Feed-forward correction is a very powerful, but somewhat involved
distorsion reduction technique. For more information,
and some worked examples, see chapter 13 "Ultra-low distorsion power
"Single Sideband Systems & Circuits".
Another aspect is to generate a low-power signal with an IM3 level approaching
This may turn out be a major obstacle, as the transmit signal path of the
need to be designed in more or less the same way as the receive path.
The cleanest SSB exciter I have used was the ITT-Standard Radio TD91,
while using some alignment "tricks", can provide a - 65 dB IM3 level at 50 mW
In the TD91 the signal path is closely related to the CR91 receiver, using
high-current U350 quad JFET
ring mixers, and class-A CATV transistors with generous bias levels to increase
the signal to the 50 mW level.
When discussing IMD levels lower than about - 60 dB, the distorsion properties
of passive components
such as crystal filters and ferrite core tuned circuits also need to be
----- Original Message -----
From: "Tom Cathey" <K1JJ@comcast.net>
To: <email@example.com>; <firstname.lastname@example.org>
Sent: Monday, December 19, 2005 6:32 PM
Subject: Re: [Amps] Designing the Cleanest Linear with RF Negative Feedback
> 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
> Hi Tom,
> The FF system would be entirely external to your FT-1KD -- It just
> connects between the rig's output the outboard linear, plus an input from
> the low level tap. Therefore, transportable to a new rig with a low level
> out -- an easy addition, if necessary. It could even be left in line with
> only a slight (1-2%) loss of power when not in use.
> Regarding the link, which looks to be more of a mechanical diagram than a
> schematic, it might not be GG. The final neutralization ckt. resembles a
> grid driven stage. The 1st-2nd stage coupling also appears to be more
> suitable for grid driven operation. Perhaps, it's 3 grid driven stages?
> Where have we seen that before...
> 73 & Good evening,
> Marv WC6W
> Amps mailing list
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