EMRFD Message Archive 6650

Message Date From Subject
6650 2011-09-13 04:30:09 Fernando Krouwel About Elecraft K2 Receiver
Hi folks, good morning:

Trying to get some ideas for a simple hi performance homebrew receiver, I went to K2 Elecraft schematic (not so different from other circuits seen in EMRFD, like easy 90, etc), on the following link:

http://www.elecraft.com/manual/K2_Owner's_Manual_Rev_H.pdf

Schematics begin on manual page 142, but for this purpose we will go to page 143, where we can see the AGC circuit on the lower left.

The designer appears to have used a separate demodulator (NE602 based) for that task.

The uncommon detail is that, having an IF frequency of 4.9136 MHz, according to this manual, it appears to use a separated AGC product detector that uses a xtal of 5.068 MHz for the BFO, and picks up IF signal AFTER the main IF xtal filter (see page 145 for the IF receiver schematic, among other circuits). Will it pick up any signal there using a different xtal frequency for this additional demod?

Does anybody, please, have any idea about how this works [or, in other words, where is my mistake or not in this possible circuit (mis)understanding]?

73´s
Fernando - PY2ETT




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6651 2011-09-13 06:04:39 Tim Re: About Elecraft K2 Receiver
My understanding is that this is all done because the main product detector is a NE602 and traditional AGC diode-rectifying detectors would not be the "right" threshold for the lower optimum level NE602. I am possibly missing the point though. It certainly is a different way of doing it than I've appreciated before.

4.9152 is main IF.

5.068 AGC BFO minus 4.9152 = 150 kHz carrier for AGC purposes.

Detection of a 150 kHz carrier amplitude is a lot easier and faster than audio frequency detecti
6652 2011-09-13 07:29:07 Ashhar Farhan Re: About Elecraft K2 Receiver
i saw the circuit. This is my understanding (if it is correct, then they are
pretty smart and so am i to figure it out!) ...
the IF is beat down to a very low 2nd IF (don't think of it as a audio). the
agc's envelope detector works at this frequency. The advantage of this could
be that the sampling rate is higher than audio hence the attack can be
quick, while the frequency is low enough to use op-amps as the second IF
amp. i suppose that using a 2nd IF exclusively for agc detection also helps
in keeping the BFO isolated . this is a halfway house between IF derived AGC
and audio AGC.
- farhan

6653 2011-09-13 07:42:07 Don Wines Re: About Elecraft K2 Receiver
Morning,

Since you have the Users Manual look on page 114 for a nice description of the AGC circuitry.

Don
K5DW


6654 2011-09-13 08:59:13 William Carver Re: About Elecraft K2 Receiver
Adding a conversion to eliminate BFO leakage? If 150 KHz is better than
audio, then 4.9152 is better than 150 KHz.

Sounds like $$$ talking (cheaper than shielding and decoupling?) rather
than a technical advantage.

W7AAZ


On Tue, 2011-09-13 at 19:59 +0530, Ashhar Farhan wrote:
> i saw the circuit. This is my understanding (if it is correct, then they are
> pretty smart and so am i to figure it out!) ...
> the IF is beat down to a very low 2nd IF (don't think of it as a audio). the
> agc's envelope detector works at this frequency. The advantage of this could
> be that the sampling rate is higher than audio hence the attack can be
> quick, while the frequency is low enough to use op-amps as the second IF
> amp. i suppose that using a 2nd IF exclusively for agc detection also helps
> in keeping the BFO isolated . this is a halfway house between IF derived AGC
> and audio AGC.
> - farhan
6655 2011-09-13 15:02:25 kb1gmx Re: About Elecraft K2 Receiver
6656 2011-09-14 04:36:39 Fernando Krouwel Re: About Elecraft K2 Receiver
Hi all from the group, good morning:

Thank you very much for clearing my mind about this subject; in fact, for a long time I was thinking it in a wrong way, now I understood the principles that guided the designers to do this way!

73´s
Fernando - PY2ETT

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6662 2011-09-14 14:07:43 kb1gmx Re: About Elecraft K2 Receiver
6663 2011-09-14 15:42:22 William Carver Re: About Elecraft K2 Receiver
6666 2011-09-15 08:06:41 AD7ZU Re: About Elecraft K2 Receiver
the corollary:

Incremental improvement too often at exponential cost

6667 2011-09-15 09:26:49 William Carver Re: About Elecraft K2 Receiver
On Thu, 2011-09-15 at 08:06 -0700, AD7ZU wrote:
>
> the corollary:
> Incremental improvement too often at exponential cost

ABSOLUTELY!
(but it's only money, heh heh)
W7AAZ
6686 2011-09-18 12:40:00 kb1gmx Re: About Elecraft K2 Receiver