[Elecraft] CW Decode and Preamp?

[Bob McGraw K4TAX]

I'd say initially that you aren't configuring the radio for the best 
decode.   Here's something I wrote some months earlier.   It might prove 
worthwhile in your case.  Of course not knowing what bands and your 
noise conditions is something you'll have to judge for yourself.  You 
may need to change the TEXT DEC value to a lower or higher value.

****************

As to RF gain, I find many hams do not understand the way to get 
enhanced performance from their radios.   And this applies to most all 
legacy radios today.   More RF Gain does not assure one they can or will 
hear weaker signals.   The story is Signal to Noise ratio, namely in the 
area of the receiver noise floor.    Since we can't change the RX noise 
floor, as it is established by design, then we need to change the signal 
coming into the radio. Typically one wants the no signal band noise to 
be some 10 dB above the noise floor of the receiver.  And to do this we 
use a combination of attenuation and RF gain reduction.

To run the math, if the RX noise floor is -135 dBm, typical of most 
modern radios, then the "sweet spot" for no signal band noise is -125 
dBm or about S-1.  The -125 dBm value is 10 dB above the receiver noise 
floor.    If the no signal band noise is -97 dBm {S-5} then we need a 
combination of 28 dB attenuation and RF gain reduction.  I typically use 
15 dB attenuation and then back the RF gain down a good bit on 75M.  
While on 10M and 6M I will switch the attenuation off and engage the 
preamp.    Running high gain and no attenuation just to see the S meter 
move upscale is just sucking up receiver headroom and certainly doesn't 
do anything constructive toward improving signal to noise ratio.

Cranking up the RF gain and adding the preamp in many cases is the wrong 
way to go.  I hear all the time about stations running the receiver 
preamp on 80/75 meters.   WOW, they must have a really really low no 
signal band noise.  Something like -135 dBm. In your dreams.

********************

Hope this helps.

73

Bob, K4TAX


On 12/12/2018 7:21 PM, Gwen Patton wrote:
> Please forgive me if this is a FAQ. If it is, just point me to where I can
> find the old answer and I won't bring it up again.
>
> I can usually get the CW decode to work if I can clearly hear the signal.
> The CWT bar flashes and the text scrolls, and it gets a lot of characters
> right. BUT...
>
> ...that's only if I have the preamp OFF.
>
> I've tried using the preamp but decreasing the RF gain. I've tried using
> the preamp and the attenuator. I even tried setting a couple of bands to
> only 10db preamplification. But if the preamp is on AT ALL, the decode
> either never gets a "lock" and the CWT bar is flickering randomly (no
> matter what the threshold setting is) with no decode, or the CWT bar is
> solid, no flashing, and there's no decode.
>
> If have the preamp OFF, the same signal will usually decode at least
> somewhat, and sometimes excellently. Most of the time, W1AW can decode
> fairly easily...though FLDIGI and CWGET seem to decode more reliably.
> They'll decode 80-90%, while mush is appearing on the PX3 display lines.
> But at least I get something with the preamp off.
>
> But there's just some times when the signal is weaker and I can't get the
> threshold to crack without the preamp. But then I get no decode at all.
>
> Am I doing something WRONG? Is there some step that I'm not understanding?
> Heck, I've seen past entries in the list where some have said they can't
> get decode unless the preamp is ON!
>
> Or is there some known issue with preamp + decode that I haven't seen? Mind
> you, my antenna, while not fantastic, is adequate for me to get to Europe
> on FT8 at 35 watts. I even got from Philly to Fargo on 50w FT8 on 30
> meters, using an indoor magnetic loop I built. So it's not my antennas. I
> have signal, but paradoxically, when the signal is strengthened by the
> preamp, I get lower decode performance. I'd expect that in high-noise
> conditions -- amplifying the noise as well as the signal and hashing them
> both -- but when the noise floor is low and the signal is just made more
> solid with the preamp, I should be able to decode it with the same
> technique as when I use it on a stronger signal without the preamp.
> Shouldn't I? Or am I simply not grasping some fundamental issue? I've read
> the manual, I've read mailing list archive messages, I've experimented with
> my own second rig on a dummy load sending a canned CW message (K2 s/n 1641).
>
> This will, I hope, be a moot point in a few months. I'm trying to learn to
> copy the CW barebrained instead of using the decoder. (I got rusty from
> when I first learned it back in 1999.) But I want to at least be able to
> check my copy using the decode function if I can until then. Between them,
> maybe I can hold a coherent QSO!
>
> Thanks,
> Gwen, NG3P
>