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Message started by Breneer on Oct 25th, 2008 at 7:42am

Title: Satellite Basic
Post by Breneer on Oct 25th, 2008 at 7:42am
Im interested in VSAT technology.

To be able to learn and troubleshoot effectively.

Could someone explain in a basic way.

1. SNR

2. C/N

3. POWER

4. VBER

5. TDMA

... ADD MORE PLS.

What are the things to look at in the digital spectrum when looking at the carrier?



Title: Re: Satellite Basic
Post by Eric Johnston on Oct 26th, 2008 at 11:52am
1. SNR
Signal to noise ratio.  Signal refers to the total signal power.  Noise refers to the total noise power in the carrier bandwidth.  Normally expressed in dB.  Typical values 4 dB to 20 dB.

2. C/N
Carrier to noise ratio.  Same as SNR.

Note that when you attempt to measure the signal or carrier level there will also be an amount of noise included, so your measurements are in fact (C+N)/N  Read more: EbNo calculation  

3. POWER
Measured in Watts (or dBW) on milliwatts(or dBm).  Power is true RMS power, like the heat developed by dissipating the radio energy in a resistor.  If you use a spectrum analyser  to measure an unmodulated  CW carrier the result is accurate.  If you use a spectrum analyser to measure a modulated carrier you MUST activate the MARKER NOISE ON function and get results expressed in -dBm/Hz.  If you put the marker on the top of the carrier just add 10log(BW) to get the total power (c+N).

Uplink power from your earth station is measured in Equivalent Isotropic Radiated Power (eirp) dBW, which is the BUC operating power (e.g. 0.5 watts or -3dBW) plus the antenna transmit gain.  

If you want a measure of transmit power put a high loss calibrated waveguide coupler in the transmit waveguide and connect an HP power meter head and HP power meter.


4. VBER
I have no idea what the V is about.  BER means Bit error rate, for example, 1 error in 1000000 bits, also written a 1E-6.     Forward error correcting codes are used.  Turbocodes are a recent major development.  A high bit error rate at the receive demodulator is converted to a low bit error rate at the output, the penalty is that the transmission bit rate is greater than the information rate. Read more Symbol rate and FEC codes

5 TDMA
Time Division Multiple Access.  This normally refers to the remote VSAT uplink.  The VSAT transmits occasional short bursts.  These bursts arrive at the satellite, carefully timed so as not to overlap in time bursts from other uplink sites.  Between 2 and 100 VSATs may share the same TDMA uplink carrier slot.  See How TDMA works.

If you monitor a sample of your uplink carrier (at say 14 GHz)
check for spurious transmissions over the entire transmit band 14-14.5 GHz.  Check close to the carrier for spectral regrowth which will interfer with adjacent carriers.  If yoy are transmitting 2 or more carriers apy attention to your intermodulation products. If you have 2 carriers, A and B you will radiate some intermodulation at 2A-B and 2B-A frequencies.  All permuations of A+B-C apply if you have 3 carriers.  Your HPA rating needs to be 4 times your composite actual operating power.

Monitoring a downlink digital carrier observe that it is on the correct frequency and at correct power level.  Look to see if it is causing interference on the opposite polarisation. Turn it off and see if there is any interference underneath.  The shape of the carrier should look normal.  Humps may be interference or ripple in the transmit or receive equipment.  There should be no spike in the middle - due to carrier leak in the modulator.  A modulation analyser is needed to detect phase problems - often due to overdrive or power supply noise in HPA or BUC.

Monitor a TDMA transmit carrier by putting the analyser in MAX HOLD mode and waiting 10 minutes till a full carrier spectrum gets filled in.  Alternatively look in zero span and fast single sweep mode and keep clicking sweep till you capture the envelope of a burst.  Good for indentifing chirp and burst timing overlap.  Chirp can be due to BUC power supply problems.  

Best regards, Eric

Title: Re: Satellite Basic
Post by Y2J on Jan 23rd, 2009 at 11:07am
Thanks Eric for the valued information, it will help a lots of us..

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