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A satellite transponder operating in multicarrier mode needs to be backed off from saturation (e.g. 4 dB input back off, 1.5 dB output back off) so that carrier to intermodulation noise ratio is kept at an acceptable level. �Normal optimum value of C/I is around 21 dB with the transponder full.

Assuming the transponder is full of similar small carriers.
If you turn off half of the carriers, the transponder input back off goes down by 3 dB (from 4 dB to 7 dB and the C/I ratio improves by 6 dB (from 21 dB to 27 dB). � �Each 1 dB of extra back off improves the C/I by 2 dB. Exact actual values will vary according to the satellite techjnology.

This is a classic transponder link budget design trade off. �If you plot overall C/N versus composite carrier level there is a broad peak, around C/I=21 dB. �That is how satellite operators come to set "4 dB input back off, 1.5 dB output back off, C/I=21 dB" as their recommended operating point for multicarrier mode. On-board linearisers enable operation nearer saturation.

The C/I is an important entry in the link budget and if you are operating carriers with an overall C/N of around 10 - 20 dB then changing the C/I will have a noticable effect.

An improvement in Eb/No when some carriers are turned off sounds quite normal to me.

If you turn off a single carrier (remote to hub) and the two adjacent carriers improve then that carrier was probably near to saturating its BUC and causing interference to the adjacent carriers due to spectral regrowth.

You mention CNC, so maybe you have a big 36 MHz carrier also occupying the same transponder, with all the small carriers superimposed. �In this case the power of the 36 MHz carrier needs to be added to the power of all the small carriers, when considering the transponder operating point. The level setting of the 36 MHz carrier is critical to the exact operating point. �

When looking for things that might be wrong consider all amplifiers in the system which carry more than one carrier.

On the receive side at the hub:

The LNB output final stage amplifier transistor (L band) may get overloaded if the dish is very large and there are many high power TV carriers from the satellite.

If there is an L band IF booster amplifier prior to a 16 way splitter then this amplifier must not be overloaded. �Suggest 8 - 10 dB backed off. �Some L band amplifiers are poor quality or have poor quality, noisy, DC power supplies.

If you have downconverters, e.g. C band to 140 MHz IF then input levels, gain adjustment and output levels are important. �Make sure that the composite downconverter output power is well down on the rated downconverter output power.

Input levels to the receiver cards should be correct. �This applies both the composite total power applied to the card and also to the power of the individual wanted carrier.

Look at the hub receive spectrum of your carriers and look to see that there is a clear noise floor in the gaps between them. �Investigate any filled gaps for possible cross-pol interference or spectral regrowth from overloaded BUCs.

IR (kbps): 2048kbps, �
Modulation: 8-PSK, � �(3 bits per symbol)
Coding: 3/4

Symbol rate = 2048 x 0.333 x 4/3 = 909 kHz
Occupied bandwidth = 1.4 x 909 = 1273 kHz.

Transponder is 36 MHz wide, so 28 carriers.

If your transmit modem filtering is tighter, then factors like 1.2 or 1.3 or 1.35 apply instead of 1.4

It is up to you how much adjacent carrier interference you accept as a trade off of total capacity. �Leave a larger gap if you have a high power spectral density carrier adjacent to a low power spectral density carrier.

Reviewing your list of results I would tackle the worst ones first.

Best regards, Eric

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https://www.satsig.net/cgi-bin/yabb/YaBB.pl Anything else >> General and other topics >> converter IF's in/out load range https://www.satsig.net/cgi-bin/yabb/YaBB.pl?num=1368009072 Message started by AOGreen on May 8^th, 2013 at 11:31am