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Low noise block downconverters (LNBs) are essential for reliable satellite communication – but not all LNBs are created equal. Some produce too much phase noise or degrade signal strength. Others stunt data rates or fail in extreme temperatures. How do you know if you’re getting a high quality, high performing LNB for your satellite terminal? Here are 10 things to look for:
1. Phase Noise
LNB phase noise (or phase jitter) is caused by the phase fluctuations of an oscillator – and can seriously impact the performance of your terminal. A low phase noise LNB is critical for minimizing bit error rate (BER), particularly with higher order modulation schemes that squeeze as much data out of the link as possible, such as 32 APSK used in DVB-S2 or up to 128 QAM for some Earth Observation systems.
Phase noise is measured at specific intervals from the carrier frequency – usually starting at 10 Hz and moving up to 1 MHz. Good phase noise LNBs have 10 kHz values of -85 dB-Hz. In other words, the phase noise value is 85 dB below the carrier value when measured 10 kHz away from the carrier frequency.
2. Noise Figure
Noise figure is a measure of the additional noise an LNB adds to a system link. The more noise an LNB adds, the worse the signal strength becomes. Signal quality is typically measured as a carrier-to-noise ratio (CNR, C/No or C/N ratio) – and modems require a minimum C/N ratio to acquire a signal.
There is no such thing as a noise figure of zero. All LNBs add noise due to the electronics in the unit – and the higher the frequency, the higher the noise will be. What is a good LNB noise figure? Look for 1.5 dB for Ka-band frequencies, 0.8 dB for Ku and X-band, and 0.4 dB for C-band.
Signal gain – the extent to which an amplifier boosts the strength of the signal from the antenna – is not usually an issue as most LNBs come with a fixed gain value of about 60 dB. However, an LNB’s gain is important to consider in two key circumstances: when dealing with long interfacility links (IFL) or large, high-gain antennas. With regards to IFL, the longer the cable that connects your outdoor satellite receiver to indoor routers or transmitters, the greater the signal loss. In these cases, an LNB that delivers higher gain is necessary.
With large antennas (e.g. at teleports) too much gain can be an issue. These antennas introduce large signal power to the first stage of an LNB, which can saturate the LNB and cause signal compression. When the signal is compressed, the output power no longer increases with the input power. This creates a non-linear response – which in turn produces signal distortion and harmonics. An LNB that can be customized for gain is critical in these circumstances.
4. Gain Compression Figures
To ensure maximum LNB linearity, it’s important to know the point at which gain compression will begin to occur. Compression is measured in two ways: P1 (1 dB compression point) and IP3 (third-order intercept point).
P1 is the point where the LNB drops 1 dB off its expected output level as the input increases. This compression point measures the beginning of the non-linear region of the LNB. The higher the P1 figure, the better. In Ka, Ku and C-band LNBs, P1 should be at least +10 dBm. In X-band, the figure rises to +15 dBm.
When an LNB becomes non-linear, it will begin to produce harmonics – and harmonics create signal distortion. IP3 is the theoretical power level at which the third-order harmonics equal that of the main carrier. The IP3 value essentially indicates the power level at which third order harmonics become a real issue. When choosing an LNB, look for IP3 levels that are +20 dBm or higher.
5. Temperature Range
Most LNBs are mounted directly on the antenna feed and are therefore subject to wide temperature variations. Survivable and operational temperature variations are often spec’d. From a service perspective, operational temperatures between -40C and +60C should be a minimum. For very hot climates, look for LNBs that can operate at temperatures as high as +90C.
6. Desense Level
Both the transmit and receive signals pass through an orthocoupler on their way to or from the antenna. Orthocouplers are not perfect devices and transmit signals will leak into the receive chain. The desense level is the effect of these transmit signals on an LNB’s noise figure. (More specifically, it is the RF power in the receive chain that will diminish the noise figure by 0.1 dB.) Desense becomes a bigger issue when transmit and receive frequency bands are relatively close together, such as X-band.
When choosing an LNB, think of the desense level as a measure of the effectiveness of the transmit reject filter in the LNB. The better the filter, the less the transmit signal can affect the receive noise figure. Desense levels in the order of -50 dBm indicate very good filters.
The Voltage Standing Wave Ratio (VSWR) measures the impedance mismatch between the antenna feed and the LNB flange. It’s very difficult to get a perfect match as feeds and LNBs are manufactured separately. The mismatch causes some of the receive power from the antenna to reflect back to itself. The smaller the VSWR, the better the match – and the more power is delivered to the LNB.
How do you limit the VSWR? With an isolator. These can be external devices, but good LNBs have internal isolators. To get the most out of your LNB, be on the lookout for this built-in feature.
8. Flat Frequency Response
It’s not good enough to simply have an average gain of 60 dB across the band you’re operating in. You also want the gain to be flat across that frequency band. (“Flat” meaning only minor variations for low to high frequencies.) LNBs with high skews (not flat) will introduce non-linearities into the receive signal and impede data throughput. Gain flatness should be +/- 1.5 dB across the entire band.
Ripple is another specification closely related to band flatness. Ripple is the peak-to-peak variation of the signal over a narrow portion of the band, say 10 MHz. Good ripple specs are in the order of +/- 0.1 dB over any 10 MHz.
9. External Reference vs. Phase-Locked Loop (PLL)
The local oscillator (LO) inside the LNB is responsible for accurately converting the RF signal to L-band. The LO is either locked to an internal 10 MHz reference using a Phase Locked Loop (PLL) – or it is locked to an external 10 MHz reference.
An LO with an internal reference usually has poorer phase noise characteristics than an LO with an external reference. In addition, when crystal oscillators are mounted inside an LNB, they are subjected to large temperature variations that can cause the crystal to drift over time. This is good enough for some applications (such as receive-only video with a large link margin), but high data rates need much better LO performance.
LOs that are locked to an external 10 MHz reference signal deliver this performance and offer superior phase noise and stability. The quality of the external reference matters as well. Like LOs, external references are measured by their phase noise and stability characteristics. Phase noise of -85 dBc/Hz at 10 kHz is very good.
10. Gain Adjustment
LNBs usually come with a fixed gain figure – which works well when system losses are accounted for and the signal that enters the modem is within its range. But often, paper designs differ from what occurs in a field installation. For this reason, LNBs that have a variable gain adjustment are a good choice. Gain adjustments can be set by the manufacturer, and in some cases a Monitor & Control port on the LNB will give the user the ability to make their own adjustments. For flexibility in the field, look for LNB gain adjustments of 20-30 dB.
There is a lot to consider when selecting a low noise block downconverter for your SATCOM terminal – even beyond what is listed here. For example, you may need to factor in other environmental conditions such as high humidity, or man-made conditions such as vibration. All LNB manufacturers provide spec sheets so you can see at a glance what you’re getting. However, these specs are usually averages, so if you need to ensure high performance, find a vendor that will individually test their LNBs and provide the actual figures for each unit. (We do this here at Orbital).
Here’s a cheat sheet for the top 10 things to look for in an LNB:
- Phase noise with 10 KHz values of -85 dBc/Hz
- Noise figures of 1.5 dB for Ka-band, 0.8 dB for Ku and X-band, and 0.4 dB for C-band
- High gain or customized gain (if needed)
- P1 of at least +10 dBm for Ka, Ku and C-band, and +15 dBm for X-band – and IP3 of at least +20 dBm
- Minimum -40C to +60C temperature range
- Desence level of approximately -50 dBm
- Internal isolator
- Gain flatness of +/- 1.5 dB across the entire band, and ripple of =/- 0.1 dB over any 10 MHz
- External reference LO (for most applications)
- Variable gain adjustment of 20-30 dB