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

To:, Tom Cathey <>
Subject: Re: [Amps] Designing the Cleanest Linear with RF Negative Feedback
From: Bill Fuqua <>
Date: Fri, 16 Dec 2005 18:05:01 -0500
List-post: <>
Better yet, if the two tones are only a hundred Hertz or so apart, you can 
take the audio output of the SSB receiver and put it into the sound card of 
your computer and do an FFT.  You can use a number of off the internet 
freeware FFT programs. In fact if you use "Spectrum Lab" (see 
<> ) to generate the tones and display the received 
spectra at the same time. Naturally you have to have a separate receiver 
and transmitter to do this.
    The receiver must be in SSB mode and with 100 Hz difference in the two 
tones you should be able to display the two tones and their IMD products 
within the 2 or so kHz bandwidth of the receiver. Their relative amplitudes 
will be easy to observe and measure.
      No need for calibrated attenuators or S meter readings. Just don't 
over drive the receiver or sound card input.

Bill wa4lav

At 04:59 PM 12/16/2005 -0500, Gary Schafer wrote:
>Hi Tom,
>You do have a spectrum analyzer in your shack! Your receiver.
>Modulate the transmitter with 2 tones and tune across with your cw
>filter on another receiver. Note the level of one of the tones on the S
>meter. Then tune to the 3rd order product and note the level difference.
>Same thing a spectrum analyzer does.
>If you want to get real accurate put a step attenuator in front of the
>receiver so you don't depend on S meter calibration.
>Now you will have a base to work from.
>Gary  K4FMX
>Tom Cathey wrote:
> > That's more valuable info, Marv - Tnx again!
> >
> > A few follow up questions:
> >
> > If I tap off the FT-1000D 10mW low level point, can I run a long  ~ 20' 
> coax
> > cable to the amplifier CA2XX module's input, or will this cause problems?
> > This is for 75M only. Maybe there is a way to do this.
> >
> > I looked at the FT-1000D's circuit that puts out 10mW, just before it goes
> > into the power amp board. I'm trying to figure if it's possibly as clean
> > as -55db there. I don't have a spec analyzer.  It uses all pnp 
> transistors -
> > a 2SC2026 base driven, driving another 2CS2026 in emitter follower, 
> driving
> > a 2CS1973 in emitter follower at 10mW out. There's much more stuff 
> involved,
> > like the balanced modulator, etc.  Is this enough info to make a guess 
> from
> > your experience of what kind of IMD we are dealing with at this 
> point?  This
> > will have a big effect on what direction I take, of course.  Or maybe I
> > could sample it into a receiver and get an idea using the same relative 
> > procedures I use for a big amp.
> >
> > OK on the sample amplifier using a 6146 input running reduced voltages and
> > 1W, low power for cleanliness. Guess NFB will not help there.
> >
> > Last question:  Let's say I did put two 4CX-350's in cascade, so had 
> lots of
> > gain to work with. What is the practical limit for conventional negative
> > feedback?  Is it a matter of running into instability, perhaps?  And I 
> take
> > it from your comments, that you would run feedback from the final to the
> > predriver, [two stages at a time only] and then where would you run the
> > second loop to cover the pre-pre driver 4CX-350 and the input 6146, for
> > example?
> >
> > 73,
> > Tom, K1JJ
> >
> > ----------------------------------------
> >
> > Hi Tom,
> >    That '5106 is probably for Cable TV use and likely cuts off around 
> 40 MHz
> > (on the low end!).
> >
> >    If I recall correctly, some of the general purpose units that go 
> down to
> > a MHz use numbers from CA28XX group.  They come in both single ended and
> > push-pull versions.
> >
> >    I've seen a few of the 350J's for sale on the web over the years.  I
> > don't believe the demand is very high for those as they have a 26V heater
> > and as such can't be dropped into a 4CX250B socket.  There is also the
> > 4CX600J/JA/JB.
> >
> >   The 6146 shown on the Hughes schematic is run deep into Class 
> A.  Note the
> > low screen & plate voltages.  I bet the numbers were just fine running all
> > of a watt output.
> >
> >   For feed forward, an error amp with perhaps another 4CX350FJ would be
> > necessary.  It is not a difficult scheme to implement but, it would double
> > the parts count for the project.  If you read about the technique on the
> > web, keep in mind that "they" are typically working with transistor amps
> > that start with distortion numbers 20dB worse than tubes.  Therefore, in
> > this case, less correction power will be required, the output combiner 
> ratio
> > will optimally be a bit higher, and power lost from the main amp will be
> > lower.
> >
> >   The subject line said you wanted "the Cleanest Linear".  To avoid
> > degrading the system, the driver should exhibit distortion specs at least
> > 10dB better than the amplifier.   Your FT-1K will still be useful for the
> > receiver.
> >
> > 73 & Good morning,
> >   Marv WC6W
> >
> >
> >
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> >
> >
> >
> >
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