Author: Anthony Dunnett
DRO and PLL – Say what? Terms perhaps we take for granted without understanding the real difference between these types of LNBFs. DRO stands for Dielectric Resonator Oscillator and PLL stands for Phased Lock Loop. It is a known fact that Phase lock loop PLL LNBF’s are more stable than DROs, as they use a more stable internal reference source by using crystal oscillators
They kind of look the same
Up to now all domestic LNBFs used for satellite TV reception use dielectric resonator stabilized local oscillators. The DRO resonates at the required frequency and is very cheap to produce. Compared with quartz crystal a DRO is relatively unstable with temperature and frequency accuracies may vary by +/- 250 kHz to as much as +/- 2 MHz
Compared with quartz crystal a DRO is relatively unstable with temperature however these have been mass produced by the millions over the last twenty years.
Originally designed for very low digital data services, the Phase lock loop down converter was standard equipment for Telco’s.
Up to now most multiple channel per carrier (MCPC) TV carriers use wide bandwidths 36, to 18 MHz so even with a 2 MHz error, the indoor receiver will successfully tune the carrier and capture it within the automatic frequency control capture range.
However the increasing use of single channel per carrier “SCPC” broadcasts where many narrow carriers are squeezed into a half or full transponder.
Good phase noise performance is desirable but not a must for the reception of low bit rate digital carriers and for digital carriers using high spectral efficiency modulation methods like 8-PSK, which reduce the bandwidth required but need more power from the satellite.
PLL LNBFs 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. If you want an LNBF for the reception of narrow carriers then the latest generation of Phase Lock Loop (PLL LNBF) provided added frequency stability over the Dielectric Resonator Oscillator type.
For DTH (direct to home) strength signals the PLL is an overkill, satellite receivers have been operating very successfully using DRO LNBF’s since the launch of Ku DTH services in New Zealand and Australia as far back as Pam Am Sat 2 launched in 1994.
There is no doubt that the domestic version Phase Lock Loop LNBF’s provide the same type of stability that broadcasters have relied upon for stable reception of “Feeds and “back hauls” using as little as 6MHz of bandwidth and symbol rates as low as 1,500 to 2000 M/s .
While the PLL domestic LNBF might appeal to the enthusiasts and feed hunters it is worth remembering that bandwidth and power restricted feeds also require a suitably sized receive dish to provide above threshold signals for services that could be 10 Db or more lower than the published signal strengths for specific transponders.
After all a 0.2 db noise figure will provide the same gain whether the LNBF be DRO or PLL. By the nature of LNBF manufacture the published noise figure will be located at a frequency somewhere within the 12.250 to 12.750. incoming signal when converted to L band. It is not normal for the published noise figure to be across all incoming frequencies .
So for the average free view installation Dielectric Resonator Oscillator type LNBF’s will be around for some time to come.