RadioBanter - View Single Post

Paul Keinanen · October 12th 06, 07:57 AM posted to rec.radio.amateur.homebrew

On 11 Oct 2006 13:33:26 -0700, wrote:

I'm dealing with a DSSS current signal at 70MHz. Before I start
designing anything, I am calculating the signal amplitude.

You did not specify the chip rate or base band bit rate and spreading
function. For GPS signals the chip rate is 1.023 MHz, and the signal
spectrum is about 1 MHz wide, so the front end selectivity should be
as close to this as possible.

For a terrestrial 70 MHz DSSS system, there are usually going to be
several independent DSSS transmitters sharing this 1 MHz bandwidth (or
whatever your chip rate is), some being close to the receiver and some
very distant.

If you want to receive the distant signal and use the whole 1 MHz
bandwidth all the way through the 140 dB gain, a weak distant
transmitter producing only 0.1 uV at your receiver antenna terminal
would be amplified to 1 V. However a local DSSS transmitter sharing
the same 70 MHz band with a different PN sequence could produce 1 mV
at your antenna terminal, which should also be amplified by 140 dB to
10 kV, which clearly is impossible ! The strong signals would saturate
the later amplifying stages and the weak signal would not get through.

A reasonable gain/bandwidth distribution for a 70 MHz system would be
to use a low loss band pass filter with a few MHz bandwidth at 70 MHz,
to take out any VHF band I TV signals and FM broadcast signals at 100
MHz. Use a low noise amplifier stage, followed by a filter with deep
sides which is matched to the bandwidth of the service (1 MHz for
GPS). Then perhaps one stage, possibly with AGC and then the
despreader, followed by a filter matching the bit rate (not chip rate)
of the signal, followed by multiple AGC amplification stages to
amplify the despread signal before the actual BPSK or whatever
detector.

In terrestrial systems at 70 MHz the noise temperature is well above
300 K, thus the antenna noise figure is much worse than 3 dB, so
trying to keep the receiver system noise figure below 3 dB might not
be too productive. It might even be better to suffer an additional 1
dB noise figure hit and use steep filters tailored to the chip rate in
front of the first LNA.

Paul OH3LWR