Frequency math, or don’t touch my local oscillator

Satellite engineers have a very poor showing on the Internet. I started writing about satellite stuff because of how difficult it has been for me to find reliable information. I’ve had to do a lot of learning by doing. Perhaps other satellite engineers enjoy their business being a black art, but like locksmiths and security alarm salesmen, they’re just waiting for the light of responsible disclosure to toss their world upside down.

And that brings me to a trend I have noticed in the satellite Internet business. There are two camps: radio guys who somehow configure a router, and IP guys who somehow align an antenna. Usually the IP guys do better in business than the radio guys. Perhaps this is because radio guys tend to be former military and unused to competition. Perhaps this is because they think all you have to do is deliver a carrier and everyone is happy. But the customer doesn’t care about Hertz, he cares about bits. And really, the space link is still a Layer 1 problem, and the harder work is above that.

On with revealing a little black magic.

Satellite systems communicate over a variety of frequencies. Modems and other terrestrial equipment commonly communicate between each other in one frequency range, yet the actual spacelink communication happens in another frequency range entirely.

The frequency bands I most commonly deal in my job managing satellite Internet services are (band, receive range, transmit range):

  • L-band, Rx 950-1450 MHz, Tx 1250-1750 MHz
  • C-band, Rx 3625-4200 MHz, Tx 5850-6425 MHz
  • Ku-band, Rx 10.95-12.75 GHz, Tx 13.75-14.5 GHz

There are a lot of ways to get confused here. The definitions of these bands varies from region to region. There are a lot of confusing terms like “standard Ku” and “extended Ku” despite there being overlap between the two ranges they define. The C-band I define above is actually called “extended C-band”, yet there is no “standard C band” as far as I can tell. And the Wikipedia articles on these things are a mess.

Satellite communication is normally a one-way process. Internet communication is by nature two-way, but from a satellite perspective we can regard it as two one-way processes. * So we’ll analyse it from that perspective.

During the entire one-way process, the physical communication signal changes frequencies four times. This is done to avoid radio interference at the satellite and ground antennas, and to avoid electrical resistance when communicating over copper.

Assuming Ku-band communication with the satellite, the devices that do the translating are:

  1. BUC, or Block Up-Converter, which steps a carrier up from Tx L-band to Tx Ku-band
  2. The frequency translator at the transponder on board the satellite, which steps a carrier down from the Tx Ku-band to the Rx Ku-band for retransmission to Earth
  3. LNB, or Low-Noise Block down-converter, which steps a carrier down from Rx Ku-band to Rx L-band

These devices work by using a fixed translation amount, called the LO, for Local Oscillator. If you’ve guessed this is because an actual fixed oscillating clock is used, you are correct! It is very important to know what the LO value is, or you don’t know what frequency your transmission will become.

Let’s say a modem starts transmitting at 1131 MHz over a copper wire. This signal reaches a BUC with an LO of 12800 MHz. 1131 + 12800 = 13931 MHz, which is then transmitted into space as radio. The satellite receives 13931 MHz, where the transponder’s translator has an LO too. Here, it is 1260 MHz, and 13931 – 1260 = 12671 MHz, so 12671 MHz is then transmitted back to Earth as radio. ** Finally this 12671 MHz reaches the customer’s LNB with an LO of 11300 MHz. 12671 – 11300 = 1371 MHz, which is what reaches the customer’s modem.

In summary:

  1. 1131 MHz (modem)
  2. 1131 + 12800 = 13931 MHz (BUC)
  3. 13931 – 1260 = 12671 MHz (transponder translation)
  4. 12671 – 11300 = 1371 MHz (LNB)

The process repeats itself when the customer transmits in return, using whatever BUC and LNB are installed in the opposite direction. It’s common for both BUCs to have the same LO, and both LNBs, which makes the math easy for me. Each group of transponders on a satellite has its own transponder translation frequency, and these are different between C-band, Ku-band, K-band, and so on. The average technical user guide for the satellites Talia operates on is 35 pages. Read carefully.

I have a large spreadsheet tracking frequency translations like this. When you call a satellite operator to complain about a problem, they want the downlink frequency (12671 MHz in the example above). They certainly don’t know or care what your L-band frequencies are, because they can be different depending on the LO of your equipment!

A spectrum analyser is a common tool for observing satellite carriers, checking for interference, and measuring transmission power. Spectrum analysers commonly operate in L-band. This means that you must spot a problem in L-band and then do the math to find the downlink frequency before reporting a problem to the satellite operator. It seems to me that internally calculating the translation values would be a simple feature of a spectrum analyser, but most cheap ones (under $10,000 USD) don’t offer this. You’d need to define profiles for your different hardware/satellite configurations, but anyway it’s something that an IP engineer would think to add and a radio engineer just expects they’ll have to do manually.

I have yet to find even a single open source package to manage frequency plans or carrier assignments, by even the simplest methods. My spreadsheet is the most advanced free tool I have for this.


* Sometimes satellite Internet services are one-way. The download link is via satellite, and the upload link via terrestrial modem. But that method is unpopular today.

** I find it interesting that satellites themselves receive in the Tx range and transmit in the Rx range.

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  1. Craig’s avatar

    A very interesting read indeed.

    As they say, learn something new each day.

    If you need a helping hand writing some software feel free to drop me the specs/idea. Might be able to help.

    Reply

  2. senthil’s avatar

    Very interesting

    But one clarification : whether Downlink link frequency is related to Transponder frequency for both Ku & c- band
    ex: For c-band Downlink link 3625-4200 MHz Transponder frequency 3000-4200 MHz

    Reply

  3. tyler’s avatar

    Senthil, I don’t understand your question. What are you asking?

    Reply

  4. Karl’s avatar

    Thanks for the post,
    One transponder program is http://www.gunamoi.com.au
    But a nice program to manage transpoders freq, would be nice with fec codec and that kind of stuf.

    But I think we have to stick to calc and the analyser when we counted wrong…

    kl

    Reply

  5. ABAYNEH’s avatar

    interesting point u explain.please tell us example of different transponder frequency.

    Reply

  6. ABAYNEH’s avatar

    please tell us different transponder frequency.

    Reply

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