[Lowfer] Homebrewed SSB Filter question - Clarification

[Jim]

> I forgot to add the most important detail in my question; but I
> think most would understand anyway, nevertheless, what I am
> hoping to accomplish is, I suppose, a Direct Conversion USB Xmtr.
> No IF freq's, or conversions up or down, just straight out of the
> DBM, thru the USB filter, amplify it to stay linear, but drive
> the PA. Not planning on doing Linear modes to start with, I'm
> hoping to try Jason, which dosen't require a linear PA, as long
> as I can keep distortion out of the picture.

I've seen a number of posts from Chris on other lists he's much more capable 
of answersing this than I am.

I guess my concern is that I don't know how "smooth" the transition 
from "linear" to "non-linear" can go. If it's a true FM (which, in a way, I 
understand JASON is) then it's probably okay.

But I still keep thinking of the two-tone test waveform you get when you test 
a SSB xmitter. It's just not "non-linear" even though it's just simply 
composed of two tones. I have no idea what a PSK or other modes like that 
would look like in the time domain--just something I haven't tinkered with.

I guess what I'm trying to say (in a very round about way) is that even though 
JASON doesn't require a linear final, that isn't necessarily the same as 
saying you can take a linear signal and drive a non-linear final. Or maybe it 
is--with an FM transmitter you go through linear stages generating the signal 
and then drive a non-linear final. But there's got to be criteria on when 
that will work and when it won't. (Or it seems like there would be.)

And it seems like in some cases, the conversion from linear to non-linear 
stages could cause unwanted side effects.

I'll be interested in Chris's thought on it because, candidly, I just don't 
know.

One thing I DO know is that you can build high-efficiency Class D audio amps 
(that's kind of what Charlie and I were batting around recently). So you 
could use that same approach to build a linear, Class D final with fairly 
high efficiency. And it wouldn't necessarily be that hard if you could use 
something like the chips from Maxim to get it done.

I'm almost wondering if that where the really interesting thinking starts 
taking place--moving linear Class D from audio to RF. Or maybe someone has 
already done it.

Whatever the outcome, I can guarantee we'll all probably learn something from 
what you're doing. I think I feel a QST article coming on--$50 a page 
baby! :-)

73 de
Jim W4JBM

http://www.geocities.com/w4jbm

"With a soldering iron in one hand, a schematic in the
other, and a puzzled look on his face..."

Working the world from the New Dog Iron Ranch!