PIN diode switch - how to change biasing - RF Cafe Forums
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Post subject: PIN diode switch - how to change biasing Posted: Sat Sep 03, 2005 12:51 am
I am working on Switching circuits at 2-4GHz range using PIN diodes. I am designing the circuit in ADS. Started with simple most series PIN diode switch, but facing some problem in that.
Problem : -
I used on series diode betweeen two microstrip lines terminated in Ports.
the diode has been biased throguh high - low transmission lines. I gave bias voltage at P terminal of diode of around 2 Volts, the diode is in forward bias. Even though I change voltage value to be -25 volts, the switch does not come in OFF condition... i.e. S21 is still less than 1 dB.
Can some please help me out here.
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Post subject: PIN DiodesPosted: Sat Sep 03, 2005 11:07 am
PIN diodes don't really turn on by voltage - they reach a low-impedance state by (forward) current. So there has to be enough current flowing in the forward direction to give the proper impedance. In the reverse (off) direction, the applied ("DC") bias must (significantly) exceed the peak voltage of the RF.
If you're sure you have those two necessary conditions, then there are only a couple of possibilities:
1. Bad diode - PIN diodes can be damaged by both overcurrent and overvoltage.
2. Coupling around the diode - parallel microstrip lines might provide that if there's enough length where they're parallel
3. Impedances aren't what you think they are. (Not too likely, I know, but worth mentioning/checking)