Offset Issue on Evolution Modems !

There are so many Remotes which are doing Offset from 10000 - 18000 KHz. What will be the issue ! My LNB stability is between 0 - 10 kHz and modms are not login on Hub.

I am unsure of your question. 

1. Stability of what?  Remote LNB or Hub LNB?
2.  Are the remotes reporting a Green receive light?  If so, are they reporting a Green Tx Light (LED)?
3. If they are reporting a Green TX, can you see them with a SPECAN at the hub?

Based on what you mentioned (that fixed it),  there was no specific modem issue, you just need to ensure that you have the downconverter stability set properly (in the Hub RFT).   The same applies for the  Remote's LNB (Remote RFT) ensure you have the stability set appropriately for the type of LNB you are using (DRO or PLL).

Quote:


A PLL LNB with internal reference that stays within +/- 10 kHz is very good.  The frequency drift as seen by your receiver will exceed this due to frequency errors at the transmit end, satellite doppler and satellite conversion oscillator stability.  Also the claimed +/-10kHz for the LNB internal reference may be a little optimistic.   Note you cannot change the LNB stability from 10kHz to 20kHz.  The stability depends on the crystal temperature and drift characterics of the crystal and its surrounding components.

When you alter the LNB stability setting in the software you are altering the acquisition sweep range of the receiver.   The wider the range the better will the receiver be able to find and lock to the carrier.  In the case of the outlink - hub to remotes - a wide acquision range is normal e.g. 1800000 (1.8 MHz) to cope with cheap DRO LNBs, also since a remote only locks to the outlink once (when powered on) it does not matter if it takes a whole second or so to lock up.   In the case of the hub TDMA burst receiver the receiver must lock to each incoming burst extremely fast ( few microseconds ) so the initial frequency error must be low.  This is done by controlling all the remote site transmit frequencies, ideally so all bursts arrive at the same frequency (and level.) < some text deleted here > The hub software may tell you what frequency errors are being observed for incoming bursts.

Your hub will have some kind of 10 MHz frequency reference.  This may be a rack with ovenised crystal oscillators or atomic based from local rubidium standard, GPS or Radio.  This reference will feed a 10 MHz supply to all modems, BUCs and external reference PLL LNBs.

If you have ovenised crystal oscillators or are using the iDirect hub oscillator as the reference then it would be worth checking and adjusting this annually compared with an atomic derived standard. Adjusting an ovenised oscillator normally needs a trim tool or may be done in firmware by altering a bias voltage in an EPROM.

Regarding your other questions above, Mike is probably best placed to help.

Best regards, Eric.

Dear M,

*Are you certain that your Hub LNB stability (spec) is 10kHz?  

Ans.Yes, i Have Checked it is 10 kHz.

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*It is doubtful your issues are related to Carrier Groom or Freq hop settings.  Are you using FH or Carrier groom?  How large are your upstream carriers?    

Ans. Yes, We are on Frequency Hopping
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*What are you presently seeing with regards to Symbol offset (remember each node will be different)?  

Yes, it is Correct no issue in Symbol Offset
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One thing I would like to be in your notice that in One RF Chain we are using iDirect iNFINITi as well as iDirect Evolution but issue is only on Evolution Network. ???

*Ref the downlink RF/IF chain (at the Hub): is that downnconverter shared? 

Ans. yes the Down converter is Shared(10KHz)

It is possible.  How much do they differ?   If you have iSupport I suggest you call the TAC and inquire.

The original post said: "There are so many Remotes which are doing Offset from 10000 - 18000 KHz."

I presume you mean " 10000 - 18000 Hz" and that you are referring to the burst receive frequency offset at the hub TDMA receiver.

Can you make a list of all the remotes and the measured offset of each one ?

Does the collection of offset values make a pattern, e.g. normal distribution curve ?

Does the pattern drift with temperature or time of day ?

Best regards, Eric.

The acquisition aperture can be changed from downconverter stability. You can modify the downconverter stability making the aquisition aperture wider (eg from 10 kHz to 20 kHz or 30 KHz).
Notice:
1.1. The configuration will be applied only to remote Hub side and the netmodems that are already in the network will remain in the network.
1.2 The acquisition time for the remote will be longer than usualy.
1.3 After acquisition (if it works)  you can change at the old value the downconverter stability.
1.4. For netmodem Infinnity, the acquisition in the network are made even if the aquisition range is narrow because after a period of time I think that change de mode aquisition from fast aquisition to wide acquisition.

I think this will not solve the problem but will give a clue about what happening.

Best regards,
Adrian

Do all the remotes have the exactly the same outlink frequency, symbol rate and translation frequencies specified in their options files ?  Minor discrepances will not affect the outlink reception at the remotes but the remotes may use the actual outlink frequency and symbol rate and the programmed (text) frequency numbers to derive the exact remote transmit frequency.

I don't know how iDirect controls the remote return link frequencies.

In the hub software are all the remotes being told the same information regarding return link frequency into the TDMA line card ?

There was mention of two networks.  Are both networks in the same transponder ?  Satellites may use different conversion oscillators for transponders on opposite polarisations for example.

If you widen the acquisition window you will slow down the acquisition time.  The may decrease the traffic throughput significantly as fast acquisition is essential for short TDMA bursts.  The TDMA line card receiver must retune at the start of every burst.

Best regards, Eric.

Do both networks use the same outlink carrier ?

I don't have any experience with iDirect so please if anyone knows how iDirect controls the remote transmit frequency and how the TDMA receiver line card deals with off-frequency remotes, please help.

Best regards, Eric.

I had a similar problem with the terminals in my network but sincerely I think you have a different situation.
Anyway I solve the problem in this way (after many attempts like swap downconverter with backup downconverter and the entire receiver chain, including the Rx HLC with standby HLC).
From Hub Chassis, I swap the Reference Clock Module (Master) with the reference clock Module (Slave), and  the frequency offset from the netmodems was more stable than in the first configuration, but not enough. So I conect an 10 MHz external reference to RCM and the network work fine.
Notice
The external reference clock must be very stable (Not working with 10 MHz from netmodem iDirect).
The only explanation what I have is the out of calibration RCM.

Other suggestion:
1. Check if the BUC from the remotes are similar for the sites what working and different for the site with problem.
(maybe are differencies between radio equipment configuration for infinnity and evolution remotes) and the upstream Tx frequency offset are to big.In this case you can put the stability for hub downconverter at a convenient value (40 kHz) and mentain this value to see what is hapend.
2. Try to swap Rx HLC with a spare one (or Tx card) because HLC have a FLL loop. Maybe you have a problem with demodulator.


Best regards,
Adrian

Dear Mr. Eric,

In theory you have perfectly right.
But practical it's very difficulty to adjust RCM frequency.
That  because I don't have any  circuit diagram for RCM and it have a lot of component on the printed circuit that don't have a clue what it do. I see where is the crystal oscillator but it isn't enough.
It is neccesary to know at least a pinout for the  RCM board equipment and it very hard to adjust the frequency when you don't know where is the pinout for voltage supply or output reference signal.
It isn't possible to make this operation with the RCM in use because you don't have any additional space to adjust any component. I can't even to measure the reference signal because the risk to damage the board is very high. I think it is necesarry an aditional printed circuit that allow you to do that outside the RCM. And the things begin to be more complicated because you need the same type connector  like that are used by hub chassis.

Otherwise, you need a very good instrument for measure frequency and at this precision it is hard to get one because is very expensive.

First I chose to check if this is the real problem and after that I will  decide what are the best way to solve the problem.

Best regards,
Adrian

According to iDirect if power is applied to a chassis that has both RCM installed, either RCM can become the chassis RCM master. The first RCM to become operational will determine if another RCM is installed in the chassis and will arbitrate to determine wich one will become the master RCM.

I notice the RCM slave track to 10 MHz reference signal from RCM master. This is accomplished by Frequency-Locked-Loop circuit on the RCM.
If the 10 MHz reference signal from master RCM isn't calibrated and have an offset frequency the master RCM don't consider that failure and remain master.

Best regards,
Adrian