Spike from Modem

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Apr 30^th, 2011 at 9:24am

Hi,

Im using satellite modem (comtech) and I found that the is CW/spike being transmitted together with modulated carrier from the same modem. The CW was noted at center frequency of the carrier. I’ve tried to power rest the modem but same result.
As such, I ve been wondering what was the issue that causing this bleed-over. I dont have any redundant modem to check.

Appreciate your help and opinion..
thanks!

Apr 30^th, 2011 at 10:37am

strange.

I would suggest two things:

1. Turn off the transmission, even better, turn off the modem, and check if the spike is still there. If it is - maybe it is not coming from the same modem!

2. In the CW option, you can choose several options, like offset, dual etc. Try to play with it and see if the CW moves.

3. Try to toggle CW on and off....

Apr 30^th, 2011 at 1:24pm

Try different spectrum analyser settings, span, narrower resolution bandwith etc. Spectrum analysers sometimes display this anomaly when no such spike is actually there.

Is your carrier continuous and do you have scrambling on? If the carrier is intermittent such as sending bursts then the start of each burst will be CW and show this effect. If scrambling is off the spectrum is more likely to have multiple spikes and jagged shape.

The modulator in the modem may have its I and Q channels unbalanced. In some modulator designs there is a small potentiometer to null out the residual CW.

Best regards, Eric.

Oct 12^th, 2011 at 12:05pm

i assume thats from 10Mhz from the modem.
turn off 10Mhz setting on the modem,
Buy 10Mhz power supply for supply BUC and then try to switch on your carrier.

thanks.

Oct 13^th, 2011 at 9:45am

How can the 10 MHz reference cause interference at the centre frequency of the transmit carrier ?

Comtech modems output the transmit carrier in the following IF band ranges:
70 MHz IF band (50 - 90 MHz)
140 MHz IF band (100 - 180 MHz)
L band IF band (950 - 2000 MHz)

Best regards, Eric.

Dec 2^nd, 2011 at 11:01pm

Hello,

If you dont have another spectrum to check with. call your provider. they will do the checking for you. During the verification, turn off both carriers and possible HPA's at both ends. if you still do have a CW (provider Watch) then this CW is not from your ends.

kindly update the forum on the results

Regards
Green A.O

Email me:eric@satsig.net

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