The RF amp stage of the receiver

Hi

The RF amp stage of the receiver , is it's purpose to isolate the locacl osc. from the antenna so that the receiver will not turn into transmitter, or it offers some amplification of the incoming chosen signal , if yes what is the "right " amplification factor of the RF amp stage ?

Thanks
Elico

BarNash said:

Hi

The RF amp stage of the receiver , is it's purpose to isolate the locacl osc. from the antenna so that the receiver will not turn into transmitter, or it offers some amplification of the incoming chosen signal , if yes what is the "right " amplification factor of the RF amp stage ?

Thanks
Elico

Click to expand...

Depends on frequency to be received.
In general, local-oscillator leakage should be reduced by RF pass-band filter. Isolation by RF preamplifier adds to this rejection.
A mixer is often a lossy element, with a conversion loss, so having a low-noise RF preamplifier is preferable.
What is your receive frequency? Is the receiver tuned by local oscillator, and how much?
Designing a good receiver can be a complex task.
You should specify its noise figure, bandwidth and the gain depending on signal modulation, and more.

The meaning that the RF amplifier (LNA) isolate the local oscillator to don't go to the antenna is a consequence of the fact that the LNA is placed where it is placed.
The main reason of existence of the LNA is the amplification, and secondary is the isolation.
Generally the gain of the LNA is about 15dB (+/- few dBs). Higher gain helps for better system noise figure but degrades the system linearity.

Many thanks
Elico

RF amplifier does not stand for a spesific RF block, you should call it LNA. An LNA is designed for only first amplification with low noise as it is called low noise amplifier. İsolation between mixer, oscillator and LNa is just a design goal, so LNA is not for isolation itself. You should investigate Friis formula to understand the concept of LNA. There is no a standart amplification factor for LNA or PA or mixer, all of them are considered as a blok for whole system and contribution to overall noise of the system. RF design is all about SNR. So you should first examine the friis formula and what LNA is for. I can give an example for you to undestand, an LNA with 22 dB gain and 5 dB NF can be useless next to an LNA with 9 dB gain and 1.4 dB NF. you should think at system level first. I hope I ve been helpful, take care!