RadioBanter - View Single Post

Mike Andrews · October 11th 06, 10:46 PM posted to rec.radio.amateur.homebrew

On 11 Oct 2006 13:41:07 -0700, wrote in om:

wrote:
I'm dealing with a DSSS current signal at 70MHz. Before I start
designing anything, I am calculating the signal amplitude. It's going
to be 5nArms. If I have a 50ohm resistor, it's -153dBm. I'd be happy to
know any solid proof that it's not doable because it's my job now.

In theory DSSS signal can work under noise level, but can it be so
much?

Harold E. Johnson wrote:
wrote in message
ps.com...
It seems that I know the Friis equation.

I am asking because it seems difficult, and maybe impossible. I've
never heard of this high gain before, but I dont' know why I cann't
stack amplifiers and I am looking for somebody who can tell me what
exactly the problem is: you see the NF is nice, and I can get the SNR
if the band is narrow enough.

It also seems that I "don't know what I'm doing". So please explain a
little if you want me to understand. Thanks.

Well, how about a little additional information? What is it that you're
trying to accomplish? A gain of 140 dB would amplify a signal of strength 8
x 10^-10 Watts (About the equivalent of a received signal of 10 dB over
"S"-9) to a kiloWatt. I don't think you're trying to do that, so what
convinces you that you NEED 140 dB of gain?

I googled and saw GPS signal is -150dBm. So it's detectable, but it's
much higher frequency. What if it's 70Mhz?

It probably isn't going to make a lot of difference whether it's 25 cm. or
40 cm; the principles are the same for DSSS decoding.

GPS signals make heavy use of PN sequences so that they can be detected
using autocorrelation techniques. They're down below the terrestrial noise
threshold, and require autocorrelation techniques for synchronization and
detection. But you probably know that already. This probably will be true
of your solution as well, and especially so at those power levels.

If you're going to be receiving and detecting signals at those power
levels, you'll need to use something other than *just* amplification to
pull the modulation out.

One presumes you'll have a known short spreading sequence that you can
use to try to sync up on the signal, or (if it's a longer sequence)
that you'll have a rough idea of the code epoch, so that you can try
to sneak up on it from behind, as it were.

The point here is that a _lot_ of the process gain comes from the
autocorrelation and despreading processes. You may not need all that
analog gain. Getting it up to ... something like -30 dBm or -15 dBm
may be enough -- or even overkill.

--
Mike Andrews, W5EGO

Tired old sysadmin