RadioBanter - View Single Post - Echos from the past, code a hinderence to a ticket

August 12th 05, 05:10 PM

wrote:
o
wrote:
Michael Coslo wrote:
wrote:

The 'phone companies have cleaned up their act so much that most lines
will not
only support 56k dialup, they'll also support DSL. Such improvements
are almost
invisible to the unsuspecting public. They happen over periods of
years.

I'm using one of those lines here. A few years ago it was originally a
DSL line Ma Bell set up as a second line for Eric. Then he bailed away
from his DSL service and moved to Comcast thus I inherited Ma Bell's
former DSL line for my dialup connection. As dailup connections go the
thing screams. His cable TV connection is only 2-3 times faster than my
dialup connection but no more than that. He's paying something like
$50/month for his cable connection and I'm only paying $15/month. AND
my dialup connection is more reliable.

There ya go!

The big draw of DSL, besides the speed, is that you can use the voice
'phone and the DSL at the same time over a single pair. The monthly
cost of two lines is often comparable to the cost of one line plus DSL.

As much as the S/N allows! See above.

Of course! As a practical matter though, we have to assume amateur
radio conditions. A real limitation there.

Which is why you'll not see the pundits doing any of what they talk
about.

Gotta love it. And they're *everywhere*.

Yup.

Hot air is free. Brains, innovation and bench time are not free. End
of.

What matters is what gets built and used.

Like all the bafflegab and excitement about DSP, SDR's, etc. Poke
around under the hood of any of the modern top-end ham xcvrs and you'll
find that their hot performance is directly dependent on their cascaded
xtal filters. Jumper the filters and put it all on their "firmware".
HA! As if. Xtal filters being at least seventy-year-old technology and
all that.

Xtal filters in ham receivers and noise blankers can be traced to the
same guy: Jim Lamb, at ARRL Hq. Mid 1930s.

One of the things that I wonder about with the need for huge s/n
handling ratios is that we obviously want a quiet reciever, with a
impressive noise floor. This means all of the "oomph" must be on the
strong signal handling side. We need an exquisitely sensitive and qui=
et
reciever, with exceptional strong signal handling capacity.

Not really.

In most situations, HF radio reception is limited by the noise picked
up by the antenna,
not internal receiver noise. Been that way since at least the 1930s.

Exactly.

It
does no good to
have an HF receiver with, say, .05 uV for 10 dB S/N sensitivity if the
antenna picks up .5 uV of noise in the same bandwidth.

Eeeek! Yo, An HF RX with 0.05 =B5V sensitivity?! Not many of those
around!

Because there's no point to building one!

Somebody put an HBR-16 through its paces and it bettered 0.5 uV
sensitivity
on 80/40/20. That's not .05 uV but the point is the same: There's a
limit to how much sensitivity is usable on HF, with typical HF
antennas.

As you go up in frequency, that situation changes.

You don't need a noise generator, lab full of gear or an EE to know if
your receiver is sensitive enough. Just do this simple test:

1) Tune the rx to an unoccupied frequency, using the mode and bandwidth
you intend to use.

2) Turn off the AGC and turn up the gain until you hear the background
noise roaring away.

3) Disconnect the antenna.

If the noise drops way down, or disappears, you have all the
sensitivity you can use in that application.

I've done that and gotten some weird results. I get non-weird results
by A-B testing between the antenna and a dummy load instead. Impedance
matching issue.

That's the more-correct way to do it, but the point is the same: If the
noise coming in from the antenna exceeds the noise generated by the rx,
you've already got all the sensitivity you can use with that setup.

73 de Jim, N2EY