Power limiter purpose in a receiver

Question

let's consider the following blocks diagram for a receiver of a pulse RADAR (but I suppose my question regards many kinds of receivers)

My book says that T/R Lim means a power limiter which is inserted after the duplexer (which is after the antenna) in order not to burn the receiver.

Well, my simple questions are:

1. Is such a power limiter realized with an attenuator (e.g. a voltage divider, or a microwave resistive attenuator etc)? Simply put, is it a resistive attenuator or a voltage/current limiter (e.g. diodes)?

2. If it is an attenuator, how does it affect the SNR ratio? Does it reduce both noise and signal powers?

3. If it is an attenuator, why should we attenuate and then amplify with LNA? Why can't we attenuate less (just less than the maximum power the system may work with) and save space and money by avoiding the Low Noise Amplifier?

Answer 1

Is such a power limiter realized with an attenuator

Usually no, as an attenuator would attenuate all signals so that means your receiver would become less sensitive. You actually want a "smart" attenuator that doesn't attenuate small signals but that does attenuate large signals.

Note that the large (harmfull) signals aren't the ones that you're interested in so there's no need to attenuate these large signals properly in the sense that the shape of the signal cannot be influenced. You just want to avoid that these signals damage the receiver so you can simply clamp them (limit their amplitude).

If it is an attenuator, how does it affect the SNR ratio? ...

Yes indeed, that is exactly the issue why you would not want to use an attenuator.

No you can't remove the LNA because without the LNA, how are you going to receive small signals?

The simplest way that this power limiter can be implemented is by using anti-parallel diodes. When the signals are small, the diodes will not conduct and barely affect the signal. For larger signals (> 0.6 V for silicon diodes) the diodes start to conduct and simply clamp the signal and protect the LNA. See also this question.