SatSig topic:

10 MHz Reference Fault

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Jun 9^th, 2014 at 3:33am

Dear all

What are exactly happening shown by this alarm log iBUC

6/2/2014 12:07:54
DRO out of lock
Tx OutputLvl low
10MHz Ref fault

???

This alarm appears every time all remote down.
All remotes were down but the there was still the outbound.
TAC said that there is trouble on the RF not on the HUB.

Need help please. Cause I have no much experienced in RF.
Cry

Thank you

Jun 9^th, 2014 at 5:50am

Hello Marcell,

Do you receive the alarm on the BUC of the hub?
If so, please describe the Tx chain - what is connected at what order from the iDirect hub to the BUC. Where is the hub located? We can send you an engineer to revise it as well.

Kind Regards,
Nimrod

Jun 16^th, 2014 at 7:04am

Thank you for the concern Oasis,

Yes on the BUC of HUB
I found that the problem was on the 10 MHz Ref.
I have fixed it by using 10 MHz Ref from another source. I use X5 modem as the source by disabling DC and enabling 10 MHz.

Thank you
Sincerely

Jun 16^th, 2014 at 8:09pm

If you are repairing a 10 MHz reference pay attention to the smoothing of the DC power. This applies to both the oscillator itself and all subsequent amplifiers / boosters.
Noise on the 10 MHz signal will degrade the entire network.
If you have splitters in the 10 MHz distribution, isolated outputs are ideal. Watch out for noisy devices that feed back noise into the network. Terminate unused ports.
Trim the frequency of the oscillator a week after installation and then annually.
Best regards, Eric.

Jun 17^th, 2014 at 4:52pm

Hello Eric,

Can you please explain what do you mean by trimming the frequency of the oscillator?

Thanks,
Nimrod

Jun 17^th, 2014 at 7:01pm

I use a frequency counter, an accurate but ancient BBC long wave radio 198 kHz off-air 10 MHz standard and compare readout with that of the device under test.

A modern test set up would be likely to use a GPS signal as a reference. If money is no object then a redundant rhubidium atomic standard is what some big teleports use, giving them a secure accurate source, independent of GPS reception, which can suffer from local radio interference and rare and brief outages due to gaps in orbit coverage.

Many teleport hubs rely on dual redundant ovenised crystal 10MHz oscillators, able to be adjusted in situ, and removed form the rack independently for repair if required.

The adjustment is normally a tiny screw adjuster on the oscillator case, alternatively a software programmable voltage.

Jun 18^th, 2014 at 10:18pm

Thanks Eric, that brings me back like 15 years ago, when we worked with such tunable equipment along with other analog systems like FDM microwave where you could tune the pilot and the order wire frequency..

Does this tuning exist with modern modems? like on the board of the modem? I dont remember I ever notice one.

Thanks

Jun 19^th, 2014 at 5:26am

SCPC modems are likely to have an adjustment facility for their internal 10 MHz osc, plus an optional input for an external reference.

Star network TDMA VSAT remotes have their frequency controlled in real time from the the hub so the TDMA bursts arriving at the hub are all on the same frequency. The adjustment process may include compensation for doppler shift and the exact frequency of the remotes will vary during the day. There is no need for a remote to be particularly accurate when initially powered on unless the outlink carrier (hub to remotes) is unusually narrow bandwidth as at the modem can take its time finding and locking to the continuous outlink carrier. Compare with the hub TDMA receiver which must tune and lock to every burst very fast.

Jun 19^th, 2014 at 9:09am

Eric,

What is the exactly function of RCM (cause there is also 10 MHz ref) in a HUB?
Sorry, I am just a new engineer

Jun 19^th, 2014 at 9:37am

Eric,

Will the 10 MHz ref of X5 work in this such system?

Thank you

Jun 20^th, 2014 at 6:40am

I would expect your design to work.
The level of the 10 MHz into the hub may be low but the hub may be tolerant of this. Check the hub specification for range of input levels acceptable.

The accuracy of the X5 10 MHz would be worth checking. Keep the X5 at stable ambient temp.

Don't mount the master oscillator next to a fan shelf as the 'spinning' magnetic field from the motor may modulate the 10 MHz reference.

Regarding the diagram, I assume there is a DC block in the side of the splitter leading to the hub, and that the DC supply to the hub BUC is ON. Beware of burn out due to accidentally feeding DC into a splitter or attenuator. My references to 'smoothed' DC refer to the +5 or +12V on the supply pin of the oscillator can. The triangle representing the BUC amplifier should preferably point downwards in this example, towards the antenna.

I have no idea what RCM stands for. Maybe remote control and management ?

Jun 20^th, 2014 at 8:56am

Marcel,

Another point worth checking - reason I asked for uplink details, is that if two elements provide the 10MHz, it may cause some problems.

Nimrod

Jun 24^th, 2014 at 5:20am

Dear Nimrod,

This is BUC captures I got.
I hope it gives you the uplink details.

I still don't understand yet about 10 MHz Ref that provided by RCM on HUB. What is the function? cause you said that using two 10 MHz Ref might caused problem.

Marcell

Jun 24^th, 2014 at 10:00am

Repeating from the first message above: "TAC said that there is trouble on the RF not on the HUB."

Have you actually checked for the presence of 10 MHz at the two TX ports on the hub ? A good starting point in diagnosis would be connect a spectrum analyser (with a DC block and attenuator to protect its input) to each of the TX ports and turn the hub supplied 10 MHz on/off in the software to see if a hub fault really exists.

Why do you have a splitter/combiner on the two TX ports ?
Are you transmitting two outlink carriers via one BUC?

If 10 MHz is available from both TX ports, and you need the splitter/combiner, then only one of the 10 MHz should be turned on in the software. If you have both on at once then when they are combined the result may vary from no output at all to twice the level (depending on phasing) and if the two sources come from different master oscillators then the level will vary periodically, as the two frequencies will be slightly different. e.g by one tenth of a Hz.

The splitter/combiner device must be broadband, including from 10 MHz to the upper end of the hub L band output (may be 950-1525 MHz). The splitter/combiner device would need to be ultra-broadband from DC to 1525 MHz if you were using a low power (e.g. 3W) BUC powered via the coax. I'm presuming, in this case, that you have the BUC DC power disabled in the hub.

Regarding your iBUC data display. I note that in one instance the 10 MHz input source is shown as low (yellow warning). The BUC upconverter PLL is still locked, so it is still working, but as risk of outage.

Jun 24^th, 2014 at 11:06am

Hi Marcel,

This is Terrasat BUC.

Maybe your cables are too long or there are too many combiners/combiner of a poor quality. Sounds like the BUC gets the 10Mhz signal too low. The BUC needs the 10MHz ref signal in order to generate the carriers accurately on the right frequency.

What is the cable type you are using for the uplink and what is the cable length?

Again - we may help you if you can describe exactly the Tx chain. Maybe it was not designed properly.

Nimrod

Jul 1^st, 2014 at 5:41am

Dear Eric and Nimrod,

Sorry to respond it late.
yes, two outlink carrier via one BUC. Two outlink carrier from to transmit linecards in a HUB. They are connect to one Satellite.

I unfortunately did not work on the problem anymore. I suggested to site person, to do what all you had suggested me. And they found that the problem was on the cable 10 MHz (on Combiner) - BUC. Bad bad quality of cable. They had replaced it with the better one.
One question Sir, please explain me how cable impacts the 10 MHz event it causes all remotes up and down? Please...

Many thanks and regards for all the assistance. It helps me a lot as a newbie. hehehe

Jul 1^st, 2014 at 7:23am

Hi Marcel,

Different cables have different attenuation at different frequencies. Maybe the older cable attenuated the lower frequencies too much and therefore the 10Mhz signal came to the BUC too low.

Kind regards,
Nimrod

Jul 1^st, 2014 at 12:01pm

This is purely educational.

BUCs need a 10MHz reference in order to work. If it is absent or low the BUC will stop transmitting rather than transmit on some wrong and unstable frequency. If it is a hub BUC then the entire network will fail as the outlink carrier will be off.

If two cables feed one carrier each, via a combiner, to a single BUC then the 10MHz reference should only be supplied from one source.

The combiner device needs to pass both 10MHz and L band (950-2050 MHz)

DC power for the BUC is sometimes provided via the coax cable. If your BUC is powered from AC mains power, turn off any DC supply option in the software. DC power can burn out any attenuators or combiners in the transmit combining network.

Remote VSAT site modems have local 10 MHz oscillators that are all locked in frequency using the broadcast outlink signal from the hub. This ensures that all bursts from remote sites arrive at the hub at exactly the same frequency.

If you transmit two carriers at the hub using one BUC then you will transmit unacceptable intermodulation interference if the composite power of the two carriers exceeds about 0.6 of the rated power in watts. Note that 0.6 = -2.2 dB output back off. "Super linear" power amplifiers or ones equipped with linearisers may be able to operate nearer saturation while keeping intermodulation levels acceptable. If you transmit more than one carrier per BUC you must have an RF spectrum analyser at the site capable of monitoring the RF output (in 6 or 14 GHz bands) to verify that your spurious and intermodulation transmissions are acceptably low. Print out your transmit spectrum and sent it to the NOC.

Beware of what will happen if uplink power control is implemented at the hub.