[Elecraft] Re： Manual notch In K3

Wed Nov 10 15:56:41 EST 2010

No receiver that offers gain can handle an arbitrarily strong signal without overloading. When used, AGC helps prevent that. 

I'm not sure where Lyle actually implements the notch in the DSP sequence of events. It might be at the second I.F. in which case increasing the I.F. gain *after* the notch shouldn't risk overload. 

But, if the notch is at audio, the only reasonable place to increase the gain is in the audio channel. 

In either case, it's unlikely the S-meter would ignore the notched out signal. The AGC system still needs to recognize the big signal and respond accordingly to avoid overloading various stages, and the S-meter reading is simply a measure of the AGC action. I suppose it's possible to have the S-meter system compensate for the increased post-notch gain to show the strength of the signals reaching the phones. 

The ideal receiver has *all* of its selectivity (including any notch function) at the antenna input. The problem with doing that is three-fold: 1) filters have loss, hence they degrade the noise figure so it's better for highest sensitivity to have some gain ahead of the filter, 2) Really good *tunable* filters that match the best fixed-frequency filters have never been developed - especially when one wants to tune a range of, say, 0.5 MHz to 54 MHz like the K3, and 3) filter performance (skirts, stop band, etc.) decreases with frequency. 

Col. Armstrong's invention of the superheterodyne format allows us to use fixed-frequency filters for optimum performance but added the need for stable oscillators. 

Everything's a compromise - even if cost is of no concern.  

Ron AC7AC

-----Original Message-----
From: elecraft-bounces at mailman.qth.net [mailto:elecraft-bounces at mailman.qth.net] On Behalf Of Johnny Siu
Sent: Wednesday, November 10, 2010 5:41 AM
To: JAMES ROGERS
Cc: Elecraft
Subject: [Elecraft] Re： Manual notch In K3

Thanks, James.

Yes, AGC off is the last weapon to use.

73 Johnny VR2XMC

----- 郵件原件 ----
寄件人﹕ JAMES ROGERS <w4atk at bellsouth.net>
收件人﹕ Johnny Siu <vr2xmc at yahoo.com.hk>
傳送日期﹕ 2010/11/10 (三) 9:09:09 PM
主題： Re: [Elecraft] Manual notch In K3

I wanted to check something before I answered this, but yes the notch filter is 
probably in the audio chain, rather than the IF.  I picked a good carrier on my 
K3 and then applied the notch. the carrier was not diminished on the P3 display 
but completely notched out in the audio from the speaker.

If I might suggest, in the situation you described apply the notch,  reduce the 
RF Gain, and turn the AGC off.  This will remove the AGC action on the weak 
signal.  I often do this on weak DX as a matter of practice with or without the 
notch. You can then advance the RF Gain to bring up the weak signal to a 
comfortable level of copy.

73s Jim, W4ATK

On Nov 9, 2010, at 10:18 PM, Johnny Siu wrote:

> Hello Group,
> 
> I attempted to manual notch a strong carrier adjacent to a weak station.
> Although I was able to notch out the strong carrier (from the audio), there 
was
> no reduction in S meter level caused by that strong carrier.  In other words,
> the weak station did not pop out from the noise because the stronger
> carrier de-sensed the K3 AGC.
> 
> Please advise whether my observation is correct.
> 
> TNX,
> 
> Johnny VR2XMC
> 
> 
> 
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JIM ROGERS
w4atk at bellsouth.net
http://web.me.com/jimrogers_w4atk

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