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converter IF's in/out load range

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Senior Member

Posts: 90
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

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

A. O. Green
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Eric Johnston
Senior Member

Posts: 2109
Reply #1 - 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
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« Last Edit: May 8th, 2013 at 10:51pm by Admin1 »  
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Senior Member

Posts: 90
Reply #2 - 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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