Don Pearce wrote:

On Mon, 01 May 2006 13:50:43 GMT, "Michael A. Terrell"
wrote:

Don Pearce wrote:

On Mon, 01 May 2006 13:24:48 GMT, "Michael A. Terrell"
wrote:

Don Pearce wrote:

Thank you! I did make a gaffe. The actual figure for the Neve noise
figure is about 3dB. That is still unforgivably poor for high end kit
- it is in fact no better than my little Behringer. Ten years ago I
was designing satellite receivers working up at 12GHz. The noise
figure I was working to was 0.3dB.


Its a lot easier to reduce the noise figure at a narrow bandwidth at
microwave frequencies.

Narrow bandwidth? I don't call a bandwidth of nearly 2GHz narrow. And
asitappens, I can achieve these sorts of figure at audio as well.

d

--
Pearce Consulting
http://www.pearce.uk.com

2 GHz at 12 GHz is 16.67% which is narrow. Don't tell me that there
is no filtering at all. A circulator or isolator has a usable bandwidth
which keeps out of band noise out of the amp. One of my KU band
receivers is aboard the ISS.

Of course there is filtering. The waveguide coupler as a bandwidth, as
does the mixer. The image filter then has a bandwidth, and finally the
IF amplifier. Despite all this filtering - none of which is lossless -
the noise figure still happens. It really isn't easy even compared to
audio. Well done for having a receiver aboard ISS, but ground
transmitters tend not to be particularly power-limited, so the noise
figure probably isn't quite the issue it is with a terrestrial
receiver. In fact there would be no point because it is pointing at
the earth, which is hot.

Audio is true broadband, from DC to whatever the upper limit is set
at by the design and limitations of the components if it is DC coupled,
and from less than 100 Hz if its AC coupled.

But achieving low noise at audio - however you want to describe its
bandwidth - is actually pretty trivial. The fact is that manufacturers
choose not to implement it because they can make something that is
"reasonable" for a lot less money. The business of achieving low noise
at audio doesn't rely on being able to juggle S parameters, physical
layout, board materials and mechanical design like microwave.


My point about microwave is that without filtering you have aditional
noise to deal with in the system. We offered IF bandwidths from 10 KHz
to 20 MHz, and video amps with matching bandwidths to keep as much noise
out of the received signal as possible.


The same receivers are used by NOAA for their weather sat downlinks.
With the improved doppler compensation they are able to lock onto the
sat before it comes over the hrizon five minutes faster than the old
Harris equipment it replaced.


--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida