reflective/obsorptive switch P1db

[7rots51]

hello

It seems obsorptive SPDT switches ICs that are available have lower P1db(about +25dbm) than SPDT reflective switches(about +40dbm or more),How I can switch an antenna between Tx/Rx parts of a transciever using SPDT reflective switch/switches? (assume that My TX power is 5W).
(f= 433MHZ)

Reflective switch RF1 and RF2 ports are short(~0 ohm impedance to GND) at OFF state.

 

[PaulHolland]

Hi look at www.hittite.com they have switched that can do 10watt @ 433 Mhz I used it and they worked perfect. (reflective type). Refelective types are open ports when not used. Absorptive types have an internal 50 ohm load (very small). See: HMC484MS8G

 

[7rots51]

PaulHolland:
[/quote] Refelective types are open ports when not used [/quote]

The hittite switch HMC174MS8 is reflective switch,In datasheet page 1 hittite says that it is short to ground when switch is in off state,so the switch is not OPEN at OFF state ,as you said?!what is your opinion?

I have HMC174MS8 from hittite and want to use it for ANTENNA switch(for half duplex operation at output of TX/RX RF module),How can I do it? How to terminate RX and TX sections when switch is in OFF state?

 

[biff44]

Most of these class of switches have series Fets at the input junction, and some shunt fets along either of the two output paths. Obviously, when one arm of the switch is set into isolation mode, the shunt fets are biased on (short circuit to ground drain to source).

But what is your point? Your transmitter should not care if it is looking into an open or a short as a load, since either will totally reflect power back to the transmitter. If your transmitter is sensitive to a non-matched load, either an open or a short might blow it up.

Most people are carefull to turn off the RF output on the transmitter BEFORE they switch the T/R output switch from insertion loss to isolation state. This protects the transmitter, AND protects the switch (which often can not handle the full RF power while it is transitioning between states). One way to do this is to put A spst switch before the transmitter input, and activate it with the proper time delays so the output SP2T switch is never transitioning under full RF power.

 

[PaulHolland]

I believe I was called .

I have seen them also with open and not shorts as in the datasheet. BTW: it does not say short to ground but I assume that is what they want to make clear. For your transmitter or receiver their is no differance between open or short, the only difference maybe is issolation you get from short !.

Your switch is simply between your TX, RX and ANTENNA. All should be matched at 50 Ohm and coupled with a capacitor to your switch. I know the HMC484MS8G works below 200 MHz since I have had contact with Hittite some time ago and they tested this for us, completely with test report etc..

You can also look at www.skyworksinc.com they also have nice switches, lower loss even but not that high power (maybe that has changed by now)

Also check your issolation of the switch since you do not want to blow-up your front-end.. You can also use a circulator instead of a switch !.

With the switches you have to be carefull that your VSWR is not to bad otherwise your switch will be blow-up. Also switch when TX is OFF ! ..

Paul.