High power T/R switch - RF Cafe Forums

 Post subject: High power T/R switch

Posted: Mon Apr 10, 2006 12:23 pm 

Hello,

I am currently working a job which requires characterizing a high power transmit / receive switch for the HF band (1MHz to 30 MHz). It is all solid state with a blocking voltage of greater than 500 V on the diodes (power exceeds 1KW) when the receive path is off. Because of this high voltage, I am hesitant to connect my VNA to the ports for measurement in the off chance that a bump of the cable or connectors may cause a transient which will damage the VNA.

Does anyone have experience with this type of measurement? Is a DC block sufficient? Where do I get one rated that high? Most are rated 50VDC. All comments and questions are appreciated.

I do plan on testing for line transients using a scope probe to make sure I don't get spikes when switching from transmit to receive.

By the way, my background is in cellular and cable. Five watts is a lot of power!

_________________
Jim Beckford
RF Engineer

 Post subject:

Posted: Thu Apr 13, 2006 9:58 am 

My experience, with 80KW radar T/R switches, was no matter how effective the PIN diodes were (and the spec was excellent, cutting edge) the greatest risk was leakage energy around the switches through ground loops. There was enough leakage energy for the pico-seconds of switch time, and enough pulse circulating current, to blow enough 2350 Schottky mixer diodes to boost HP's share price.

Somehow, I recommend you break up the ground paths to ensure that the ground loop leakage paths between input (Tx) and output (Rx) have as high a resistance as possible.

Using a scope probe can be misleading for the same reason: where do you clip the ground of the probe? In one place you get a great reading, in another you could break down crying when you see the trace. There just doesn't seem to be a true ground zero anywhere on the circuit.

 Post subject:

Posted: Thu Apr 13, 2006 10:39 am 

Thank you. I hadn't thought of the high power RF leakage, but looking at the driving logic, I see the client has a delay between switching out the PA and switching in the receiver. I'll pass along the idea about separating the ground loops as he is in the middle of a new board turn.

I have talked to him concerning the high DC voltage. The new layout will allow the bias to be set down to 50V for s-parameter extraction. The purpose of the s-parameter sweep is to verify that there are no resonant responses (unlikely) or connector, core, PCB losses in the basic design. Certainly, at high power, things may change but the idea is to get the basic design correct. This, of course, assumes that the diodes have little change from 50V to >500V reverse bias.

Thanks again.

_________________
Jim Beckford
RF Engineer