Syllabus Sections:-

RF. amplifier and pre-amplifier

4l.1 Recall the operation of the RF amplifier.

Understand that external RF preamplifiers do not always improve overall performance and will reduce the dynamic range by an amount approximately equal to the gain of the preamp.

Understand that overloading will cause intermodulation and spurious signals.

Operation

The operation of the RF amplifier is to increase the RF voltage of the input signal from a few micro volts to say milli volts.

External R.F. preamplifiers

External r.f. preamplifiers do not always improve overall performance as they can over drive the RF Amplifier or RF stage - the higher levels signals produce the harmonics and distortion.

The effect is to reduce the dynamic range equal to the gain of the pre-amp. Thus it is often better to have a mixer and RF preamp stage and keep the dynamic range.

Filters

We will need to have either a band pass or tuned band pass filter such as a parallel tuned circuit in front of the RF amplifier so that only the wanted frequency is passed through.

Overloading by nearby broadcast signals

If you had a link coupling from your aerial into a tuned circuit that then would go onto the first RF amplifier or mixer as shown below.

You could have the wanted Amateur Radio signal and also the local broadcast station present on the link winding and the first tuned circuit.

To limit the broadcast station signal you have to make sure that the coupling is such that its signal is a minimum or you may need a second RF tuned circuit.

Dynamic Range

If you have a dynamic range of 70dB that is the range between the weakest signal your receiver can handle and the maximum signal - if you add an RF amplifier of 6dB gain then that original dynamic range could be reduced by 6dB and the performance is degraded and this is why you see transceivers with a switch to switch in or out the amplifier.


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