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VSAT technology and installation >> iDirect Forum: hubs and terminals >> Re: Rx_acq range

Message started by Eric Johnston on Nov 12th, 2010 at 1:24pm

Title: Re: Rx_acq range
Post by Eric Johnston on Nov 12th, 2010 at 1:24pm
LNB type NJR2836S,  ref: has 12.25 - 12.75 GHz input, local oscillator 11.3 GHz and output 950 - 1450 MHz.  The LO is PLL type with internal reference. Stability is +/- 3 ppm.

Given the LO is 11300 MHz and with +/- 3 parts per million stability the error can be as much as +/- 33900 Hz or +/- 40 kHz.

Quite what you need to set the acquisition range in the iDirect options file I don't know.  I have never been sure if the acquisition range was double sided or not, in this case maybe 80 kHz or maybe 40 kHz is needed.  Someone from iDirect needs to tell us.

Best regards, Eric.

Title: Re: Rx_acq range
Post by Eric Johnston on Nov 12th, 2010 at 5:15pm
Trial and error is no good unless unless you can heat and cool the LNB over its full environment range (e.g. in a lab).

If 40k acquisition range means +/-20k then it might work fine when initially tested and then for several months, but fail in service during very cold winter or very hot summer.

It would be best to find out from iDirect officially if a 40k rx-acq range means +/-40 kHz or +/-20kHz.

You could try testing yourself by using say 40k acq range and repeatedly tuning the rx modem to all 5k increment frequencies over a range of say +/- 100k and record which frequencies work. You may be able to find a freq offset reading in the rx modem.  If so, record that as well. Try to do the tests quickly if the carrier is drifting while you do the tests.

Personally I would add at least 20k extra either side as safety margin for satellite conversion LO and doppler plus much more if your transmit modem and BUC 10MHz reference is not locked to an atomic reference clock (GPS or similar).  If you have extremely low symbol rate carriers you may have problems if there are several such carriers adjacent.

If the possibility of locking to the wrong carrier is not relevent in your situation then use a wider acq range.   It may take longer to lock, say .8 sec instead of .5 sec, but lock up time is irrelevent in an SCPC system that only locks several times per year - hopefully !

Best regards, Eric.

Title: Re: Rx_acq range
Post by dot on Nov 18th, 2010 at 12:29pm
I believe that Rx_acq range in the options file is twice the LNB stability value. If your LNB is +-500 KHz then Rx_acqrange will be 1000000.

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