Thanks for the input...........................I wish I had some ability
programming microcontrollers. I really need to learn how to work with at the
very least, PICs. Anyway, this generator would sell for the under 150 dollar
range, putting it a couple of steps above some of the offerings in this
range.
I do like that HP equipment...........I had an 8650B for awhile and it was
very good. I replaced it with a Boonton 103D and a Wavetek 2407. I've got an
older spectrum analyzer (8558B with an 853 mainfraime). An older 8444
tracking generator allows me to measure the response of filters. I did build
the N2PK Vector Network Analyzer last year, and it does serve me very well
when it comes to doing sweeps of very narrow bandwidth filters. It is pretty
nice, since I can look at the forward transfer function and the phase
response at the same time. Using a directional coupler, I can view forward
response and return loss at the same time................I use this checking
antenna resonance.
It would be nice to come up with a different audio oscillator for the
modulation section. As long as I can keep the distortion fairly low, that
would be ok. Let's see....................the Exar chip...........that
would be an XR2206. I originally used this device for an FSK generator that
I designed for a customer. They wanted to have something testing caller ID
units. I also had some Burr-Brown 4523s these were a nice Quadrature
function generator chip. I don't know what happenned to those.
So.........you worked for Exar. That must have been a pretty cool job.

Pete

wrote in message
oups.com...
The problem with delivering something that appears to be a bit of test
equipment, is you are competing with used test equipment. When it comes
to test equipment, 20 year old HP wins over brand spanking new brand X
every time. Most of the used gear I've bought over the years is still
used in real labs. I could get it calibrated if I had the need. I have
to think really hard if I ever bought any new test equipment.

If your box was digital through and through, then maybe you might get
some sales. I'm not sure which Exar chip you are using (BTW, I used to
work there), but the Twin-T is an analog topology and thus has all the
bad features of analog as well as the good. It should be possible to
make a completely digital AM or sideband modulator with a DSP chip and
high speed DAC. The coordic is how the sine wave is computed, though at
the time nobody really talked about it since there were still patents
on coordic processors from the scientific calculator manufacturers. One
of the best books I've seen on the subject was a masters or PHD thesis
by a Stanford grad whose full name escapes me. First name Ahmend, which
might as well be Joe or Bill nowadays.

A coordic is a lookup table technique that converges on the the sin and
cos. You make a sine wave generator by accumulating phase, then taking
the sin/cos of this phase value. If the phase accumulator were 8 bits,
you would use a scale where 256 is one revolution. This way as you
accumulate phase, the sine function automatically wraps around after
reaching one revolution. You control the amplitude of the sine wave by
the initial value of the coordic. It's really much easier than it
sounds, at least the algorithm, Programming the DSP is another story.

I suspect the Wellbrook has a jfet amp because of the positive ground.
I think today most CATV circuits are bipolar to simplify the biasing.
For certain frequency ranges JFET has lower noise.You can parallel them
up for lower noise. There are a few companies still serious about
Jfets, but they are becoming less and less since the money is in high
bandwidth.
http://www.calogic.com/ comes to mind as a jfet company

Another idea would be to make VHF/UHF amplifiers using SiGe.

I do have one design that uses a JFET as a source follower............this
way, one can take advantage of the unloaded Q of the circuit. This is
followed by an RF Micro Devices RF2044. This device is a GaAs MMIC that has
20dB of gain with a NF of 4dB. Frequency range is from DC to 6GHz and linear
power output of this device is +17dBm. 1dB compression point is
+18.5dBm.

Pete