Cheap source of 8Mhz square wave? - Page 1
Forum section: electronics / projects

author: miceuz
author_rank: Frequent Contributor
timestamp: December 09, 2014, 11:36:45 am
content: For a project I need 8MHz square wave source with ~5mA source and sink capability, not particulary stable or accurate, but cheap.The only solution I came up width is PIC10F320 for 0.42$ in hundreds. Does anyone have a better solution? Seems somewhat wasteful to use a whole MCU just for an oscillator.

author: Dajgoro
author_rank: Frequent Contributor
timestamp: December 09, 2014, 11:42:34 am
content: Ignore the 6502 part.Another version:

author: Zero999
author_rank: Super Contributor
timestamp: December 09, 2014, 11:54:06 am
content: If a crystal is too expensive, just use an RC or LC oscillator.https://www.fairchildsemi.com/application-notes/AN/AN-118.pdfhttp://www.edn.com/design/other/4363948/Oscillator-has-voltage-controlled-duty-cycle

author: dannyf
author_rank: Super Contributor
timestamp: December 09, 2014, 12:10:50 pm
content: QuoteYou can always avoid that waste by spending more on a simpler solution, like an active crystal oscillator.

author: miceuz
author_rank: Frequent Contributor
timestamp: December 09, 2014, 01:25:26 pm
content: Quote from: dannyf on December 09, 2014, 12:10:50 pmThus the question.I have considered the above mentioned options - the crystal alone in hundreds is ~0.22$ and I'm not sure if I can achieve 8Mhz using RC+inverter oscillator.The only alternative seems to be an inverter ring oscillator, but propagation delay seems to vary quite widely making the oscillation frequency vary at least twice or even ten-fold with varying temperature or voltage.

author: Zero999
author_rank: Super Contributor
timestamp: December 09, 2014, 01:47:20 pm
content: Quote from: miceuz on December 09, 2014, 01:25:26 pmIt's doable but not very stable. LC is probably better. For Colpitts L = 22uH, C = 33pF for 8.35MHz or L = 10uH, C = 68pF for 8.6MHz but the F will be slightly lower than that, due to parasitics, so you may need to lower C slightly.

author: DmitryL
author_rank: Regular Contributor
timestamp: December 09, 2014, 02:57:59 pm
content: Quote from: miceuz on December 09, 2014, 11:36:45 amI do! I do!As soon as you haven't specified any certain requirements for stability and accuracy, then anything will do for youI would suggest RC-oscilator and 74S logic from a trash bin. Good enough ? That's cheap, as you wanted, less than 40 cents.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: December 09, 2014, 04:10:35 pm
content: Look into ceramic resonators, they should be cheaper than quartz. Then there's the whole programmable silicon oscillator thing, but I don't know if they're that cheap.

author: mikerj
author_rank: Super Contributor
timestamp: December 09, 2014, 04:45:14 pm
content: MAX7375 would be perfect apart from Maxims typical pricing strategy. Searching for "silicon clock" or "silicon oscillator" might yield something cheaper.

author: Zero999
author_rank: Super Contributor
timestamp: December 09, 2014, 07:06:04 pm
content: How square does the wave have to be? Will a sine or distorted sine do? How about a common base oscillator with a source follower buffer.A smaller inductor than I was suggesting before, should be cheaper.

author: Kjelt
author_rank: Super Contributor
timestamp: December 09, 2014, 07:12:19 pm
content: Quote from: miceuz on December 09, 2014, 01:25:26 pmUse a ceramic resonator, such as MURATA CSTLS8M00G53-A0 they are with an expensive distributor as Farnell € 0,0963 /1000 pieces so you should be able to find a cheaper source somewhere on this globe esp. if you don't need an A brand

author: dannyf
author_rank: Super Contributor
timestamp: December 09, 2014, 07:18:20 pm
content: You have to figure out what you are trying to do.If it is one off, it doesn't matter if it is $0.44 or $44 a piece.If you value frequency stability, RC oscillator is out;If you are trying to minimize current consumption, crystal oscillator is out;If you are trying to generate random numbers, go with an RC oscillator;....Without knowing what you want, impossible to answer

author: Zero999
author_rank: Super Contributor
timestamp: December 10, 2014, 01:54:26 pm
content: Quote from: Kjelt on December 09, 2014, 07:12:19 pmI agree, plus a single inverter IC is cheap too. It also has the advantage of not requiring any external capacitors. All that's needed is the resonator, the inverter and a bias resistor, very cheap.http://www2.mouser.com/ProductDetail/Murata-Electronics/CSTLS8M00G53-A0/?qs=sGAEpiMZZMsBj6bBr9Q9aavJxGXZ4o2iufjO98nYUxA%3dhttp://www2.mouser.com/ProductDetail/Diodes-Incorporated/74AHCT1G04SE-7/?qs=sGAEpiMZZMuyBeSSR239IfLrDdP8YV%252b4Jx8wNZCJiB0%3d