaced by 2 or 3 resistors, once defined proper bias. I do not believe there could be real grounding issues, we are at 7MHz. I have done 2nd mixer replacement, using the I7SWX 2T H-Mode Mixer (FST3125 and FSA3157)in several commercial equipment (like FT1000, IC775, FT2000, TS940, IC765 to describe some) having IF input from 40 to over 70MHz and having to reposition the new mixer on a different mechanical position, did not discovered grounding issues.
8309 2013-03-02 18:32:14 Jerry Wolczanski Re: KK7B MicroR2 problem To recap, my buddy is a little computer shy....and has problems with IMD on his MicroR2 (my original note below). I printed and bundled up everybody's advice and sent it off via snail mail.
One of my early letters to him, I suggested he try some RF filtering at the front end.
I got a letter from him today. He thought a front-end bandpass filter was worth trying, since it did not require any mods to the board. So, using the EMRFD he built a triple-tuned filter, 100kHz bandpass, centered on 7050kHz. He included the design info:
Ro = 50 ohms
F = 7.05
B = .1MHz
q = 1
k = .707
Qu = 200
L = 1.0uH
A little tinkering with parts on hand and ugly-construction did the trick. He also had a signal generator and scope which he used to tweak the design.
Bottom line: The filter fixed his IMD. He tried it with and without the filtering during the evening hours when the problem manifested itself.
So thanks to the authors of EMRFD and all who responded to my original note. I learned plenty siting here on the sidelines and reading the mail!