Weak signal

Up Spectravue
 

Very weak to extreme weak signal communications in mode's like EME

The last years I have been active in EME and other weak signal mode's like aircraft scatter on 23 cm

I found very good help in detecting and finding weak signals using Spectran but even better using SDR

Read my article I prepared for the EME 2014 conference on Weak signal communications using SDR

Do's and Don'ts in weak signal communications

Be sure to have enough gain in your system, this is to prevent loosing performance in your system

Calculate overall performance and check it by measuring

Calculations can be done using VK3UM calculator

Measurements can be easily done using SDR Spectravue program

For moon- and sun noise measurements use Continuum mode from Spectravue
Work as hard as possible to gain each 0.1 dB performance
Weak signals communications is a fight for each 0.1 dB
As general rule of thumb: losses before the pre-amp will degrade the echo's by a factor 2
Calculate overall system Noise figure including transceiver
Be sure to have good image rejection of at least 20 dB
Be sure that phase noise of your LO at your IF range is well below the noise floor at your mixer
For 10 GHz use an IF of 432 MHz or higher
For 5760 MHz when using a good filter an IF of 144 MHz is just acceptable
When turning on and off the pre-amp you should have at least a 20 dB rise in noise floor
Too much gain is hardly ever a problem
Use Spectravue and FFT averaging to find extreme weak signals
Signals with a S/N of -18 dB@3KHz are just strong enough to copy in CW
Using Spectravue S/N of -18 dB@3KHz show as S/N of 6 dB@12Hz so clearly visible
With enough overall gain your receiver is just a detector, performance like phase noise is not very important
(as long as there are not very strong signals in your system)
Check your dynamic range taking into account your operating mode
On EME you don't need a very large dynamic range; 20 dB for most setups will be enough
In other modes you will need more dynamic range; 23 cm tropo will need at least 80 dB
If possible use frequency referenced oscillators coupled to a stable 10 MHz source
If possible use automatic Doppler correction
 

 

Signals in Spectravue for different S/N ratio's

I will show you signals with different S/N ratio's in Spectravue

Click on the picture to listen to the audio of these signals, switching on and off

All these Spectravue pictures have been made using an FFT Ave of 5 and 12Hz resolution bandwidth

4dB@12 Hz  = -20dB@3KHz

6dB@12 Hz  = -18dB@3KHz

8dB@12 Hz  = -16dB@3KHz

10dB@12 Hz  = -14dB@3KHz

Automatic Doppler correction

 

Easy noise figure calculator

 

 

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