[SOLVED] Tunable bandpass using PIN diodes

Hello.

I am new to this forum.
I have a small question to ask.
I am currently trying to build a tunable bandpass filter using PIN diodes.
I've done this using varactors but I've been requested to do it using PIN diodes.

I am using a series L, shunt LC resonator configuration.
What troubles me is how the diode comes into the picture to create the discrete steps for switching Fc.

Any help or suggestions will be greatly appreciated.

Thank you very much.

LC filters usually require L or C tuning elements as high Q resonators are used.
This is why varactors and sometimes magnetically tuned inductors are used in tunable filters.

A PIN diode is essentially an artificially degraded diode (by inserting a lossy I layer into a P/N junctions), so it does not rectify a RF signal. By adjusting a DC current through the P-I-N junction, the device behaves as a variable resistor at RF. Therefore it is used preferably as attenuating element.

If your filter can be tuned by a variable resistor, then a PIN diode can be used. PIN diodes are mostly used at frequencies > 10 MHz. I have not seen a PIN diode as a tuning element in a RF filter as it sounds having not too much sense in high-Q LC circuits. But it is not impossible to try it, maybe in a RC or active-filter circuit.

I think you can change the resonating capacitor in a capacitor bank circuit that consists of different valued capacitors in parallel and hence your center frequency of selective resonant circuit is changed.
Or, you may also use active device if you frequency is not too high to build bandpass filter with sliding capacitor values...
There are some technically feasible solutions with PIN diodes...Research the literature...

I agree with the capacitor bank and pins leading to each capacitor. You would either fully bias the pin diode on (maybe 50 mA of DC bias) so that it has a very low series resistor, or fully bias the pin diode off (maybe 20V reverse bias) so that the diode and its attached capacitor are completely out of the picture.

You should be able to achieve a much higher IP3 point with only PIN diodes as opposed to varactor tuning.

In all of these things, you find that if you try to tune over too broad a band, the filter 3dB passband width changes dramatically.

Rich

That was my approach but maybe I was not pursuing the right goal or maybe something different was being done rather than what I was supposed to do.
Do you have any suggestions as for a literature or link where I can find some information regarding this?
A colleague of mine said that varactor themselves are not that slow but like you said, IP3 is very poor.
And as you go down on frequency, capacitor value gets incredibly high, making varactor diodes practically useless.
Thus, the PIN diode solution I am pursuing.

I would greatly appreciate any source of information that could at least show me some pointers that I could use to follow.
Thank you very much for your replies.

From all you wrote I do not see what "low frequency" is for which you intend to use PIN diodes to tune filters.

PIN diodes are not useful below ~10 MHz. On the other hand I know that there are varactor diodes to tune SW radios, so their capacitance is ~ 300 pF and can vary by voltage down to ~ 150 pF.

There are "active" filters for frequencies from zero to >100 MHz where operational amplifiers are available. Tuning is typically done by varying resistance, now digital potentiometers are available, too.

Better approach to low-frequency filtering is by digital signal processing, now the common technology.

Instead of talking, I would suggest you to study. There are heaps of good literature, textbooks and papers. Try google!

Study is the thing I do every moment I get to.
There's a lot of documentation online that I have downloaded in the past month about tunable filters.
Problem is that all of them are > 1 GHz frequencies.
If my situation were in that range, believe me when I say this, you wouldn't even know I exist.
But, the frequencies I am working with go from 20 MHz to 500 Mhz.
At those frequencies, as you know, microstrip is complicated to build if not impossible due to size limitations.

However, I am interested in finding out more about what you said.
Varying the resistance is something that I have enough resources to follow.
Any website or publication you may suggest?
I would greatly appreciate it.

You also mention DSP as a common technology used for this.
I have not worked with DSP so far.
I will make note of that and see what I can find about it.

I have donwloaded some IEEE papers as far as my subscription allows me.
There are others but those you have to pay, which I don't mind.
Though I would like to know if the information there will be useful before buying.

PIN diodes can be slow because you actually have to inject charge into or remove it from the I region of the diode.

But people forget that Varactors have a similar effect. If you have a varactor tuned to 10 volts, and suddenly change the bias to 1 volt, you have to inject/remove charge from the back biased junction, and that takes time. The depleted region of the back biased diode will not change until you inject/remove that charge. You might have an RF capacitance of a few pF, but the effective "bulk" capacitance you have to change when changing bias voltage will act like the varactor is thousands of pF.

Exactly.

That's the reason I have to dive into PIN diodes.
Yes, they take time to switch depending on the width of their I region.
Still, it makes them much faster than the varactor counterpart.

I have recorded speeds of up to 400 microseconds when switching from 1 to 10 V and viceversa.
It's decent, but still kind of slow compared to other technologies.

You should be able to drive (low rf power) PIN diodes on or off in under a microsecond without too much trouble. You need a driver designed specifically to do that.

If you need faster than that, you can use GaAs pin diodes (maybe a nanosecond or two), or FETs (maybe 10 nS).

As friends mentioned, pin diodes are used to on and off capacitors in a capacitor array to create a tunable structure. It is called PIN diode switched binary capacitor array. You forward bias the diode to turn it on and you reverse bias(with relatively high voltage) to turn it off. There is a related paper: Design of a High Power Digital Tunable Filter(you can find it in IEEE database). You may find related documentation in tunable filter company, polezero.com.

Binary Capacitor Array!

Thank you very much for that keyword.
That's what made my day! I assembled a small circuit using two capacitors.
One with the array one without so I could see the changes.
And when I tested it, it worked perfectly!
I can see the main tone and small bandwidth move in frequency without one affecting the other.

Again, Thank you very much for your help!

A new thing I have learned and I am deeply grateful for your advice.
I will try to see if I can collaborate on another post so I can pay back the favor.

Thank you very much!