On Jan 5, 3:10*pm, "Joel Koltner" wrote:
"K7ITM" wrote in message

...

As you think about all this stuff, it becomes easy to see why nobody
has yet built the perfect receiver. *;-)

Hey, I was at a conference some five years or so ago now where some high-level
muckety-muck from Intel got up there and claimed that within a few years we're
be connecting antennas straight to ADCs and radios would henceforth be 100%
digital... :-)

Of course, it is a bit easier to build a good radio when you're operating in,
e.g., the cell phone bands and by law you control the spectrum (no big
intereferes), you manage the power of all the transmitters dynamically
(limited self-interference), etc.!

Well, I guess it was about four years ago now we introduced a 100kHz
to ~35MHz receiver that practically does that. There are gain stages,
attenuators and some selectable filtering in front of the ADC, but our
customers want to be able to "listen" to the whole band at once, so
you can switch the filtering all out if you want. I did most of the
hardware, up to the signal processing. We've gotten feedback from
some customers that it's the best receiver (for that purpose) that
they can buy. It's particularly spur/residual-free: I added some
copper tape to one to see just how good I could make it, and the worst
residual is about -144dBm at the switching frequency of one of the
switching POL regulators (around 600kHz). Worst residual above 1MHz
is about -154dBm. I think it's fair to say that's pretty hard to do
with a general-coverage superhet design. But neither the hardware nor
the software that goes with it are inexpensive enough to worry that
it's going to replace other ways to do it any time soon--and the
performance doesn't quite equal what you can do with a really good
single-signal superhet design. Check in again in a few years,
assuming that there's enough economic incentive for ADC designers to
give us a little bit better parts. There are some claims out there
for really stellar ADC performance with Josephson junctions, but they
require cryogenics, and from what I've heard not all the claims are
substantiated...

By the way, the gain that's available in front of the ADC in this
design is mostly there because customers expect to need the gain,
based on previous experience. In a perfect world where they really
understood what's needed, I could have gotten by with maybe 10dB
maximum available voltage gain between the antenna and the ADC. I'm
more worried about how to gracefully handle big signals--much more
worried. What do you do about the plethora of short wave broadcast
signals, several of which can each be up to perhaps 0dBm out of your
antenna, or the fellow just down the street (or on the same ship,
etc.) who keys up a transmitter and feeds +20dBm to your receiver --
WHILE you want to keep listening to the signal that's only -110dBm at
your receiver?

Cheers,
Tom