An LNB is a combination low-noise amplifier and block down converter. It converts the feeble 3,700 to 4,200 MHz satellite signal into a stronger 950 to 1,450 MHz signal which is easier to send over coaxial cable.
Phased locked loop LNB uses a phased locked loop (PLL) oscillator to make the LNB's frequency output as stable as it can be made at reasonable prices. Both Wegener and Cumulus Media Networks recommend PLL LNBs for use with their satellite receivers.
"Digital ready" LNBs are marketed as being "second best" to PLL. Yes, they are cheaper, but they also have poor stability specifications when compared to PLL LNBs.
What does "noise temperature" mean anyway? A very simplified explanation is as follows: Atoms vibrate in any substance that is above absolute zero in temperature. That means the atoms in the materials in the LNB amplifiers vibrate. These vibrations create "noise". The signal from the satellite is so minuscule, you do not want to add a lot of "noise" to the signal. So you want to find an amplifier with a low "noise temperature".
Typical LNBs are designed for a noise temperature between 20 degrees Kelvin and 80 degrees Kelvin. If you have an LNB at 120 degrees or 150 degrees, it's worth a few hundred dollars to replace it. Keep the old one on the shelf as a spare; it's cheap insurance.