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.pdf
ST ("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