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2 Carriers on a Single Transmit Antenna
9th Aug, 2017 at 11:27am
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We would like to ask assistance regarding the requirements for 2 Carriers on a single transmit antenna.

Plan Configuration:
Two Modulator going to L-Band Combiner to SSPA Input

Bandwidth of the First carrier is 14 MHz (start/stop 6089MHz - 6103MHz)
Bandwidth of the Second carrier 2.5 MHz

Equipment:
- Antenna: 4.5 Meter - Single TX and Single RX port
- Amplifier: 400W SSPA with L-Band Input | Frequency Range: 5.850 GHz to 6.425 GHz
- Modulator: Sencore with L-Band output @ 1196MHz

Question:
1. What will be the L-Band Frequency of the 2nd modulator? 1208.25MHz?
2. For the L-Band Combiner, can we use an 2-Way passive Splitter that is within the range of L-Band (950MHz to 2150MHz)?
3. For the Amplifier: At multi-carrier Setup the Backoff power will be 200Watts (53dBm). Please note that on our current setup (single carrier operation) we are transmitting at 50.5 dBm (112W). Is the amplifier capacity is enough for multi-carrier operation?

Thank you so much for your help.

Best Regards,

Tristan
  
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Re: 2 Carriers on a Single Transmit Antenna
Reply #1 - 10th Aug, 2017 at 5:33pm
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1. What will be the L-Band Frequency of the 2nd modulator?

Find out what is the local oscillator frequency of your upconverter, from L band to C band.  Modern 400W C band amplifiers often have an option for an embedded L band to C band Block Up Converter.

Regarding your existing first carrier, you say it is 14 MHz wide between 6089-6103 MHz, so its centre frequency is 6089 + 7 = 6096 MHz.

The L band modulator frequency of Carrier 1 is 1196 MHz.

Your upconverter local oscillator frequency is the difference. i.e. 4900 MHz.
( Note that other manufacturer BUCs may have different LO frequncies. )

The satellite operator will tell you what centre frequency to use for your second carrier, somewhere in the range 5850 - 6425 MHz. They need to allocate you a frequency slot wide enough for your second carrier.

Calculate the required L band modem output frequncy for the second carrier by subtracting 4900 from the wanted C band frequency.

2. For the L-Band Combiner, can we use an 2-Way passive Splitter that is within the range of L-Band (950MHz to 2150MHz)?

Yes. It will cause about 4 dB loss so the output level from the first carrier modulator will need increasing by 4 dB or the gain of the BUC/400W amplifer might alternatively be adjustable. Don't operate either the modulators or the BUC/400W amplifer at extreme low or high gain settings. Ideal modulator operational outputs should be several dB down from maximum output so you can't increase them accidentally by 10, 20 or 30 dB!

3. For the Amplifier: At multi-carrier Setup the Backoff power will be 200Watts (53dBm). Please note that on our current setup (single carrier operation) we are transmitting at 50.5 dBm (112W). Is the amplifier capacity is enough for multi-carrier operation?

If both carriers have similar modulation and FEC and go to similar sized antennas at the far end the power needed for the 2.5 MHz carrier will be in proportion to the existing 14 MHz carrier.

Existing carrier: 112W = 50.5 dBm
Second carrier: 112 x 2.5/14 = 20W = 43 dBm
Total power: 112 + 20 = 132W = 51.2 dBm
Note 2.5/14 = 0.17857 = -7.5 dB

You will be operating at 51.2 dBm, so backed off below 53 dBm, so your transmitted intermodulation interference should be acceptable.

Get permission and be talking to the NOC when you bring up the new carrier. Start with the second modulator set at its minimum output in CW mode and check yourself and with the NOC that the spike is in the right place. Turn it off and on to be sure that it really is you. The NOC will allow you to increase slowly and tell you to turn the modulation on. They will talk you to the correct power.  Expect your two modulators to have output levels differing by 7.5 dB.
  

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Re: 2 Carriers on a Single Transmit Antenna
Reply #2 - 15th Aug, 2017 at 9:07am
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Hi Sir,

Well noted on this. So we can use a 2-Way passive splitter considering the loss.
As of this time we are waiting for the transmission plan from the satellite operator.

Thank you so much for the help.  Smiley

Best Regards,
Tristan
  
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