What is this RC oscillator called? - Page 1
Forum section: electronics / general

author: king.oslo
author_rank: Frequent Contributor
timestamp: November 04, 2011, 07:38:37 pm
content: Hello there,I was reading page 11 of this datasheet:http://ww1.microchip.com/downloads/en/DeviceDoc/21455C.pdfI want a higher Fosc than their example. I am trying to work how to get that.I searched for rc oscillator on google, and it became obvious that there are many kinds of rc oscillators. Unfortunately I was unable to find the same rc circuit as in the datasheet, and I was therefore unable to decide out how it works.Help!Thanks.M

author: Balaur
author_rank: Unknown
timestamp: November 04, 2011, 08:07:10 pm
content: If you just want to select component values for a specific frequency, just follow the recommendations from the paragraph 7.5 and equation 7-2.

author: king.oslo
author_rank: Frequent Contributor
timestamp: November 04, 2011, 08:16:10 pm
content: As far as I can tell, the only recommendation made is for 48kHz. The formula for Fosc is 0.45/RC. Personally I have no clue what value RC is. This is the reason why I asked the question in the first place.M

author: armandas
author_rank: Frequent Contributor
timestamp: November 04, 2011, 08:18:02 pm
content: An RC oscillator is just that, a resistor and a capacitor. I think you are looking for aPierce oscillator.

author: armandas
author_rank: Frequent Contributor
timestamp: November 04, 2011, 08:31:42 pm
content: Quote from: king.oslo on November 04, 2011, 08:16:10 pmRC is a time in which a capacitor C will charge, through R, up to 63% of its final voltage. You should be able to substitute the values given in the datasheet:Code:Fosc = 0.45 / (100k * 100p) = 45000Hz

author: IanB
author_rank: Super Contributor
timestamp: November 04, 2011, 08:39:49 pm
content: Quote from: king.oslo on November 04, 2011, 08:16:10 pmRC is R x C.For example, if R is 100k (100 x 10^3) and C is 100 pF (100 x 10^-12), then RC = 100e3 x 100e-12 = 1e-5. Therefore f = 0.45 / 1e-5 = 45 000 Hz = 45 kHz (which is not 48 kHz, but apparently it is close enough).

author: king.oslo
author_rank: Frequent Contributor
timestamp: November 04, 2011, 08:53:25 pm
content: So if I choose 100k and 22pF, I get 200KHz?Is there an advantage of using a big resistance vs small capacitance?Is there a limit of how high I can go? 1MHz would be great!Thanks!M

author: amspire
author_rank: Super Contributor
timestamp: November 04, 2011, 09:06:13 pm
content: You could use 22pF, but it may be more practical to use 220pF or more. 22pF is down in the range where stay capacitances, like rearranging wiring, our putting the box lid on can change the frequency. 220pF and 10k would be easier to work with.Richard

author: Zero999
author_rank: Super Contributor
timestamp: November 06, 2011, 09:26:55 pm
content: Quote from: king.oslo on November 04, 2011, 08:53:25 pmYes lower power consumption, at the expense of stability and predictability.

author: qno
author_rank: Frequent Contributor
timestamp: November 07, 2011, 07:24:46 pm
content: I think this is called a relaxation oscillator.It is based on a single RC network as feedback in a comparator circuit

author: king.oslo
author_rank: Frequent Contributor
timestamp: November 07, 2011, 08:43:43 pm
content: Thanks guys! Beautiful!M

author: ciccio
author_rank: Frequent Contributor
timestamp: November 07, 2011, 08:53:55 pm
content: Please refer to the end of paragraph 7.5 of the datasheet: to reject both 60 an 50 Hz noise the oscillator frequency should be 48 kHz. You can calculate C that should be 12 pF (with R = 100k).I've used a lot of 7107s, about 20 years ago, even before Maxim began to sell them, and the datasheeet was alway not-too-clear. I remember that the Teledyne version contained some errors the made me crazy..Best regards