Further to JX Sat’s recent article on how LNBF’s operate , it seems that many enthusiasts and installers alike seem to be gauging the LNB, or LNBF’s potential by its output gain figure.
This is not how LNB’s or LNBF’s are graded. The expression low noise refers the quality of the first stage input amplifier transistor.  The quality is measured in units called Noise Temperature, Noise Figure or Noise Factor.   Both Noise Figure and Noise Factor may be converted into Noise Temperature.  

The lower the noise Temperature the better, which means   C band LNBF or LNB with a 13K noise figure should be better than a 17 k .Like wise a Ku LNBF or LNB with a .1 db noise figure should be better than a .3 db.Most LNB’s or LNBF’s publish with in there specifications an impressive output gain figure of 65db or in some cases a massive 70db .whilst these figures look and sound impressive 65db output gain is pretty standard.

This output gain is required to amplify the incoming micro wave signals after processing and output the 500MHz block of L band signals (950 to 1450 or 1750 MHz) to a level where it can be connected via a coaxial cable with differing lengths. Some times 20m or longer cable runs are required before the L Band signal reaches the input connector of the satellite receiver.

The diagram shows the input waveguide which is connected to the collecting feed or horn.  As shown there are vertical and horizontal probes that pass through the wall of the waveguide. These probes function is to collect the incoming vertical or horizontal signals depending on which polarity is selected. 
The satellite signals first go through a band pass filter (which attenuates the signal by -0.5db) which only allows the intended band of microwave frequencies to pass through.  The signals are then amplified by a Low Noise Amplifier (This device determines the over all noise figure of the device) (It also adds about 20 db) before the signal is passed on to the Mixer. 
At the Mixer, all that has come through the band pass filter and amplifier stage is severely scrambled up by a powerful local oscillator signal to generate a wide range of distorted output signals. (A further 12db of gain is introduced) These include additions, subtractions and multiples of the wanted input signals and the local oscillator frequency.  
Amongst the mixer output products are the difference frequencies between the wanted input signal and the local oscillator frequencies. The mixer output is termed an IF signal which is still sitting at a fairly low level. These are the frequencies of interest.  
The second band pass filter selects the desires frequency block (typically 500MHz) Since these frequencies  are  of a specific bandwidth they  can be easily amplified significantly to a level where the output can be passed via a coaxial cable to the satellite receiver .
“Equate this L band amplifier to a modulator .A modulator normally accepts an audio video input. It changes the A/V in an RF frequency and amplifies it; however the amplification is not normally enough to distribute the RF signal around a large cable distribution system so another amplifier (distribution amplifier) is driven by the modulator. By adjusting the RF gain control on the modulator the distribution amplifier is being driven so it can pass a satisfactory amount of signal around the network.” The final output levels are controlled by the distribution amplifier its self and are normally adjustable.
This is the job of the L band amplifier in an LNB. It provides enough signal to over come cable losses, may be DiSEqC switching which will attenuate the signal by up to 4Db and additional splitters attenuating the signal by another 3 Db or so. Add all these losses up and you can see why you need 65 to 70 Db L band output. It is fortunate that today’s modern Digital receivers can operate on inputs of -25 to -65Dbm so the losses need to be very large before the STB will exhibit pixilation.
The final output signal gain of LNB’s does vary depending on the manufacturer. However it can be seen from the above that whether the output is 65 or 70 Db, it makes little or no difference to the overall performance which is governed by the low noise amplifier situated at the front of the LNB not the output amplifier which is the final amplifying device in the chain.
All LNBs used for satellite TV reception use dielectric resonator stabilized local oscillators.  The DRO is just a pellet of material which resonates at the required frequency.   Compared with quartz crystal a DRO is relatively unstable with temperature and frequency accuracies may be +/- 250 kHz to as much as +/- 2 MHz at Ku band.  This variation includes both the initial value plus variations of temperature over the full extremes of the operating range. Fortunately most TV carriers are quite wide bandwidth. Even with 2 MHz error the indoor receiver will successfully tune the carrier and capture it within the automatic frequency control capture range.

If you want the LNB for the reception of narrow carriers, say 50 kHz wide, you have a problem since the indoor receiver may not find the carrier at all or may even find the wrong one.   In which case you need a rather clever receiver that will sweep slowly over a range like +/- 2 MHz searching for the carrier and trying to recognize it before locking on to it.  Alternatively it is possible to buy Phase Lock Loop LNBs which have far better frequency accuracy.  Such PLL LNBs have in internal crystal oscillator or rely on an external 10 MHz reference signal sent up the cable by the indoor receiver.  PLL LNBs are more expensive.
The benefit of using an external reference PLL LNB is that the indoor reference oscillator is easier to maintain at a stable constant temperature.
All modern DRO LNBs are sold as 'digi-ready'.  What this means is that some attention has been paid in the design to keeping the phase noise down so as to facilitate the reception of digital TV carriers.  The phase noise of DRO LNBs is still far worse than for PLL LNBs.   What good phase noise performance is really needed for is for the reception of low bit rate digital carriers and for digital carriers using high spectral efficiency modulation methods like 8-PSK, 8-QAM or 16-QAM modulation, which reduce the bandwidth required but need more power from the satellite, a bigger receive dish and better quality PLL type oscillators in both the transmit and receive chains.

Testing suspected Faulty LNBF’s
1: Check with a current meter that it is drawing DC current from the power supply.  The approx number of milliamps will be given by the manufacturer. Normally up to 300ma
 2: Badly made or corroded F type connections are the most probable cause of faults.  Remember that the centre pin of the F connector plug should stick out about 2mm, proud of the surrounding threaded ring.
3: Use a satellite finder meter.   If you point the LNB up at clear sky (outer space) then the noise temperature contribution from the surroundings will be negligible, so the meter reading will be If you then point the LNB at your hand or towards the ground, which is at a temperature of approx 300K then the noise  reading on the meter should go up,
4: Note that LNBs may fail on one polarization or on one frequency band and that the failure mode may only occur at certain temperatures.
The DC voltage power supply is fed up the cable to the LNB. By altering this voltage it is possible to change the polarization. Voltages are normally 13 volts or 18 volts, it is essential that these voltages switch. Otherwise the LNBF may become stuck on one polarity.

Perfect weatherproofing of the outdoor connector is essential, otherwise corrosion is rapid.  Note that both the inner and outer conductors must make really good electrical contact.   High resistance can cause the LNB to switch permanently into the low voltage state. Giving the effect of only one polarization working. Very peculiar effects can occur if there poor connections or corrosion. The use of weather proof boots help cut down water ingress and preserves the connection integrity