Tech Talk – D.R.O vs P.L.L LNBF

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.

MCPC 27MHZ Multiple channels encoded, sent up on a single carrier to satellite, sent back down to earth and unencoded for us to watch

However the increasing use of single channel per carrier “SCPC” broadcasts where many narrow carriers are squeezed into a half or full transponder.

SCPC -8Mhz Single channel encoded – up to satellite and down again

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.

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