RF input, output question

[nec4b]

If you have a RF transceiver with 1 antenna, how do you prevent that transmiter circuit does not interfier with the receiver circuit? For lower frequency circuits i guess i could use JFET as switch. I know that there exists an element called a "circulator" that allows signal to travel only in one way. Can you get such element in electronic shop?

best regards

 

[vfone]

If your transceiver is half-duplex (never transmit when receiving) you can use a switch, which is the best option.
If your system is full-duplex (transmit and receive in the same time) you can use a duplexer or a circulator.
Circulators you can find here:
**broken link removed**

 

[flatulent]

One further item on the full duplex is that the two frequencies are usually separated by a large amount. You have band pass filters at the transmitter output and the receiver input. These are frequently designed with some zeros at the frequency used by the other half of the system. RX filter will have notches at TX frequency for example.

 

[nec4b]

Thanks for your help. I'm thinking of half duplex operation. Does it matter what kind of switch do I use (relay, single transistor, double transistor switch), regarding the noise?

 

[vfone]

Choosing the type of the RF switch depends by power, insertion loss, switching time, frequency, linearity, and noise. In general at medium powers the most used is FET or BJT type, and for higher power handling PIN diodes.

 

[Lecroy2607]

You can get circulators in RF surplus shop. Be carreful they are specified for a defined frequency. I never see circulator for frequencies below VHF.
If the output power is high you will probably need pin diodes in the receiver
chain because there will be still enough power for the front-end.

There is a good application note about circulator and isolator in Philips semiconductor website.

 

[Frank_H]

You can use a Diplexer filter. One pass band for the transmitting freq. and one pass band for the receiving freq.

regards.