Programmable capacitors - Page 1
Forum section: electronics / projects

author: jpanhalt
author_rank: Super Contributor
timestamp: January 20, 2018, 09:43:37 pm
content: 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):http://www.psemi.com/pdf/datasheets/pe64102ds.pdfIxys (non-volatile, 6.5V programming, great range)http://www.ixysic.com/home/pdfs.nsf/www/NCD2100.pdf/$file/NCD2100.pdfST ("tunable" requires HV bias) (01/21/18 link fixed)http://www.st.com/content/ccc/resource/technical/document/datasheet/ec/36/b9/b9/92/7e/47/86/DM00214282.pdf/files/DM00214282.pdf/jcr:content/translations/en.DM00214282.pdfThe 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.This unit does not have a lower limit on frequency:http://azmicrotek.com/products/frequency-control/capacitive-tuning/azt71/Unfortunately, telephone calls and e-mails to Arizona Microtek have not been answered yet. Moreover, a unit to fit my needs, the AZT70 has lead spacing of 0.35 mm, which is smaller than some bacteria. I have no problem with 0.65 mm, or maybe 0.5 mm, but 0.35 is a bit small for my DIY.1) Is anyone aware of other, non-obsolete options?2) Although not spec'd below 50 MHz, can the Peregrine be used below that, e.g., at 4 MHz to 8 MHz to trim an MCU crystal? 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 with its programmable capacitors.Thanks, John

author: John Heath
author_rank: Frequent Contributor
timestamp: January 20, 2018, 09:59:56 pm
content: Lightening strikes have such a broad range of frequencies mostly in the audio range so maybe a low Q antenna could do the trick. Just a wire running about 20 feet with a little neon bulb to ground in case the lightening is a little too close for comfort. Form there a standard audio mixing board should do with EQ sliders to fine tune the VLF lightening frequencies. I use the audio card in my computer to keep track of local and distant lightening strikes.

author: jpanhalt
author_rank: Super Contributor
timestamp: January 20, 2018, 10:12:33 pm
content: My question is not about the AS3935. It is working quite well (within its design limits).While the programmability of the AS3935 led me to ask this question, the question is whether there is a non-microscopic, programmable capacitor IC on the real market that can be used to trim MCU crystal/ceramic resonators at lower frequencies. Non-volatile memory is a plus, but not required.

author: David Hess
author_rank: Super Contributor
timestamp: January 21, 2018, 02:56:26 am
content: Quote from: jpanhalt on January 20, 2018, 09:43:37 pmCapacitor switching can be done discretely with transistor or diode switches and capacitors but this may be difficult at high impedances; it works great at low impedances. At lower frequencies, variable transconductance circuits can be used including operational transconductance amplifiers and paraphase amplifiers which gets into using a gyrator.QuoteTest it and find out. Usually a MCU crystal would be trimmed electronically using a varactor diode but a switched capacitor can certainly be used.

author: Zero999
author_rank: Super Contributor
timestamp: January 21, 2018, 11:17:03 am
content: I don't see why it shouldn't work at lower frequencies than 50MHz.What's wrong with the idea of using a varactor diode?What range of capacitance is needed? A quad analogue switch and four capacitors could give 16 different capacitance settings, but you need to keep an eye on the on resistance and parasitic capacitance.

author: jpanhalt
author_rank: Super Contributor
timestamp: January 21, 2018, 11:27:54 am
content: Thanks for all the replies.ST Link is fixed, hopefully.The MXA1474 is obsolete -- at least I didn't find it at DigiKey or Mouser.The PIC chip I am using does have a built-in DAC. Simply hadn't thought of using that and a variactor. Worth looking at.Finally, yes, I was also tempted to just try the available chips at lower frequency. That is no big deal of course. However, with all the people on this forum, why re-invent the wheel? So, I thought I would ask.Range of capacitance needed is just enough to trim a resonator/crystal. Empirically, that is usually less than 6 pF. My current device is trimmed with 4.x pF.Regards, John

author: David Hess
author_rank: Super Contributor
timestamp: January 21, 2018, 09:19:52 pm
content: Quote from: jpanhalt on January 21, 2018, 11:27:54 amUsually the problem with varactors is that they require a high voltage range to achieve their maximum capacitance range. So operating one at 3.3 volts may be a problem. In a set and forget application, they also have a temperature coefficient which will limit accuracy.There actually is another way but I am only mentioning it for thoroughness. An inductor's value can be varied by controlling its saturation with a DC current so before varactors, they usedsaturable reactorsfor variable frequency tuning and trimming crystal oscillators.

author: Zero999
author_rank: Super Contributor
timestamp: January 21, 2018, 09:34:03 pm
content: Quote from: David Hess on January 21, 2018, 09:19:52 pmHow about using a crappy ceramic capacitor, the type with a lower capacitance, at higher bias voltages, as a varactor? I've heard about that being done before, especially at lower frequencies.

author: jpanhalt
author_rank: Super Contributor
timestamp: January 21, 2018, 09:44:30 pm
content: The SMV1247-SMV1255 have a nice capacitance range for trimming resonators at 0 to 3.3 or 5 V. (https://www.digikey.com/scripts/DkSearch/dksus.dll?Detail&itemSeq=249635240&uq=636521451282634753)I will probably try a switched version to see if it works at typical PIC resonator frequencies. At least, they are set and forget. If not, I will try back-to-back varacters and a resistor for isolation. My PIC (16F1829 or others) has a 5-bit DAC which should be adequate.John

author: David Hess
author_rank: Super Contributor
timestamp: January 21, 2018, 10:15:39 pm
content: Quote from: Hero999 on January 21, 2018, 09:34:03 pmThat should work very much like a varactor but the temperature coefficient will be monstrous and the lowest value capacitors which would be required for a crystal are usually C0G/NP0 dielectric which has a very low voltage coefficient of capacitance. It might be feasible for much lower frequency applications.