On Mon, 18 Oct 2004 13:30:45 -0400, Mike Coslo
wrote:
There are also digitally based methods of generating an RF frequency
of a purity that exceeds all the current Ham radio equipment
commercially available, but those methods are not widebanded trash
generators. Clearly, digital methods are not necessarily interfering
methods - except in the hands of commercial interests with less than
amateur talent in marketing dominating their engineer's efforts.

Why aren't we using them then? Is the nasty need for channelizing going
to rear up?

Hi Mike,

Heavens no. There are several means to generate a clean RF source
digitally. These devices (a chip with a rock) can change phase, or
frequency in the time it takes to change a register (microseconds).
These are largely based on look-up tables feeding D/A converters.

Memory is cheap you may well know, I habitually carry a 128MB flash
drive in my pocket as my briefcase, and a 64MB voice recorder as my
stenographer, and my 32MB Palm Pilot as secretary. Grand total cost
for my "digital" office is under $300.

Another method of high purity RF source generation is direct feed of a
pulse train into amps with only the slightest of filtering. These
pulse trains are weighted to look like a binary sine wave (a few bits
on early, a lot on in the middle, and a few on in the late stage).
This work has been long available in the pages of Steve Ciarcia's
magazine "Circuit Cellar."
http://www.circellar.com/
I believe it was written up by Don Lancaster (a name that should be
resourced for ideas that approach problems obliquely). The length of
the pulse train, and the weighting of the bit positions leads to
exceptionally pure RF (spurs at least 60dB down).

The only problem here in this last method is moving that pulse train
fast enough for HF (128 bit trains to drive at HF will require you to
clock them through at UHF rates).

The only problem with the look-up/ADC method is that with my last look
at these products, they were roughly limited to 16MHz (with 0.1Hz
resolution) - so perhaps some mixing is called for at the higher
bands.

If there is a digital mode that uses less bandwidth, sounds at least
acceptable to enough people, and allows me to tune my radio in a normal
fashion, I'll hop right on it.

You need to look at Digital AM (specifically Harris transmitters).

And of course, we have to remember that there is an inertia based on
the need to have people to talk to. If I have the nifty new digital mode
of communications, how much fun is it going to be if I only have three
other people to QSO with?

This is why Spread Spectrum died in Ham radio. The FCC mandated we
transmit on a specific "gold code" and painted the research into an
isolated technical corner.

I guess I hadn't looked at it from that perspective, but I agree, there
must be some confusion as to what exactly digital is.

It is a Marketing term like fractal antenna - wholly spun from the
imagination to create the impression of advanced technology when it
has been around since the early 1800s (anyone ever here of Ada?).

I have a nice mid-80's IC745 that is indeed digital in all the places
where it serves the purpose!

Exactimundo!

73's
Richard Clark, KB7QHC