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Re: #bitx40 #ubitx #bitx40 #ubitx

 

A solution for every passband (my bandwidth):

... audio range passband...
Benefit: saves your ears, $0.02 diode, trivially easy to implement.
Bad: only protects your ears... not RF sections from excessive signal and overload. Not a true AGC signal for "s" meter, only an audio strength.
Comment: Save the $0.02 diode, turn the damn volume down and live with the IF overload.

... IF range passband...
Benefit: Saves ears, prevents IF overload, AGC signal for "s" meter (can be calibrated to real signal strength by reference).
Bad: More complicated/ more parts, RF level attenuator and AGC response, more expensive than the $0.02 diode,
Comment: can be triggered by anything in the passband and that can be good and bad.

... Full range integrated Passband (front end through audio section)...
Benefit: True AGC on all levels, no overload or distortion of any stage. AGC signal for "s" meter (can be calibrated to real signal strength by reference).
Bad: Very complicated, impacts every RF/IF/Audio stage. Lots of mods and parts.
Comment: A CW ops dream.

I think you may get what you pay for.


Dr. William J. Schmidt - K9HZ J68HZ 8P6HK ZF2HZ PJ4/K9HZ VP5/K9HZ PJ2/K9HZ

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-----Original Message-----
From: [email protected] [mailto:[email protected]] On Behalf Of Tim Gorman
Sent: Tuesday, March 27, 2018 10:40 PM
To: [email protected]
Subject: Re: [BITX20] #bitx40 #ubitx

Jerry,

You pretty much nailed it. While hi-tech solutions are of course important to develop, the target audience for the ubitx are probably more interested in simple modifications that anyone can implement which provide some increased functionality.

tim ab0wr

On Tue, 27 Mar 2018 18:40:29 -0700
"Jerry Gaffke via Groups.Io" <jgaffke@...> wrote:

Detecting RF for AGC would have to happen after the 12mhz crystal
filter.
Not really¡­ it can happen anywhere in the chain. The issue is the
bandwidth of:
1) the attenuator and 2) the detector used for the AGC.
With the right parts, anything is possible.
AGC should detect what's in the passband of the receiver, nothing
more, nothing less. If RF into the AGC detector is wider, then any QRO
signal in the band will quiet the receiver. I can vaguely imagine ways
to make it work with the AGC detector in front of the IF filter, but
they aren't pretty. I'm fine with just detecting the audio on a
$109 transceiver.

Let us know how your AGC scheme works once finished.
Though I'm more in Don's camp, agonizing about whether a $0.50 PIN
diode array is worth the expense when a $0.02 FET is almost good
enough.

On Tue, Mar 27, 2018 at 06:03 pm, K9HZ wrote:




Detecting RF for AGC would have to happen after the 12mhz crystal
filter.







Not really¡­ it can happen anywhere in the chain. The issue is the
bandwidth of: 1) the attenuator and 2) the detector used for the
AGC. With the right parts, anything is possible.




Would be tough to do on a uBitx without having that detector get
swamped by 12mhz BFO energy given the total lack of shielding.







I would certainly not argue with this¡­specially with one that is
wide banded and sensitive.




Nobody has tried.






Um¡­ no. I currently am using a AD8367 ultra wide band variable gain
amplifier with a AD8361 true RMS detector fed back to the VGA via an
AD820 high speed Op Amp configured as a combination LP filter and
integrator with a ¡°voltage¡± offset (RF Gain control). Using a few
components, I¡¯m able adjust the time constants in the integrator for
different AGC reaction speeds (basically a pot for continuous
slow-medium-fast-none). I think this sort of scheme is used in some
VHF receiver applications. Not finished yet¡­ (not ready for prime
time- looks like a mess), but is very promising as a true RF AGC
that rivals the AGC circuits used in some of the world¡¯s best
receivers. Stand by.











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