EMRFD Message Archive 4449

Message	Date	From	Subject
4449	2010-03-13 09:00:24	drmail377	NPO Capacitors, Lowest Microphonics in DC RX?
I think I saw a post here about how NPO capacitors exhibit lower microphonics in direct conversion receivers (or in-general). Anyone care to expand on why? I suspect it might be due to their differential physical construction. Regards, David
4452	2010-03-14 09:52:02	Tim	Re: NPO Capacitors, Lowest Microphonics in DC RX?
4453	2010-03-14 10:40:11	Art	Re: NPO Capacitors, Lowest Microphonics in DC RX?
Not to long ago I remember reading a web article about using surface mount caps as microphones. Apparently they work very nicely. Wish I could remember where I saw the article, I didn't bookmark it at the time, and now I can't locate it!!! Regards, Art >
4456	2010-03-15 11:20:30	Glen	Re: NPO Capacitors, Lowest Microphonics in DC RX?
Thanks for the link to Kemet, Tim - like you, have mostly experimental and not theoretical knowledge of weird dielectric properties. Haven't heard microphonics from NP0 caps. But not all small-value caps are free from piezo effects.... Once tried to use a 10pf 6KV ceramic cap to neutralize a 3 KV DC vacuum tube. Could get a superb balance, but at only one plate voltage. As plate voltage changed, so did the capacitance, unbalancing the neutralizing network. In this case, even an air-dielectric cap would give a similar result. An example of electric field forces at work. If you push ANY of our components hard enough, you'll find non-linear effects. Just ask anyone setting up test equipment to measure high IP3 Just based
4457	2010-03-15 11:51:43	Tayloe Dan-P26412	Re: NPO Capacitors, Lowest Microphonics in DC RX?
I have used high value ceramic caps in the front end of a DC receiver, and I really do not find the microphonics to be all that bad. If you tap the parts themselves, they are clearly microphonic, but in normal use, they do not seem objectionable. In addition, when I have found parts that have been microphonic (like the toroids in the LO filter in the NC2030 DC transceiver), the secret to removing the microphonic quality seemed to be to simply to dampen the vibration of the part. This can be done with a bit of hot glue or wax to bridge the part to the board, dampening the vibration. For the purposes of dampening, a soft substance (hot glue, wax) would be preferred to a hard substance (epoxy). - Dan, N7VE ________________________________
4458	2010-03-15 12:57:45	Neil Douglas	Re: NPO Capacitors, Lowest Microphonics in DC RX?
Glen, Jack Smith, K8ZOA, has measured a number of capacitors with changing polarisation voltage. http://www.cliftonlaboratories.com/capacitor_voltage_change.htm Even the common 100n decoupling capacitors are reduced to 80% of their capacitance at 5V and 60% at 12V. I used a ceramic 10U as a microphone to trigger a laser pulse cartridge I made for a friends shotgun a few years ago. The noise of the hammer falling triggered a pulse of red laser light that shone out of the gun barrel. NeilD G4SHJ _____
4495	2010-03-31 13:47:05	davidpnewkirk	Re: NPO Capacitors, Lowest Microphonics in DC RX?
4496	2010-04-01 23:19:51	drmail377	Re: NPO Capacitors, Lowest Microphonics in DC RX?
Thanks all for the replies & links. Great info. I have experienced microphonics with surface mount caps. It seems SMT parts are more mechanically coupled to the board compared with through-hole parts. Board vibrations stress the dielectric in SMT caps more than in through-hole parts. YMMV. Best 73's, David