[Elecraft] K3 Rx IMD

[Joe Subich, W4TV]

Ramiro,

> Anyway, since I changed AGC-F to AGC-S or even better AGC-off, I have
> no longer received duplicated traces on the screen. If I change to
> AGC-F I get consistent duplicated traces. I do not certainly know if
> the spurs are generated out or inside the K3, I tend to think that
> the latter.

Try setting AGC DCY = SoFt as recommended by Lyle, KK7P.  You are
seeing the effect of the BEAT between the two signals modulating
the DSP AGC.  The SOFT AGC Decay was designed specifically for the
situation when the two signals were within the AGC loop filter!

I believe you noted that the effect was almost non-existent when
the two signals were 100 Hz apart vs. 10 Hz.  This is the nature
of nearly every receiver with DSP based AGC.  The DSP is so much
FASTER than conventional analog AGC that these effects (AGC induced
IMD) begin to appear.

Fortunately Lyle and Wayne recognized this issue some time ago and
have provided the tools to deal with it.

73,

    ... Joe, W4TV


On 11/25/2010 4:56 PM, Ramiro Aceves wrote:
> Thanks Geoffrey for the interesting information about IMD measuring.  I
> see that it is a difficult task, it implies using carefully made
> equipment and care. Everything is out of my possibilities at the moment.
> I have made these easy experiments just trying to have an explanation of
> what occured in my K3 when tried to see aircraft reflections and got
> duplicated reflections. While they are not perfect, these tests have
> teached me implications of AGC-F/S and have duplicated a IMD problem
> under more "controlled" circumstances.
>
> Trying to obtain a decent test gear is a difficult task for me, and
> perhaps it makes no sense to invest time and money on it. I am not a
> receiver tester, just a curious person who likes to know why things
> happen.
>
> Anyway, since I changed AGC-F to AGC-S or even better AGC-off, I have no
> longer received duplicated traces on the screen. If I change to AGC-F I
> get consistent duplicated traces. I do not certainly know if the spurs
> are generated out or inside the K3, I tend to think that the latter. But
> who knows, perhaps the K3 receiver is not as perfect as many of us think.
>
> Thank you, will keep in touch.
>
> Ramiro. EA4NZ.
>
>
>
> Geoffrey Mackenzie-Kennedy escribió:
>> Ramiro,
>>
>> Some very good information about Combiners is published on Jack Smith's
>> website (Clifton Laboratories), also how to perform "IMD" tests.
>>
>> The reason for using a lowpass filter between each generator and the
>> combiner is to prevent the second harmonic of generator "A", for
>> example, getting into generator "B", mixing with the fundamental of "B"
>> to produce a third order product 2A - B. Similarly to keep the second
>> harmonic of "B" out of "A". Any mixing of other harmonics can also
>> result in "alien" third order products.
>>
>> If you build a "pretty" version, I would suggest that each generator be
>> built in its own metal box, with all DC supply leads filtered. What you
>> do not want is the generators talking to each other :-)
>>
>> Good luck with your project.
>>
>> 73,
>> Geoff
>> GM4ESD
>>
>>
>> Ramiro Aceves wrote on Thursday, November 25, 2010, at 7:52 AM:
>>
>>> No problem. I cant resist what you propose. I am going to build Xtal
>>> oscillator. I have a 4 MHz cristal here. The the combiner.
>>> Keep tuned!
>>>
>>> Ramiro
>>
>>
>>
>>
>
> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:Elecraft at mailman.qth.net
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
>