I read an article recently comparing the benefits of “mid-stage” amplifiers with conventional amplifiers using input level control attenuators. It was an article penned by a well-known distributor and was less than honest in its appraisal of the benefits of the “mid-stage” design without showing the negatives as well. The conclusions were as expected extolling only his version of the facts.

You wouldn’t expect the MATV industry to be a follower of fashion; in fact most of the principle players are beyond their 50’s heading fast towards the cardigan and slippers era.

Yet there seems a definite fashion swing towards launch amplifiers with built-in “mid-stage” level control. “Mid-stage” attenuation can have benefits but it is not the only way to control amplified signals and can actually degrade the possible signal quality available. It can also introduce the danger of cross-modulation and inter-modulation components if the uncontrolled pre-amplifier is overloaded. Turning the “mid-stage” attenuator will not reduce the input signal to a manageable level.

Most supporters of the built-in pre-amplifier/ “mid-stage” attenuator design claim it is the only way to amplify DTT signals and that other methods are inferior. This is evidently misleading.

The principle of pre-amplifying the incoming signal before setting the launch output level required is not disputed. The question is, where is the best place for the pre-amplifier?

Amplifiers work best when running near maximum where their inherent noise figure has least effect on the amplified signal.

A pre-amplifier built-in to the launch amplifier might be convenient, but normally the MATV head-end is some distance from the receiving aerials so the down-lead cable attenuates the incoming signal before reaching the pre-amplifier (see Fig.1). This consequently reduces the available carrier to noise ratio unnecessarily before the signal is pre-amplified.

The reason for their support of this method is that in-line variable attenuators reduce the carrier to noise ratio of the required signal as the amount of attenuation increases.

So in conventional designs where the attenuator is at the input to the amplifier, there is a risk of degrading the carrier to noise ratio of the incoming signal. Their solution is to increase the signal level before control. There is no dispute about this. Increasing the signal level before control will reduce carrier to noise problems.

But why position the pre-amplifier at the bottom of the down-lead cable in the first place? The best place surely, for the pre-amplifier is at the top of the mast next to the aerial as soon as the signal is received and where the carrier to noise ratio is at its best (see Fig. 2).The message is “amplify the signal before you lose it, not afterwards”.

Using a high quality screened masthead pre-amplifier with high output capability before the down-lead cable losses are incurred will give better results than putting the pre-amplifier in the same casing as the launch amplifier. Using a masthead amplifier has several benefits.
1. Pre-amplifies the signal before down-lead losses.
2. Installer chooses the level of pre-amplifier to suit the job.
3. Reduces the amount of gain in one box.
4. Generally cheaper for a better result!

So you see, “mid-stage” pre-amplifiers are not new. A good masthead amplifier and conventional launch amplifier with input level control forms the equivalent in technique to a “mid-stage” design and will work better in most cases.

The important measurement is not to arrive at the input of the launch amplifier with too much signal. So a launch amplifier with a 120dBuV maximum output (subject to the number of channels) and 40dB gain cannot have more than 80dBuV of signal input. At least with a masthead amplifier, we can choose one with the right amount of gain to achieve this required level.

After all it’s only a fashion!