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7350 series drifting TX freq.

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Ex Member
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Apr 1st, 2010 at 11:45am  
We are having an issue with the 7350 iDirect series, using software version 6.0.9.7. The remote terminal is drifting in frequencies more than 20 Mhz. When we turn off the RX enable we are able to put up a cw at the right frequency, and transmit at the right frequency. Once we turn on the RX enable and start talking to the hub, the tx freq will drift coming out of the modem. Checking the TX freq with the command will show the proper freq, but what is coming out is not that frequency.
We have swapped out the modems on both the remote and the hub, boot files have been created and pushed multiple times. We were up and running for several days, then it started flaking out.
Any ideas or simular problems would be helpful. Thank you.
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dot
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Reply #1 - Apr 1st, 2010 at 2:56pm  
Jeffrey

Is this a TDMA remote working from a hub or an iSCPC link? The reason why the CW is stable when you disable Rx is that you are effectively stopping the remote form recovering clocking from the received carrier. This indicates that the instability is on the distant end. You further proved this by getting a stable CW while running on the local ovenised oscillator. I'd suggest you look at the hub side. If this is indeed a hub then you have an issue with the RCM or external 10 MHz reference or something similar. If it is an iSCPC link then you have more complex challenges.
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Ex Member
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Reply #2 - Apr 2nd, 2010 at 2:18pm  
Dot,
The TDMA is working from a hub. We tried the same project on a different hub system, with the same results. The only connection between the systems is the timing. We will check out the reference to see if it is a timing issue. Thank you for your suggestion!
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Eric Johnston
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Reply #3 - Apr 2nd, 2010 at 6:16pm  
You can disconnect the BUC from the antenna, and attach to it a high loss crossed waveguide splitter and dummy load.  Then carry out tests with a 14 GHz spectrum analyser without interfering with everyone else in the transponder.

You might also check that the outlink symbol rate, as entered in the remote site receiver options file, is exactly the same as the symbol rate set at the hub for the outlink carrier.

I would insert a splitter in the BUC cable and look on a spectrum analyser to see if it is the L band carrier that is drifting off frequency or the 10 MHz reference to the BUC that is drifting ?  If neither then the BUC LO PLL is not locking.  Is the 10 MHz reference low level ?  Is the power supply to the BUC good ? Is the transmit level of the L band carrier reasonable or is the modulated carrier at far too high a level so it overloads the BUC dramatically and generates spurii on the input as well as output. ?   Do you get the frequency error symptom if you reduce the drive level by 20 dB ?

In modulated mode, with freq drift, is the 'drift' at some fixed frequency or is it wandering randomly ?  Is it sweeping  repetitively ?

In CW mode, can the frequency drift symptom be induced by changing the drive level ?.

Is there anything unusual about the site ? Very short or very long IFL cable ?  Cable splitter affecting 10 MHz pass  ?  10 MHz HF radio station antenna mast next door ?

Is the same 10 MHz reference feeding all relevent parts at the hub ?  Are other sites on the same outlink OK ? Do those sites have exactly the same options file, regarding frequencies ?

Best regards, Eric.
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« Last Edit: Apr 3rd, 2010 at 12:21pm by Admin1 »  
 
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