Preamplifiers and Pre-selectors: Problems Encountered and How to Solve Them

Preamplifiers and pre-selectors serve different functionalities; however, they both serve an overlapping function so are considered to be always together. Preamplifiers are a wideband RF amplifier that is placed between the antenna and the receiver. In other cases, these are positioned at the antenna and use to combat transmission line losses. In some, it’s mounted right at the receiver antenna input.

A pre-selector – HF, UHF and VHF pre-selector are just some examples – is a tuned circuit that goes through the needed frequencies. As the name implies, a pre-selector selects RF signals that will be used to the receiver input beforehand.

Pre-selectors can or may not be amplified. There are pre-selectors that offer both active and inactive features depending on the switch settings. Basic pre-selector installation is right at the antenna terminals of the receiver, or through a small coaxial cable to allow operator access.

The Problems Preamplifiers and Pre-selectors Often Have

Preamplifiers and pre-selectors that are amplified have a latent attraction with one another due to them making the receiver appear more sensitive. But, there are issues that can cause receiver performance to decline when amplifiers are utilized ahead of the receiver antenna input.

One issue that may happen is that the amplified signal may place the receiver into a distorted zone of its operating curve, particularly above its regular maximum allowable signal levels. In the event that this takes place, desensitization or intermodulation may occur. Receiver performance is significantly damaged when this happens.

Another issue that’s likely to happen is when the amplifier increases the level of both the noise and the signal equally. If there are no enhancements in the signal-to-noise ratio (SNR) when both the preamplifier and pre-selector are placed, then there’s nothing to be gained when using the device.

Related to the above presented SNR problem is the fact that every amplifier has the power to create noise. This noise is visible in later stages (or the receiver) as a validated signal and rightly amplified along with the normal signals. If noise levels produced by the external amplifier are too much, then the overall SNR of the receiver declines.

System noise figures are controlled by the first stage in the cascade line. If the additional amplifier holds a noise figure that is significantly lower than receiver noise figures, then improvements may happen as a result. Otherwise, there will either be no changes or a noise situation decline.

What to do in the event these problems happen?

Use a widespread pre-amplifier if the weak receiver signal performance is bad on all major portions of the available frequencies and when there are no strong local signal sources ahead of the receiver. The chosen preamplifier should hold a noise level that’s lower than the receiver. Furthermore, the gain should be lesser as is required to finish the task. Utilizing larger gains may lead to receiver performance decline.

Pre-selectors, may it be HF, UHF or VHF pre-selector, are useful when you require the help with the use of less frequencies, or on a particular band. They may need tuning, which can cause a problem for some; however, they provide enhanced weak signal performance in crowded bands or where there is a strong local signal preventing the entry of weaker signals.