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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: Gary Schafer <>
Date: Fri, 16 Dec 2005 16:59:42 -0500
List-post: <>
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 IMD 
> 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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