how to limit LNA input to be <0dbm (choosing pin diode)

[mmohdnor]

pin diode limiter

Hi, can somebody help me?

I would like to limit the RF power coming into my LNA input. The maximum input it can receive is 0 dBm (1mW). I want to protect it from a nearby RF signal that is 20 to 30dBm (1 watt).

The operating freq range is 430-450mhz.

I'm not familiar with choosing a limiter before. (ppl said i have to add one, so here i am)
All I know is that it's going to be a dual pin diode.
So how do i choose the right one for it? What parameter should I look for in these diodes?

What value of forward voltage pin diodes should i choose in order to protect it from 20 to 30dbm signal? What about power dissipation?

thanks in advance.

 

[melc]

pin diode limiter design

You are trying a weird thing. Protecting 1mW input from 1W signal !!!
You need resistive attenuators dimensioned for 1-2W and better you need to know what are you doing...
:lol:

 

[Mazz]

pin diode receiver limiters

Could you please explain better the problem?
What do you mean with "nearby"?

Before taking into account the protection, you should care about desensitization of the LNA.

Take a great care.

Mazz

 

[mmohdnor]

lna limiter

Hi guys, thanks for taking the time.

here's the path for my receiver :
Antennna > Limiter > Pre-selector Filter > LNA input

(initially i put the limiter after the filter)

my transmitter is transmitting at 433.92mhz (ISM band) with a max of 13dbm. And there's another RF transmitting at 430mhz with a max 30dbm.

My receiver and the 30dbm transmitter is on the same board, so i need to protect my LNA input which can only accept 0dbm max.

ppl r saying to me that i need to put a limiter which is just a dual fast switching pin diodes. i dunno how to select this limiter that can limit power coming from the antenna (which might be the 30dbm power at 430mhz i mentioned)

* the pre-selector is already centered at 433.92mhz with a passband of 480khz ...

Mazz,
what do u mean by de-sense? sorry, newbie here.

"nearby" as in there's another signal ~4mhz away

 

[vfone]

diode 0dbm

If your transmission on 430MHz is at different time than the receiving on 433.92MHz, and if you use the same antenna for both RX/TX, the only thing that you need is an antenna switch SPDT.
You can make one of these using PIN diodes:
https://www.microsemi.com/literature/pinhandbook.pdf

 

[Mazz]

design pin diode limiter

If your tramission is at same time of reception, you have no chances.
The TX signal will be an interferer for your LNA, and the required IP3 will be veeeeeery high, so you'll be desensitized. The intermodulation product reduce the sensitivity of your receiver.
I hope it is clear.
vfone made the right suggestion.
Mazz

 

[mmohdnor]

pin diode application

thanks a lot vfone & mazz,

seems like this is what I'm looking for ... maybe i can grab something from the pinhandbook ...let me browse thru the handbook a little bit

by the way, the antenna for my band is different (e.g not sharing the same antenna) ....

few more infos that might help u guys understand better of my design :
- the receiver is actually a superheterodyne receiver with a low side injection.
- IF filter is centered at 10.7mhz and a passband of 280khz
- the IP3 of my LNA input is -7dbm
- the IP3 of mixer input is -18dbm
- using Maxim MAX1473 super-het receiver

** i'm actually quite new to super-het receivers ...

thanks again

Added after 6 minutes:

oo yeah ...
recently i found something might be useful to others having the same problem (e.g : choosing limiter or pin diode)

an application note from Skyworks Inc.
APN1018PIN : Limiter Diodes in Receiver Protectors
h**p://www.skyworksinc.com/products_display_item.asp?did=5501

 

[vfone]

lna fast switching

Unfortunately the application note from Skyworks it has a few mistakes, like many other app notes from this company. Seems that they have a lack of qualified application engineers to write their notes.

If in your design you are using different antennas for RX and TX, this is an advantage because the isolation between antennas it helps to protect your LNA, and you need less attenuation in front of it.

 

[7rots51]

pin diode to limit rf input power

If you use circulator you can have TX and TX at the same time without any problem for LNA of RX block.

 

[mmohdnor]

rx input limiter

Unfortunately the application note from Skyworks it has a few mistakes, like many other app notes from this company. Seems that they have a lack of qualified application engineers to write their notes.

If in your design you are using different antennas for RX and TX, this is an advantage because the isolation between antennas it helps to protect your LNA, and you need less attenuation in front of it.

what kind of mistakes are there in the app notes do u mean ?