Are phase-locked loop (PLL) LNB’s a good idea?

A PLL-based LNB with the telltale cooling fins

A PLL-based LNB with the telltale cooling fins

It seems that the new rage in C-band LNB’s are the PLL (phase-locked loop) models, which are made in China and imported into North America. They can typically be identified by the presence of a heat sink (cooling fins) on the side of the LNB.

There have been various reviews on them but most tend to be positive. They seem to perform on about the same level as some of the older LNB’s they replaced, which are sometimes referred to as DRO (dielectric resonator oscillator) models. I am talking here about single-piece LNB-feedhorn combination units that use voltage to switch the polarity, not the much older LNB’s that bolted onto a separate feedhorn, and that utilized a small motor to switch polarity.

However I have noted that some of the positive reviews for the PLL-based LNB’s seem to come from people who are trying to use very small (for C-band) dishes of perhaps four to six feet in diameter. They will sometimes also use a conical scaler ring, particularly with a four foot dish, and aim it at a very strong C-band signal in the hopes of getting at least a few of the strongest stations. Sometimes this actually works, at least when atmospheric conditions are right, in other words when it’s not pouring rain. I personally would not care to rely on such a small dish for C-band, but if you live in a place where a larger dish might attract unwanted attention from the wrong people, I can see the appeal of at least trying to get something on C-band.

If you have a larger dish, and particularly if it’s a ten foot diameter dish or larger, you may want to think twice about using a PLL-based LNB. The reason is that phase-locked loop circuity is subject to saturation issues. This means that if you are locking onto a very strong signal, you could potentially have problems because the signal is a little too strong. And with a very strong signal, you probably don’t really need the extra stability offered by the PLL circuitry.

I mention this because I know someone who purchased PLL-based LNB’s, and started noticing he was having occasional signal breakups on certain very strong channels (those that maxed out at or near 100% quality readings in TVHeadEnd from time to time). In the case of one particularly strong channel, received on a ten foot dish, he would occasionally see flashes of purple lines through an entire recorded program. But on another, slightly weaker channel on the same satellite, he’d never see that problem.  This did not happen with any degree of consistency; some nights it would never happen at all, and the on other nights it would make an entire recorded program nearly unwatchable.

Eventually he replaced the PLL-based LNB’s on his ten foot dishes with some older units that do not have the PLL circuitry. Without changing the position of the dish at all, he noticed that the signal quality varied a bit, but on some transponders it actually increased by a couple of points. It seems that the particular older model of LNB he used favored the higher C-band frequencies a little more than the lower ones. But he says that the stability seems better; that even on the channels that have slightly lower signal quality numbers, the stability appears to have improved. Since he only did this relatively recently, it’s too early to say definitively that going back to the older LNB’s actually solved all the breakup issues (at least those not attributable to a quirk in TVHeadEnd, where it acts a bit flaky for a day or two any time the server has to be rebooted), but he says so far he hasn’t seen the issues he’s seen in the past.

Obviously, if it’s a saturation issue, than those running these LNB’s on smaller dishes, where the received signal strength would be lower, would be far less likely to experience that problem. And the increased stability of a PLL-based circuit might actually be quite helpful on a very weak signal, much more so than on a strong one.

Of course, that assumes that the PLL-based LNB’s actually are stable. There is a Wiki page entitled “C Band PLL LNB External reference modification”, which says this:

Since being consumer products these LNBs provide only moderate frequency accuracy and stability. Below we will describe how to modify these LNBs to achieve superior accuracy and stability with relatively little effort.

And then that page shows how to modify these LNB’s by feeding 25 MHz from a stable frequency source into the LNB. Two different methods are shown, but unless you have some experience with building and modifying electronic equipment, you’re probably not going to want to use either one. In this case, “relatively little effort” is quite relative; if you don’t know which end of a soldering iron to hold then I doubt you’ll want to try the suggestions shown.

EDIT: One other potential problem that has been noticed in cold weather climates is that the performance of these devices can fall off when it gets cold outside, which means that if you live in the northern part of the USA or Canada (you know, those places where icy roads are common in the winter months), you may find that some transponders that can be received with no issues from late spring through mid-autumn suddenly have breakups in reception from late autumn through mid-spring.  I  don’t know if this is a problem common to all such devices, or only to a subset that for some reason is especially sensitive to temperature.  I will also note that I once experienced a similar issue with a non-PLL LNB, although in that case the LNB just stopped working altogether when it got below about 40 degrees Fahrenheit.  The presence of the cooling fins on the PLL-based LNB’s make me suspect that the designers were more concerned about the effects of overheating than of frigid winter temperatures. (End of edit.)

My opinion is that if you have adequate signal strength, you may find that an older style, non-PLL LNB works just as well for you, and maybe even better in the sense that you might experience fewer “glitches” in your video and audio. However, if you are dealing with a weak signal situation, and have taken extra time to aim the dish correctly, check the skew and focal point of the LNB, and do any other tweaks that might improve the signal and you still can’t quite get a high enough signal quality reading for stable reception, then a PLL-based LNB might help. However, a larger dish would almost certainly help a lot more, if you are able to get one and put it up.

But that said, I would not discourage anyone from trying both types to see what works best for you. In your particular situation, with your dish, you may find that one or the other type performs better. If you have one of the older style LNB’s and it is working well for you, and especially if you have good signal quality readings, I would not rush out and buy a PLL-based LNB to replace it. But on the other hand, if you have already purchased a PLL-based LNB and you are not having any problems with it, then be happy and, as the Beatles sang, “let it be”!

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