high power pin diode limiters

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Colon

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Hi,

I think I know what is going on here, but I could do with some reassurance and direction if possible please.

We are trying to make a high power PIN diode limiter, for use in the HF band. Our spec says we need to able to limit RF output power at 26dBm for up to a 46dBm input!

We have looked at various ideas but one we are pursuing at the moment is a reflective shunt PIN diode. To test this I have a setup with a PIN diode (MA4P709-150, 10uS carrier lifetime, 2degC/Watt) shunted to ground and biased with a DC current. The idea eventually is to RF power detect and apply feedback to apply the appropriate bias current to keep the output at 26dBm. For now, I am just checking that it can attenuate properly at high power.

The current test sets the bias at around 0.2mA, which gives Rs for the diode at about 50 ohms, giving about 4dB of attenuation. This works well up to around 30dBm of input power, at which point the circuit starts to behave oddly, oscillating very slowly (sub 1Hz). In this mode voltage on the diode anode goes negative for large parts of this cycle, the voltage seemingly generated by the diode.

From what I have read just now (I'm quite new to PIN diodes) the test is reaching the point where the charge produced by the RF current is getting close to the stored charge created by the DC bias current. At this point the RF current should start to modulate the bias, and this seems so as I can see the bias current is not steady anymore.

At the moment I am not sure that this design will work. I could alleviate the situation by maybe combining multiple diodes or even changing to a series diode (rather than shunt, as the load will reduce the RF current through the diode) but I think I will still fail to meet the spec, even with a much longer carrier lifetime. I just think we are asking too much.

Am I missing a trick here? This is a demanding spec, and the boss agreed to meet this order without proving it out first so we've got to do what we can I guess. Any thoughts?

If need be, I can post a pic of the circuit when I get to work in the morning.

Thanks
James
 

For high suppression, use cascaded PIN attenuators. You can google "white papers" on PIN attenuator design for specific details.
 

I do not think you understand the concept. That is a very thick I region diode (7 mils). It is designed to reflect high power signals when there is a DC bias current of 200 mA going thru it. You WANT the rf resistance to be as low as possible! biasing it up at 0.2 mA will not work for thermal and isolation reasons.
 
+ 26 dBm = 400 mW? across 50 ohm = 4.4V rms, Irms = 88 mA . Just thinking aloud! At a place where I used to work we had HF band diode attenuators that could handle 1Vrms with -120 dB ip3's. Yours is the next step up. You do not mention phase mod/ ips .Which you can get with pin diodes. My immediate thought is to use a pair of pin diodes with a centre tapped transformer arranges such that the spurious "RF detected current" is balanced out (so non flows). Or use a pair of WB transformers to run your attenuator at a higher impedance point say Z = 200 = 1/2 the current, 2 X the voltage.
Frank
 
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Ok, so the concept is that we have to limit signals from the antenna to +26dBm and as it's near a transmitter we are likely to see signals up to about 50dBm potentially. The guy originally assigned to make the limiter started off with a clipping limiter but that wasn't working so well so we are looking at a reflective PIN diode limiter.

I was just testing the PIN diode with it set to 50ohms for around 4dB of attenuation, which it would be if there was 30dBm of signal present. Obviously, if it is towards 50dBm it will have <1 ohm and be biased differently. And, it has to work from 1-30MHz.This is why we chose a long carrier lifetime of 10us and generally this means a thicker I-region, although I understand lifetime is dependent on I-region volume rather than thickness.

We are still having trouble. I spent today trying a self biasing limiter with an inductor ground return (and holding the diode in reverse bias). But, due to a thick I-region, this was limiting too high. If I could find a PIN diode with sufficient carrier lifetime to work down to 1MHz and have the right thickness of I-region to limit around 26dBm, I think we'd have a winner.

I then tried replacing the inductor with a schottky which successfully reduced the limiting threshold to a reasonable level. However due to the fact that the diodes are not reversed biased at any point our intermod distortion was poor when not limiting.

James

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That's interesting. I never thought of transforming the manipulate the voltage. A higher voltage with make it limit earlier though, and only by 6dBm, but I will give it some thought.

I'm not sure I quite get your suggestion about the center tapped transformer, but I'm nearly there. Could you give a bit more detail?

Thanks
James
 

Colon:

I am quite new to PIN diode used as RF limiter and I want to learn something here. So correct me if I misunderstand your problem.

Your input is a 46dBm HF signal (3~30MHz), and you want to make sure the RF output stays below 26dBm, is this correct? So you shunt the PIN diode with the RF antenna so that it will guide some powers back into your non-RF parts as a feedback. is this the idea?

If so, I had a thought that might or might not be relevant (naive). Have you considered harmonics? e.g. a 30MHz high power signal flowing through the diode will generate 60MHz, 90MHz, 120MHz ..... harmonics. If you have more than one frequencies as the input, the diode is actually acting like a parametric amplifier which mix your f1 and f2 signals to f1-f2 and f1+f2, 2f1, 2f2, ........ Since you said it is high power input. If your RF part has a matched impedance at these frequencies, it will radiate them out for sure. Just something I am curious of.
 
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When I worked in a weather radar company, I bought some limiter + attenuator, from +80dBm limiting about +10dBm. That is waveguide structure at X band.
But in S band it's coaxial structure with about seven stages.
It seems yours is SMT on board.

- - - Updated - - -

When I worked in a weather radar company, I bought some limiter + attenuator, from +80dBm limiting about +10dBm. That is waveguide structure at X band.
But in S band it's coaxial structure with about seven stages.
It seems yours is SMT on board.
 

You are correct that you need a thick I region to work at low RF frequencies. But one problem is such a thick I region diode wants a lot of DC current to "turn on".

You might try the thick diode at the high power input, and then quarter wavelength away a thinner I region diode. The thinner diode turns on first, and reflects a standing wave back to the thicker diode--helping to turn it on. You would make the thin diode 1/4 wavelength away with a combination of transmission lines and inductors/capacitors.

You could do an active system, where there is the thick I region diode in the main line, and after it a directional coupler leading to a schottky diode detector. The detector drives an op amp that then feeds back DC current to the diode to turn it on when the right power level is reached.
 
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