What is buc and lnb

Conversion gain is a measure of the difference between the power of an input signal and the power of an output signal. Gain is given in dB, and larger numbers are better.

Gain between 50 dB and 60 dB is generally considered sufficient for most satellite communication purposes. In the case of LNBs Low-Noise Block Downconverter , this means amplifying the very weak signals received from a satellite approximately to 1, times so that they are sufficiently strong enough to be deciphered by a modem. In the case of a BUC or SSPA this means amplifying the power that is received into the device by approximately to 1, times in order for the outgoing radio waves to have enough energy to reach their targeted satellites.

As you can imagine, this results in a great deal of energy and explains why it is unwise to stand in the path of a transmitting antenna.

In a perfect universe, gain would always be linear; the power coming out of a system would continue to increase at the same rate as the power going in and higher gain would always be better. Unfortunately, this is not the case. At some point, the output power begins to drop off relative to the input power and trying to increase gain past this point ultimately leads to distortion, saturation and ultimately damage to the device.

A bit more on this topic will be presented later in this article. Gain variation, or gain flatness, is a measurement of the difference in gain across the output frequency of a product and is measured in decibels peak-to-peak dB p-p , either across the entire operating band of the device or over any 40 MHz 40 MHz is generally the bandwidth of a single satellite transponder. This can also be measured across a temperature gradient or over time. The closer this number is to zero, the better, as very low numbers mean you will witness consistent behavior of the device across its operating parameters.

Output power is one of the primary specifications for BUCs and SSPAs, which makes perfect sense as higher wattage products provide higher data rates and higher throughput. The cost of products increases as wattages get higher, and those costs can be quite high indeed for higher power units.

Unfortunately output power is also one of the most unclear specifications for these devices, mainly due to a lack of standards. Sometimes power is specified at the point where there is a 1dB difference between the theoretical linear gain and the actual gain: P1dB. Sometimes power is specified at the point before gain deviates from linear: Plin. There is no easy way to convert the output power at one of these points to output power at a different point. Any time you are comparing BUCs and SSPAs, read the specification sheets carefully to understand what the output power is at these three points.

If this information is not in the spec sheet, ask the vendor to tell you; this is the only way you can be certain you are comparing apples with apples.

Note that power is not specified for LNBs. As the years passed we sold fewer and fewer transceivers and more and more BUCs, so lets take a look at the difference between the two. The VSAT installation has the antenna mounted outside with the transceiver mounted as close to the antenna feed as possible.

The modem and other electronics are connected by cable between the operations center and the antenna. If this cable was required to carry the broadcast frequency of 4 to 6 GHz it would need to be thick and very expensive.

In order to cut down costs satellite engineers devised a system where the modem could communicate with the transceiver using lower frequencies. Intermediate frequencies can be carried over long distances between the antenna and indoor equipment using cheap cables.

To illustrate the difference between the two technologies let us first review the 70 MHz modem and transceiver combination, using C-Band as my example, but apart from the frequency range everything remains true for KU band as well. The receive frequency of 4 GHz is collected by the antenna and fed to an LNA and then passed to the receive port of the transceiver.

The Local Oscillator is what drives the mixer and allows the device to perform the frequency conversions. In the case of 7. BUCs must have the LO frequencies stabilized by a 10MHz reference frequency that is overlaid on the signal and transmitted through the signal cable.

This reference frequency must be accurate and with low phase noise. It is wise to check that the signal is clean and has not been contaminated by some local radio transmission. A very long cable run can cause an erratic BUC current, when this happens consider putting the DC supply close to the BUC and use a Bias Tee to inject the voltage into the signal cable.

There can also be problems for the LNB, as the return outer conductor drops varying voltages and these are superimposed on the LNB supply volts. The BUC and LNB are often partially connected to one another and to the earth ground at the antenna, so there is scope for strange voltages. Lightning and safety rules come first and the consequences of this may mean that you need an extra thick earth ground cable between the antenna and indoors equipment.

Note that 70 MHz transceivers can only transmit and receive over a small part of the satellite bandwidth, if it was required to transmit carriers MHz apart it is necessary to use two up-converters. Alternatively you can get transceivers with MHz center frequency for a much wider range of transmission frequencies. If you get a transceiver make sure you understand how the frequencies are calculated.