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Need a KU Band light weight 4W BUC

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Jun 12th, 2009 at 5:33pm  
TX signal isn't strong enough with current 3W BUC.  Can't get a 1.8 meter dish because of my location.
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Reply #1 - Jun 12th, 2009 at 6:25pm  
To be more specific, I need a NJT5017F.  Must be able to ship to APO address.
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Eric Johnston
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Reply #2 - Jun 12th, 2009 at 7:17pm  
Increasing the BUC power from 3 to 4W increases the C/N of the signal by just 1.2 dB.  

How many dB are you short of what is needed ?  What is the hub C/N now ?

What was the result of the CW power measurements made by the hub versus modem output levels ?  Can they plot the input - output curve of your BUC and check what is the -1 dB compression point.  

Typically your operating power is 0.5W (C/N=9dB in clear sky) and it peaks at 2W during a 6 dB rain fade. (4x = 6 dB uplink power control range).  

Note that it is quite possible to overdrive the BUC and  reduce hub signal quality with increased modem output power.  

Are you sure you can't get an extra 1.2 dB by better pointing of the dish or by removing any dish distortion.  Check the rim of the dish is flat to 1.3mm.  Remember also that the transmit beam is narrower than the receive beam.

Alternatively you may be able to reduce your transmit bit rate by half, which needs 3 dB less power.  Talk to the hub to see what in-route bit rate cards they have.

Increasing dish size from 1.2m to 1.8m increases TX gain by 3.5 dB but only if both dishes are not distorted and pointed perfectly.

Best regards, Eric.
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Reply #3 - Jun 12th, 2009 at 7:42pm  
Eric, I think that our max C/N was around 5db.  This was at -5db of transmit power on the modem.  The test was done during fairly clear weather.

When I was with TS2 they had us transmitting at max 0.0 tx power and we still only got about 4.5 C/N.  This also greatly reduced our receive SNR.

I'm positive that the dish is pointed as well as it can be.  I haven't check to see if it is warped.  But I'll try anything at this point.  The current dish I have doesn't allow for adjustments as shown in the link you provided but I do have a spare dish that has that type of set up.  If I find that my dish is warped I will swap it out.

Thanks for the help
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Eric Johnston
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Reply #4 - Jun 12th, 2009 at 9:08pm  
All this needs hub assistance.

Modem output power of -5 dBm is still rather high as that imples 15 dB loss in your cable to the BUC, which would give -20 dBm input to the BUC - which is typically all that is required to saturate the BUC.  Find the BUC spec if possible.  Do you have an abnormally long cable ?

It is worth finding out what happens at lower drive levels.  Once the hub C/N starts dropping dB for dB you know you are on the linear part of the curve.  Go back up till it start to compress, then stop.  That defines the -1dB gain compression point and this value should be set as the upper limit of the UPPC range.  Preferable C/N at this point would be 15 dB. Then set the normal clear sky power 6 dB lower so C/N=9 dB in clear sky conditions.

It is worth checking the dish for distortion with the fishing line test.  If the sides need pulling back I have secured a 1.3m length of steel, with holes at each end, across the back and used steel cored cable ties to gently and exactly pull the sides back till the strings just touch.

If necessary ask about reducing the uplink bit rate.  If the hub has line cards for lower bit rates that might help.  Since you are probably on a shared TDMA uplink it won't make any difference to your general uplink bit rate - your bursts will just be a twice as long.

To get from C/N=5 dB to C/N=8 dB you need twice the power (watts) or +3dB.

Peak the dish pointing by halving the distance between equally degraded measurements on either side.

Best regards, Eric
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« Last Edit: Jun 13th, 2009 at 11:12am by Eric Johnston »  
 
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Reply #5 - Jun 13th, 2009 at 12:39pm  
It's hard for me to speak to what the hub is or isn't doing.  Partly because of lack of experience.  I do know that I've tried service with two different Hubs and both have complained about my TX signal.

I can speak to the fact that I've shortened and re terminated all of my cables at least 3 times.  They are approximately 20ft in length and have no shorts. (tested with multimeter)

I tried the string test like you mentioned and the dish is not distorted at all.  The strings actually just touch one another.

I will ask about reducing uplink bit rate.

I was comparing my BUC/LNB assembly to that of another spare we have.  I have some pictures if you have time to look at them.  One is from a Bentley Walker system and ours is from TS2.  The LNB's are different (we have been using the NJR2784H the other is NJR2754H) but I believe the BUCs are compatible (using NJT5037F the spare is P/N 1025739).  The Bently Walker assembly looks a lot different and has a much larger feed on the front.  Not sure if this is based on how it sits in front of the dish or not.  Anyway, if it's possible to mix/match these assemblies to fix our problem let me know.  You may even be able to tell me if ours is assembled incorrectly. 

Otherwise I'm going to get a 6W BUC.
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Eric Johnston
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Reply #6 - Jun 13th, 2009 at 2:39pm  
BUC NJT5037F has a gain of 53 dB.  Its nominal P1dB output power is 3W or +35 dBm.  So maximum input level to the BUC should be +35 dBm - 53 dB = -18 dBm.

The coax cable typically has a loss of about 20 dB per 300ft.  So a 20ft length has loss of just about 2 dB.

So the maximum required output power from your modem is -16 dBm.

With -5 dBm now as the modem output you are very seriously overdriving the BUC.

Get the hub to try much lower output levels from the modem like -30 dBm, -25 dBm, -20 dBm, -21 dBm, -20 dBm  etc to find out what happens.

There is the possibility that you have damaged the modem.  It is like operating an audio amplifer far above it maximum output.  Your output will be seriously distorted and interfering with other peoples' services either side.

Don't worry about about lack of experience; having read this you probably know more than the hub staff.

With higher power BUCs (e.g. 6W , 10W etc) it is really critical not to overdrive them.  Always start with very low powers and go up in small steps, verifing linear operation and STOP when it goes non linear.  The output transistors can very easily be damaged by voltage spikes occuring with overload.  The spectral regrowth (distortion) will also interfere with other people.

Feed horns are associated with particular dishes and should not be swapped about.  Cheaper systems with small feed horns may not have such good sidelobe performance and may receive interference from adjacent satellites.

Best regards, Eric.
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Reply #7 - Jun 13th, 2009 at 3:51pm  
When testing we started with an initial power setting of -15 dBm but as soon as the terminal was in data transfer it would slowly creep to it's max power setting of -5.  Same symptom with TS2.  Initial power was -8 and would go to 0.0 dBm.

Not sure where to go from here.  Should I request another test with much lower power?  I have the other dish that goes with the assembly that came with a larger feed.  That's why I was curious if I could just swap out the LNB and try that set up.  You can just look at the other ODU and tell that it is better hardware.

I also have a spare iDirect 3100.  Is there a way to tell if my modem has been damaged other than trying the spare and seeing if that gives better performance?

We were constantly having interference issues with our current setup pointing to NSS6.  Our azimuth had us pointing in the direction of at least 10 other terminals.
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Eric Johnston
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Reply #8 - Jun 13th, 2009 at 5:44pm  
Below is my understanding of what the hub should do.  Each hub may have their own procedure and setting values.  I have not been trained in this, it is just the jist of what I have heard.

First, the hub needs to test to find the P1dB compression point of your BUC using a CW carrier.  Having determined that, they then record that as the maximum value ever to be allowed.

Then with the modulation ON they adjust the normal power setting to give C/N=9 dB at the hub in clear sky conditions.  The normal setting should hopefully be quite a few dB below the P1dB value.
Then set the upper limit of the UPPC range to say 6 dB more than the clear sky value, provided this does not exceed P1dB value.
The hub then saves these values on the hub database and uses then to tell your modem what settings to use.

If your site started at -15 dBm and was passing data at -15 dBm then there is no way an increase of a further 15 dB should be contemplated.

My guesses for your site are clear sky = -22 dBm, max during rain -16 dBm, and P1dB = -16 dBm.

This all may be wrong and there maybe a fault with the BUC but the hub should test properly and diagnose, compare with other similar sites etc.

If any iDirect hub operator is reading this please would they add their comments..

Best regards, Eric.
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Reply #9 - Jun 13th, 2009 at 6:02pm  
Is there something configured at the HUB that tells the modem to increase it's power?  I have been under the impression that the modem has an initial TX setting and that once it is 'in network' that it was supposed to increase to max power.  Shows how much I know.

I have another system not far from here (same hardware) that does the same thing.  Starts at -8 and creeps to -2 dBm.  I've not had any problems with it though and it keeps a constant 8-9C/N.
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Eric Johnston
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Reply #10 - Jun 13th, 2009 at 8:53pm  
The return link burst quality is measured at the hub and the hub then sends commands to the remote site to adjust its tx power to maintain a quality of C/N=9 dB at the hub, so when it rains either at the site or at the hub the remote site BUC transmit power is increased temporarily.

The nominal clear sky value and the range of UPPC increase must to be set at the hub by the hub staff when a remote site is commissioned.  The upper limit must not exceed what the site is capable of, thus the need to test for the P1dB comprssion point to set the never-to-exceed upper limit.  That needs to the typed in as well by the hub staff.  All this is clearly spelled out in the manual "iDirect Remote Installation and Commissioning Guide". A copy is here https://www.sky-stream.com/images/file/PROC_Remote%20Installation%20Guide%20iDX%...

Best regards, Eric.
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« Last Edit: Sep 10th, 2011 at 3:06pm by Admin1 »  
 
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TDMAMike
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Reply #11 - Jun 15th, 2009 at 12:19pm  
Quote:
Is there something configured at the HUB that tells the modem to increase it's power?  I have been under the impression that the modem has an initial TX setting and that once it is 'in network' that it was supposed to increase to max power.  Shows how much I know.

I have another system not far from here (same hardware) that does the same thing.  Starts at -8 and creeps to -2 dBm.  I've not had any problems with it though and it keeps a constant 8-9C/N.


As Eric mentioned above, there are prescribed UCP (Uplink Control Parameters) for each network inroute group (whether it is a single carrier or bundled multiple carriers).  

All modems operating in an inroute are instructed to meet a certain TDMA Nominal C/N, at the line cards.   In most cases the default is 9dB for Ku (lower for other bands).  Which is why your nodes are operating at 8-9dB.   With a configured 9dB in mind for the UCP, if bursts from a single remote or several remotes operating in that inroute arrive high or low (ex: 10.5dB or 6.5db) the NMS will issue them power offset in an effort to get them back to 9dB (9 dB is what is cosndiered the center of the sweet spot...in most cases the sweet spot is normally 1dB wide but centered on 9dB...therefore the sweetspot for Ku actually covers from 8.5 to 9.5db.).  It CAN be made wider (say 2dB) at the risk of mild acquistion CRC Errors (they are common with a sweet spot that is too wide...but easily diagnosed by the hub). If you look in the control panel for each remote you will see that their upstream SNR remains centered on the UCP Sweet spot (the TDMA nominal value set by the HNO).

With the above in mind, getting a 1dB compression test is a must.  A good 1dB compression test will identify your max power limits (read: for your operating location, and your RF components)  thus preventing the NMS from issuing power offsets to the point where you saturate the BUC and the signal/carrier becomes distorted (saturation will yield Hub traffic CRC errors).   Power offsets are common when  the Hub and/or the remote site(s) are under weather.  The NMS will ONLY up the power to a specific node, if THAT remote site is under weather.  It is site/remote specific (one site under weather will not cause the NMS to adjust power on the entire inroute group/all of the modems).  However, if the Hub side comes under weather, then obviously the NMS will issue power offsets to the entire inroute group to bring them all up on power due to the attn of the downlink - as seen by the e/s hub.
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« Last Edit: Jun 16th, 2009 at 12:39am by TDMAMike »  

Regards, &&&&M
 
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Reply #12 - Jun 16th, 2009 at 11:36am  
   


   i have used and new 4watt kuband buc 4sale

   call me on 08038547600
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Reply #13 - Jun 19th, 2009 at 6:08pm  
Just wanted to close out this post and let everyone know that we are finally up with a stable service.  We never did figure out what was wrong with our TS2 terminal nor why 3 different service providers couldn't get it to work.  We haven't found anything wrong with the hardware???

We are now on a Bentley Walker system, using the same 3w BUC on AM22.  Our tx power usually averages -19dBm but can vary between -20 and -16 depending on the weather.

Thanks for all of the help!
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