SatSig topic:

BUC and TX power problem

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Dec 28^th, 2007 at 2:33pm

I am kind a new in this, and desperately I need you professional help.
I have LinkStar system, 1.2m antenna. I am using service from Benley Walker, system sometimes works sometimes not. Provides ask me to change the cable and to check power at the end of it, and I did it, it shows 25.02V. And then they told me that I have to try to change BUC, because my TX power is to low, it show something like -2.90.
So I would like to ask you is there any way to check BUC, and is it maybe position of BUC incorrect.
At the BUC I could find only Part# VCD-022275-000 and Rev – 01.

Thank you All

Dec 28^th, 2007 at 7:54pm

Here are some reasons for low transmit power:
1. Low DC supply due to poor connection - often intermittent prior to complete failure - the contact arcs and burns out. The wire pin end may disappear. The spring inside they F connector socket may be damaged - burned oxidised. A tiny smear of silicone contact grease applied to the pin, plus good waterproofing is suggested..
2. Faulty BUC due to being overdriven due to modem output too high. Always start with modem output very low and increase cautiously. When the output (as seen by the hub) is sufficient or ceases to go up in linear dB steps, back off by 1 dB. BUCs should never be operated above their rated power.
3. Cable too long so low drive, even when modem is set to maximum output.
4. Poor dish pointing. It is easy to get a 'good' receive signal as the outlink signal from the satellite to you has a big link margin, so you get a good signal even if you are badly pointed. The transmit beam is narrower than the receive beam so a small mispointing loss on receive will correspond to a big loss on transmit. It is important that you point the dish to the exact centre of the receive beam. This requires significant skill as there is negligible change in level around the centre of the beam. If you measure your receive quality BER do this measurement very many times while turning the nuts in 1/6th turn increments across be beam and then wind back to the centre by halving the count.
5. Polarisation. If this is 45 deg wrong then the transmit level into the satellite will be 3 dB low. You will also be causing serious (total) interference to the service on the other polarisation and your receive service may have failed completely.
6. Water or moisture in the transmit waveguide.
7. Poor smoothing of DC supply - faulty power supply from the modem.
8. High phase noise on the 10 MHz reference can cause high error rate but not low level.

Best regards, Eric.