[Amps] Designing the Cleanest Linear with RF Negative Feedback

[R.Measures]

On Dec 16, 2005, at 1:59 PM, 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.

Amen, Gary.  For most of 'em, the s in S-meter stands for sucky.  
Step-attenuators are typically way more truthful.

> Now you will have a base to work from.
>
> 73
> 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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>
>
>
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Richard L. Measures, AG6K, 805.386.3734.  www.somis.org