Guru
Joined: 05/03/2018
Location: NetherlandsPosts: 5992
| Posted: 07:09pm 20 Jun 2020 |
Hi Grogster,
CN or C/N means carrier to noise. It is a measure of the signal quality (how much usefull signal is there peaking out of the backgound noise).
The C/N is measured when the transmitter output is a continuous wave (in most cases that means they switch of the carrier modulation).
If you have a measurement device that shows C/N and you have no access to the transmitter to switch off the modulation, the C/N is a integral of all transmitter power in the channel bandwidth divided by the total noise power in the channel bandwidth.
In digital modulation the demodulator and descrambler chip does this calculation for you based on raw ADC samples and their clouds mapped on the IQ diagram. Advantage is that you don't need any extra hardware to do this, disadvantage is that you get absolute nonsense as long as the demodulator is not locked.
For the device you have this technique is most likely used.
This implies that the best method of aligning a antenna or dish is to start azimuth and elevation alignment by peaking the signal level (don't look at C/N yet).
Once the antenna has the right azimuth and elevation, you optimize the C/N by keeping the antenna aligned at the transmitter, but rotating it around the beam axis to find the tuning for horizontal or vertical modulation. This is achieved when best suppression of the opposite polarity is achieved, and can only bee seen in the C/N, not in the signal strength.
If you have an antenna this means rotating the actual antenna. If it is a dish, you optimize by rotating the feedhorn or LNB.
Good luck...
Volhout
Edited 2020-06-21 05:14 by Volhout