[Elecraft] Using an External Receiver Downconverter with the K2

Wed Dec 24 16:57:01 2003

I have intended to write this up for the Elecraft and QRP-L lists for some
time but keep putting it off.

I don't have time right now to do a detailed description but let me hit the
highlights.

I have brought out two "IF" signals from the K2. The first comes from the
input to the Noise Blanker as a convenient point to tap. The circuit is
simply a 22 pF ceramic cap and a length of RG-174 going from the cap
(connected to the NB input) to one of the two BNCs added on the rear deck in
the XVERTER holes.

 That was done in order to allow the K2 signals to operate the PRO2 Spectrum
Scope at the price of losing some of the K2 front-end magic. Remember that
in the K2, the IF filter actually performs dual service as a roofing filter
and an IF filter.

The IF signal from this point will be at various frequencies depending upon
mode and band. Finding these for your particular K2 and the downconverter
receiver is the challenge, and writing up that procedure has been the
delaying point for me. A tap at this point would not necessarily be all that
beneficial for the 746PRO without the SS. But it works great on a PRO or
PRO2.

The second tap comes from the output of the AGC amplifier! Trying to take a
signal from the IF filter output, even at the input of the NE602 second
mixer, proved to be too detrimental to the receiver operation. In the K2,
AGC voltage is derived not at the IF frequency but rather yet another NE602
downconverts the IF signal to around 154 kHz. This signal is then amplified
and rectified to become the AGC control voltage applied to the IF amplifier.
This signal has the full benefit of the IF/roofing filter front end and yet
is isolated from the IF frequency circuitry to avoid interaction and the
introduction of coupling that the K2 should have.  Obtaining AGC voltage
this way is an elegant solution to what is usually a difficult problem with
superhet receivers. And it gives us an excellent, isolated tap point for the
external receiver.

The circuit here is a little more complex. First, you remove R5, which
controls the gain of the op amp following the AGC NE602, and replace it with
a 50K minipot. This pot value can be adjusted to set the AGC gain. From pin
7 of the op amp (the output) the circuit comprises a 0.005 cap and a 47K
resistor in series with the RG-174 to the other BNC on the rear panel.

The 47K value was chosen to match the PRO2 S-meter readings with those of
the K2. The cap value is not critical but it needs to pass a 154 kHz signal.

I just don't have time (Christmas Eve with guests!) to explain the details
of finding the IF signal on the PRO dial, but the IF output will be in the
4900 kHz range and the AGC output in the 154 kHz range. The mode for the
external receiver is the same as that of the K2 on some modes and bands and
opposite on others. Sorry to be vague here, but you just have to start
tuning until you find the frequency and mode that causes the CW tone from
the external receiver to match that of the K2. That is the frequency to use
for that band and mode. Set the K2 IF for maximum bandwidth.

This signal is really clean and "narrow" and is a perfect candidate for the
PRO DSP IF magic. One drawback: at that low frequency, the preamps do not
operate. But everything else does and having the IF DSP capabilities make
the K2 seem like a new radio. The notch filter alone is reason enough to
make this mod

Well, that should be confusing enough, so I'd better quit. Rather than load
up the list with a lot of conversation on this, perhaps you should address
any questions to me privately. If my response is of general interest, I'll
post to the reflectors. I am also posting this to the Elecraft reflector.

Thanks for the interest!

73, George W5YR
[email protected]

.
----- Original Message ----- 
From: "Mark Rauchfuss" <[email protected]>
To: "Low Power Amateur Radio Discussion" <[email protected]>
Sent: Wednesday, December 24, 2003 10:05 AM
Subject: Re: newer simpler rig for me, but which one

> Hi Jim,
>
> This sounds like an interesting approach. George, can you please contact
me
> off-line to discuss how you accomplished this? I am interested in the
> interfaces and circuit diagrams.
>
> Thx & 73s,
>
>
> Mark
> ----- Original Message -----
> From: "James R. Duffey" <[email protected]>
> To: "Low Power Amateur Radio Discussion" <[email protected]>
> Sent: Tuesday, December 23, 2003 11:30 PM
> Subject: Re: newer simpler rig for me, but which one
>
>
> > Rod - Congrats on your new Argonaut.
> >
> > This is not such a far fetched concept:
> >
> >  "A K2 with the IF DSP/GC-RX functionality of the Icom 746pro. :-)"
> >
> > George, W5YR, who occasionally frequents this list, uses a similar
setup.
> He
> > takes the IF from the K-2 after the crystal filter and pipes it into his
> > 756PRO2, thereby getting all of the advantages of the K-2's excellent
> front
> > end, and all of the good back end pass band tuning and DSP of the Icom.
> >
> > For all of its excellence in the front end, the K-2 is still a bit
behind
> > the times in the back end processing and really benefits from the setup
> you
> > suggest. - Dr. Megacycle Kk6MC/5
> > ______________________
> > James R. Duffey KK6MC/5
> > Cedar Crest NM 87009 DM65
> >
>
>