Hans says:
>1954 SINGLE SIDEBAND PRINCIPLES AND CIRCUITS
>by PAPPENFUSS et al if it is still avaiable, who knows?
It's actually 1964. There's an awful amount in there about applying NFB in tube
linears, over one, two and three stages too. Not an easily available book, but a
super one. I wouldn't be without mine.
Tom said:
>You can add negative feedback to a grid driven tetrode with the
>addition of an unbypassed resistor in the cathode.
This is good - it's appeared in several places - I last saw it in (magazine)
print in Technical Topics in RadCom some 10 or so years ago....... I use that
system on my pair of 4-250As in AB2. As Rich says, you don't want too much
resistance. My 4-250As have electronically regulated screen and grid supplies
(shunt regulation on the grid bias, of course), a swamping resistor grid to (RF)
ground of 200 ohms and a 4:1 transmission line xfmer, and resistors from each
side of the filament to ground of about 12 ohms, 2 watt carbon. Those resistors
have 0.01microfarad moulded mica caps in series; the filaments are fed from the
xfmr through a ferrite cored toroidal choke so that the cathode current metering
doesn't get upset. On a 2 tone test (for what it's worth - I think we all agree
that 2 tone tests are not the be-all and end-all of measurements) it comes in
with 3rd order IMD at about 34 down on PEP (28dB down on each tone of 250
watts). The NFB certainly pulls the high order IMD down. There's about 3600volts
on the plates, 300 on the screens.
Interestingly, using noise modulation and looking at the Out of Band emissions
(as per ITU definition of Out of Band), the IM products appear to be a lot
further down; I guess that this is the result of the statistical nature of band
limited noise and the average power of an IMD product in a further limited band.
Not sure that I can easily relate noise measurements to 2 tone IMD - it's not
clear in ITU Recommendation SM.328, either.
Jeff said:
>Linearizing the envelop might improve things, but it doesn't do anything
>about the phase component...might even worsen it.
Envelope elimination and restoration was the Kahn patent approach of, I believe,
1953. More recently (mid 1980's), the work of Vlad Petrovich at Bath University
used the polar loop approach; he was getting 3rd order IMD down 70dB with a
Class C PA. However, it really needed some specialised IC's to get best results.
More recently (Petrovich died in December 1998), work has been on Cartesian
feedback, where the output signal is fed back in I and Q is fed back and
differenced with the I and Q input. There's been a fair amount published on the
technique, and Securicor Wireless Technology in the UK have supplied a lot of
220MHz ACSSB land mobile stuff into the USA using that principal. There's a good
introductory article in Microwave Engineering Europe for November 1988. There's
a whole slew load of patents on it, too.
73
Peter G3RZP
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