[Amps] Designing the Cleanest Linear with RF Negative Feedback

[Tom Cathey]

Hi Gary,

Tnx for the IMD measurement info. Sounds easy enough. Actually I've been 
doing a similar thing, but using a recorded voice to play thru the rig and 
measure the bandwidth and levels. I will make a setup like that later on, or 
one suggested by Bill/LAV.

Anyway, I did a test tonight and measured the RELATIVE levels - no IMD raw 
numbers yet. I tested the FT-1000D 10mW output, the barefoot FT-1000D 200W 
power amp, and a pair of 3-500Z's.  I used an FT-102, well isolated, as a 
receiver with a pad attenuator so that I could set each signal to a 
normalized S9+40 over. Each transmit signal was dumped into a Bird dummy 
load. The FT-1000D uses an aftermarket 2.8kc ssb TX filter.

I have detailed readings, but here's a rough summary:

When I ran the 3-500Z's at ~1200W out, with S9+40 on freq, the trash was 
S9+5 at 3kc up.  At 3.5kc up =S9.    4kc up=S7.   4.5kc up = S3      I could 
still hear the trash up 5kc, but not moving the meter.

The FT-1000D barefoot is advertised at -36db IMD for 150W, I believe. This 
is running at 100W out:

On freq: S9+40.

3kc up= S8.    3.5kc up=S7    4kc up=S2     5kc up= barely heard anything. 
Better.


NOW... the FT-1000D with the low level 10mW output tap:

At 3kc up I could barely hear any trash! At 3.5kc it was totally gone.  I 
would estimate it was upwards of -55db++  3rd order assuming the FT-1000D at 
100W was -36db.

I did notice that if I ran the drive levels and audio up so that I was 
getting the max 10mW out, it did deteriorate the IMD.  Running everything 
about 1/2 full level was VERY clean. [~5mW out]  Tuning across the recorded 
voice was so sharp it was like hearing a CW note. In contrast, tuning across 
the 3-500Z's showed a definate side channel trash that would show up on a 
quiet band.

The bottom line is after seeing how clean an ssb signal can be, there is no 
doubt that I want to pursue building a -55+ db 3rd order IMD amplifier using 
the FT-1000D 10mW output. It appears WELL worth the effort!

73,
Tom, K1JJ


> 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.
>
> 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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