PLL vs DRO LNB - Which is better?

LNB's, every satellite TV system uses them. Universal, Ku band, C band, Ka band, single output, dual output, four outputs or even eight outputs, there are lnb's for every need. But essentially, little has changed in lnb technology for almost twenty years. 

One new development has materialized however, the massification of PLL (Phase Locked Loop) oscillator technology. Before we go further let me quickly explain about this oscillator stuff.

LNB's are electronic devices whose purpose is to amplify and convert the extremely weak and extremely high frequency satellite signals into stronger and lower frequency signals capable of being carried over a coaxial cable to the satellite receiver inside your home. To convert a frequency to another frequency an oscillator circuit is required.

Most lnb's use what is called DRO (Dielectric Resonator Oscillator) oscillator technology. And PLL oscillators in lnb's are nothing new, but until about five years ago use of PLL controlled oscillators was restricted to professional use (and expensive) LNB's. This has changed, and now PLL technology can be found on many low cost mass market lnb's. But is it worthwhile? Let's compare both technologies.

DRO technology lnb

Conventional LNB's use dielectric resonator oscillators (DRO) to generate the local oscillator frequencies necessary to down-convert the satellite signal (Ku, Ka or C band) to the IF band used by satellite receivers. These DRO's use pill-like ceramic components glued to the lnb board as part of the oscillator circuit. If the lnb is dual band it will have two DRO's, one for each band. The DRO oscillators are tuned when the lnb is assembled but they are affected by temperature and can drift in frequency. In a mass market lnb, this frequency drift (up or down) can be as high as 3Mhz.

PLL technology lnb

Unlike conventional DRO lnb's, PLL lnb's use a digital (PLL) circuit to generate the local oscillator frequencies needed to down-convert the satellite signal (Ku, Ka or C band) to the IF band. An lnb down-converter PLL circuit is comprised of a chip (IC) and a crystal oscillator. The IC generates the oscillator frequencies from the crystal and then mixes, down-converts and amplifies the resulting IF signal. Commercial mass market PLL lnb's have local oscillator frequency drift from 300Khz to 500Khz.

Benefits of PLL lnb

The main benefits of PLL oscillator technology are frequency accuracy and stability. Since a PLL oscillator is much more accurate than traditional dielectric resonator oscillators (DRO), a receiver connected to an PLL lnb can lock (tune) to a particular signal faster. And while a DRO frequency may drift +/- 3Mhz with temperature, a mass market PLL lnb will usually reduce this drift to a much smaller 300Khz. That's ten times less! 

Local oscillator frequency stability can be an issue when receiving signals. This is specially true when receiving narrow bandwidth low bitrate digital satellite broadcasts. Receivers struggle more to tune (lock) to such transponders, specially ones that use DVB-S2 and low FEC rates. 

For example, assuming your receiver accepts bitrates as low as 1Mbit/s, reliably locking to a 1Mbit bitrate transponder may prove difficult or impossible with a "traditional" DRO lnb. If the receiver is even able to lock to such a signal (probably only after a few good seconds) it will keep loosing signal lock. A PLL lnb will enable the receiver to tune and lock to such a signal reliably, provided the signal is strong enough.

Another benefit of PLL technology is size. Since PLL circuits don't use the relatively big dielectric resonators, the down-conversion circuitry is greatly simplified and much smaller, leading to smaller and lightweight lnb's.

Is it worthwhile to exchange a DRO lnb for a PLL lnb?

It can be, but not always. A PLL lnb is not necessarily better than a DRO Lnb. There are excellent DRO lnb's. You must be aware that other factors besides oscillator stability affect signal quality. For example, it can happen that you install a more stable PLL lnb but its frequency response is not as good as the previous DRO lnb. If you are having trouble receiving low bitrate transponders (like SCPC) then, a PLL lnb could indeed improve reception. However, if all the channels you are interested in are being broadcast in wide bandwidth transponders with high symbol rates (for example 15Mbits or more), then likely little improvement would be gained with a switch to a PLL lnb.

If you would like to share your experience with PLL lnb's please leave a comment!