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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 8th, 2013 at 11:31am |
Title: converter IF's in/out load range Post by AOGreen on May 8th, 2013 at 11:31am
Dear All,
Ques 1. How many Carrier IF's input/out can a Down-converter/Up-converter tolerate (accommodate) for a setup of Carriers IF Range=120Mhz -170Mhz IR (Kbps): 2048kbps, Modulation: 8-PSK, Coding: 3/4 SCPC CNC Ques 2. What could cause ENBO degradation on C band apart from weather, No saturation on transponder? Ques 3. I have 20+ Carriers on a transponder. when I turn off some carriers, I observe gain in EBNO on the other carriers *for 5-6 carriers turn off= gain of approx 0.5dB across the remaining 20+ carriers. (0.5dB x 20 carries= approx 10dB gain) *for 6-8 carriers turn off= gain of approx 1.0dB the remaining 20+ carriers (1dB x 20 carries=approx 20dB gain) The satellite operator checked for saturation on the transponder and notified me that the transponder has not reached compression when all the carriers are active. there is still 3 more dB to compression Although most carriers are 1+ above nominal below is a table for the carriers .....DL DL EIRP Delta Hub 19.2 Remote 17.3 CNC 21.36 23.5 2.14 Hub 20.5 Remote 18.6 CNC 22.66 24.54 1.88 Hub 20.7 Remote 18.9 CNC 22.9 25.64 2.74 Hub 20.6 Remote 18.7 CNC 22.76 26.13 3.37 Hub 20.8 Remote 19 CNC 23 25.04 2.04 Hub 20.9 Remote 19.1 CNC 23.1 24.27 1.17 Remote 13.1 Hub 14.7 0 0 Hub 20.9 Remote 18.6 CNC 22.91 26.29 3.38 Hub 20.8 Remote 18.5 CNC 22.81 25.6 2.79 Remote 18.6 Hub 20.9 CNC 22.91 25.59 2.68 Hub 20.9 Remote 18.7 CNC 22.95 23.04 0.09 Remote 10.2 Hub 12.2 11.61 -0.59 Remote 17.2 Hub 18.4 CNC 20.85 23.35 2.5 Hub 21.7 Remote 19.6 CNC 23.79 24.87 1.08 Remote 16.9 Hub 18.8 CNC 20.96 17.24 3.72 Hub 19.6 Remote 18.5 CNC 22.1 24.9 2.8 Remote 15.8 Hub 18 CNC 20.05 21.13 1.08 Remote 19.9 Hub 18.2 CNC 22.14 21.4 0.74 Hub 19.4 Remote 16.9 CNC 21.34 22.62 1.28 Remote 14 inactive Hub 12.7 inactive CNC 16.41 0 0 Remote 16.7 Hub 15.4 CNC 19.11 19.38 0.27 Remote 12.2 Hub 14.4 CNC 16.45 15.03 1.42 Remote 15.3 Hub 16.8 CNC 19.12 18.45 0.67 Remote 12.7 inactive Hub 14 0 CNC 16.41 0 inactive Remote 17.9 Hub 20.8 inactive CNC 22.6 0 0 Remote 18 inactive Hub 20.9 inactive CNC 22.7 0 0 Remote 16 Hub 18.2 CNC 20.25 20.41 0.16 Remote 9.1 Hub 10.5 CNC 12.87 13.97 1.1 Remote 11.6 inactive Hub 13.8 inactive CNC 15.85 0 0 Remote 15.6 Hub 17.1 CNC 19.42 24.07 4.65 Remote 15.4 Hub 17 CNC 19.28 20.41 1.13 What could be wrong? Your inputs will be highly appreciated Regards A. O. Green |
Title: Re: converter IF's in/out load range Post by Eric Johnston on May 8th, 2013 at 1:52pm
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 |
Title: Re: converter IF's in/out load range Post by AOGreen on May 10th, 2013 at 1:10pm
Dear Eric,
Thank you. I will look into everything again and keep you posted A. O. Green |
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