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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>
>
>
>
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