Programmable Capacitors

[jpanhalt]

Been playing on and off (mostly off) with the AS3935 lightning detector that includes programmable (SPI) capacitors for antenna tuning. The available choices for dedicated programmable capacitor IC's seem quite limited on DigiKey. Here is what I found:

Peregrine (volatile memory):
https://www.psemi.com/pdf/datasheets/pe64102ds.pdf

Ixys (non-volatile, 6.5V programming, great range)
https://www.ixysic.com/home/pdfs.nsf/www/NCD2100.pdf/$file/NCD2100.pdf

ST ("tunable" requires HV bias)
https://www.st.com/content/ccc/reso...df/jcr:content/translations/en.DM00214282.pdf

The ST Microelectronics unit is analog tuned with a bias voltage, so that leaves only 2 "digital" choices. Peregrine is nice, has SPI interface, but does not have non-volatile memory. Ixys uses a special, but not particularly complicated interface, has non-volatile memory and a wide range of values, but requires 6.5 V for programming. That would add a bit of complexity.

Neither device is spec'd below about 50 MHz.

1) Is anyone aware of other, non-obsolete options?

2) Although not spec'd below 50 MHz, can they be used below that,e.g., at 4 MHz to 8 MHz? My suspicion is that the intended use is for higher frequencies, so they are not characterized at lower frequencies, and/or at lower frequencies, the change in capacitance with frequency is small. Alternatively, that limit might mean they use some device (like a PIN diode) that will not pass lower frequencies. The AS3935 , however,works at 500 KHz.

John

 

[jpanhalt]

Half-solved:
I think I have found one that may not be obsolete: https://azmicrotek.com/products/ (click on frequency control)
Those devices seem very similar to obsolete devices by Maxim (Max1474) and Intersil (X90100) and are non-volatile.

Here's the DS: https://azmicrotek.com/products/frequency-control/capacitive-tuning/azt71/

Still unanswered is whether the devices at Digikey, particularly the Peregrine, can be used at lower frequencies.

John

 

[schmitt trigger]

Unfortunately I do not have any advice on programmable caps other to what you have found. The only other thing I could think of would be a varactor diode but of course it is analog and requires a high voltage (~27) volt bias.

How do you like the AS3935 so far? I've built my own litle weather station and I had been looking into the device...but was never fully convinced that it could add significant features and it required a lot of effort to develop.
But i could be wrong.

 

[MikeMl]

Speaking of Varicaps, does anybody know of some that would have a range of at least 3:1, but at a nominal capacitance of a few hundred pF that is readily available?

I have built some tuneable receiving loops for 1.8 and 7MHz, and would like to replace the mechanical tuning capacitor with a remotely-tuned Varicap.

 

[jpanhalt]

The AS3935 is fun to play with. As you know, it is SPI and that makes it quite easy. I bought the MikroElektronics piggyback board for the chip. Adafruit (?) also has a board. The specially designed antenna is available from CoilCraft, and it is not at all hard to make your own, which I have done. The recommended design is very underdamped. The antenna I made for my own receivers has damping.

It is pretty easy to tell what the AS3935 calls lightning from oscilloscope tracings from my receivers. And so far, the AS3935 seems pretty accurate for detection. That is, a good "strike" is usually accompanied with a visible flash, but not all flashes (at night) are detected as strikes. Ranging is a bit slow to react, so I have a routine to clear history on demand. I have an idea of what the algorithm used in the AS3935 is, and when I am reasonably sure or at least have something that correlates with it, I will post it.

John

 

[OBW0549]

Speaking of Varicaps, does anybody know of some that would have a range of at least 3:1, but at a nominal capacitance of a few hundred pF that is readily available? I have built some tuneable receiving loops for 1.8 and 7MHz, and would like to replace the mechanical tuning capacitor with a remotely-tuned Varicap.

I'm not too familiar with what's available in Varicaps, but I doubt there are many that go that high in capacitance.

This is a bit unconventional, but you might consider trying a rectifier diode in place of a Varicap; they have big junctions and therefore lots of junction capacitance. I did a quick test with a 10A10 and got 120 pF at 100 mV reverse bias, dropping to 45 pF at 10V reverse bias.

I don't know whether this would suit your needs, but it might be worth considering.