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Re: [Amps] Designing the Cleanest Linear with RF Negative Feedback

Subject: Re: [Amps] Designing the Cleanest Linear with RF Negative Feedback
From: "" <>
Date: Mon, 19 Dec 2005 20:07:17 GMT
List-post: <>
Hi Tom,
   First, I've never seen a feed forward correction system implemented for HF 
so, I can't point you to any done circuits.

   It may be because it's a futile undertaking.  I don't know of any amateur HF 
transceiver with good enough numbers to make it worthwhile.  You need to start 
with something like -75db IMD, carrier, and opposite sideband rejection.  FF 
will not fix the existing low level signal.  Does your FT-1KD generate its SSB 
in the DSP chips? 

   It would also require some effort to make it work over 2-30 MHz.   
"Narrowband" for 80M would be a snap.

   You've got the low level output from your rig in place.  Add a delay line 
(coax) plus a phase & gain adjust (a couple of pots and a C) to compensate for 
the FT-1KD power amp. group delay.

   The TI opamp I mentioned is fine for an input loop amp. for 80M.  You'll 
have to check if there is an inversion between the low level and high level 
output of the FT-1KD to see whether this needs to be configured as an inverter 
or non-inverter.

   You'll need a -30dB directional coupler on the FT-1K output for the input 
loop.  For full (2-30MHz) range operation, a cross coupled transformer type 
would be indicated.  For 80M only, just about anything would do.

   The correction loop amp. can use another couple of TI opamps (for 80M only), 
a phase & gain adjust, plus say a 25W transistor amp. running at about a watt 
for good numbers.

   A second delay line will feed the -10dB output coupler which can again be a 
cross coupled transformer type. Or for 80M only, again just about anything 
would do.

   You'll need to provide a SWR detect and hook it into the FT-1KD ALC, to 
protect against the output ever coming loose as in that event lots of power 
will reflect back into the correction amp.

73 & Good morning,
   Marv WC6W

Attachment:   Check the enclosed drawing that no one else will find attached... 
 I used round numbers for illustration, the actual values may vary a couple dB. 


-- "Tom Cathey" <> wrote:
Hi Marv,

Gee, that feed forward correction method sounds like something that would be 
ideal for the FT-1000D. As you said, it could be later moved from rig to rig 

So far all I've seen on the web are ivory tower block diagrams, etc. Is 
there a tested circuit available somewhere to build up or a commercial 
surplus unit? Even something that has suggested chip choices with a circuit 
I could then modify to my use would help.

For this particular project, I don't mind building up the two stages to go 
from 5mW to 250mW using a chip and a tube, but this feed forward correction 
idea could open the door to setting up other clean linear systems from 
exisiting stand alone linears I have now in the shack..

Just to double check... it requires TWO inputs:  The 5mW, and  the 1000D 
200W module output for sampling, correct?   And the big linear final would 
have it's own negative feedback loop back to the tube driver, if used, 
independent of the feed forward system? [assuming we don't use your 200 ohm 
swamped system and still use a tube to drive the final]

Tom, K1JJ
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