# Problem with PIN Diode Limiter - RF Cafe Forums

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KH
Post subject: problem with PIN diode limiter
Unread postPosted: Tue Mar 15, 2005 11:39 pm

I have a passive limiter circuit made from an antiparallel pair of PIN diodes connected between RF and AC ground. (There is an adjustable DC voltage to set the threshold.) When I ramp the RF input power from -5 dBm to +15 dBm, the output power tracked the input power linearly. It seems to me that the limiting diodes are not turning on themselves when the peak RF voltage is greater than 0.7V.

Does the PIN diode rectify the RF voltage when the frequency is greater than 1/(2*pi*transit time)? The diodes had tau=200ns. And I did my test at 500 MHz.

Thinking that maybe the PIN diodes didn't rectify the signal at high frequencies, I replaced the PIN diode with a Schottky antiparallel pair and the Schottky pair worked as expected up to 1.8 GHz. Above 1.8 GHz, the insertion loss increased even when the peak RF voltage was less the 0.2V.

What should I do to make this limiter circuit work? My specs are 1 MHz - 6 GHz, less than 1 dB insertion loss, and +12 dBm limit.

I thought about an active limiter circuit using a coupler, schottky detector controlling a bias circuit for a single shunt pin diode. I'm short on time and board space to make anything elaborate.

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Guest
Post subject: Re: problem with PIN diode limiter
Unread postPosted: Fri Jun 03, 2005 12:41 am

KH wrote:
I have a passive limiter circuit made from an antiparallel pair of PIN diodes connected between RF and AC ground. (There is an adjustable DC voltage to set the threshold.) When I ramp the RF input power from -5 dBm to +15 dBm, the output power tracked the input power linearly. It seems to me that the limiting diodes are not turning on themselves when the peak RF voltage is greater than 0.7V.

Does the PIN diode rectify the RF voltage when the frequency is greater than 1/(2*pi*transit time)? The diodes had tau=200ns. And I did my test at 500 MHz.

Thinking that maybe the PIN diodes didn't rectify the signal at high frequencies, I replaced the PIN diode with a Schottky antiparallel pair and the Schottky pair worked as expected up to 1.8 GHz. Above 1.8 GHz, the insertion loss increased even when the peak RF voltage was less the 0.2V.

What should I do to make this limiter circuit work? My specs are 1 MHz - 6 GHz, less than 1 dB insertion loss, and +12 dBm limit.

I thought about an active limiter circuit using a coupler, schottky detector controlling a bias circuit for a single shunt pin diode. I'm short on time and board space to make anything elaborate.

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Guest
Post subject:
Unread postPosted: Fri Jun 03, 2005 7:05 am

Do you have return path to ground? The input power is rectified by the diodes, but if there is no return path to ground the diodes cannot turn on ( no current).

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Guest
Post subject: PIN diode limiter
Unread postPosted: Wed Jun 08, 2005 11:35 am

I'm puzzled.

You normally use PIN diodes to act as a variable resistance, based on the DC current flowing through the diode.

When used as a limiter, the circuit depends on charge build-up in the intrinsic layer, which lowers the resistance. It's not obvious that you can change this behavior by adding DC bias. The limiting level is usually much higher than 0.7V. In any case, adding DC bias would have to be done separately for each diode, as that interferes with one diode acting as the other diode's DC return.

For a good discussion of the use of PIN diodes as a limiter, check out http://www.plextek.com/papers/pinlim.pdf .

Good luck!

Posted  11/12/2012

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