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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