Harold E. Johnson wrote:
wrote in message
ups.com...
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?

Well, I have no idea of what a DSSS signal is, so I can't be of much help.
Whatever it is, it better be in a very narrow bandwidth. At that signal
level, it will take a 50 Hz bandwidth to achieve minimum discernable signal
(MDS) with a front end NF of 2.4 dB.

If you're talking about using autocorrelation to bring it up out of the
noise, all bets are off depending on how much time you can sacrifice.

If you're planning on bringing this signal to -10 dBm, better prepare for a
lot of shielding and decoupling, not to mention filtering and frequency
stability at 70 MHz center frequency.

W4ZCB

Some comments:

DSSS is direct sequence spread spectrum. It is wideband on the front
end.

It is probably not a valid comparison to compare DSSS detection levels
with GPS. GPS encodes much redundancy into the signal to enhance S/N,
and unless the DSSS encoder does something similar we can't use the
same detection level numbers. To say DSSS works under the noise level
assumes some gain due to encoding, and probably also assumes an
information bandwidth much less than the spread spectrum bandwidth.

Here are the main reasons you can't cascade a lot of gain at the
operating frequency. Others have mentioned this--so this is just a
summary with clarification:

1. Stability. A lot of gain at a single frequency is difficult to do
without causing stability problems. Even careful shielding between
stages doesn't`always work. At 70 Mhz, about 40 dB is the best I can
do reliably and even then I would use two separate metal enclosures,
each with 20 dB of gain. Above, say, 60 dB all kinds of things go
wrong--enclosures don't shield well enough, power supplies don't
decouple enough, interconnects talk to each other, etc. With
single-chip MMICs and LNAs it's really easy to build something and see
for yourself.

2. Intermodulation and blocking. Any unwanted signal at the antenna
would be amplified--assume you are trying to detect a -150 dBm signal
and someone in the house has a wireless headphone, which transmits at
about 72 MHz. With 140 dB of gain your amplifiers would saturate
completely and pass none of the wanted signal.

Radio design uses a careful balance between gain, mixing, AGC and
filtering to minimize stability and overload problems. For narrowband
signals you want to apply selectivity as soon in the signal train as
you can. For spread spectrum you do the same: a filter as narrow as
you can get it and still pass the spectrum, then your despreader at the
lowest level you can make it, then a narrow band filter that just
passes the information, and finally more gain and the detector.

3. Components
Some components such as crystal filters and mixers work better at
lower signal levels before a lot of gain is applied. This is really
just an extension of reason 2.


Hope this helps.

Glenn Dixon AC7ZN