Schmidt Trigger Stabilization  - Page 1
Forum section: electronics / projects

author: nonsuchpro
author_rank: Contributor
timestamp: September 03, 2016, 04:41:10 am
content: Hey all.Got a quick question. I'm sure the answer won't be so quick but... :pAnyway, what is the simplest way to stabilize a Schmidt trigger frequency so that when it turns on it goes to the desired frequency and stays put?Right now, if I make the standard Schmidt T. oscillator, it gets close but as the circuit warms/cools it changes the frequency.Thanks in advance for your help!Rob

author: edavid
author_rank: Super Contributor
timestamp: September 03, 2016, 04:52:38 am
content: Quote from: nonsuchpro on September 03, 2016, 04:41:10 amIt'sSchmitt, not SchmidtYou could make your oscillator out of a comparator instead of a logic gate.Or, use a 555 instead.

author: nonsuchpro
author_rank: Contributor
timestamp: September 03, 2016, 06:38:34 am
content: QuoteGotcha on the spelling. I just did a spellcheck all.I agree about the 555. That's a fairly simple one, however, I'm using the Schmitt in the audio range and I think it sounds much better. You can call me crazy, but it does. Maybe I can use the 555/556/558 to keep the STrig in check?

author: Zero999
author_rank: Super Contributor
timestamp: September 03, 2016, 08:32:22 am
content: Lol, it shouldn't make much difference, as they both output a sqarewave. One possible explanation is the duty cycle of the 555 will be slightly off 50% which may sound more pleasant to you.Schmitt trigger logic gates are not designed to make stable oscillators. The thresholds are not accurately controlled and will shift considerably with temperature and supply voltage so the frequency will change.As far as the spelling is concerned: my spell checker accepts both Schmitt and Schmidt as valid spellings. I think they're both names: Schmitt is the person who invented the Schmitt trigger and Schmidt is just another name, probably famous for some othe reason.Please post the schematics.

author: nugglix
author_rank: Regular Contributor
timestamp: September 03, 2016, 09:15:14 am
content: Quote from: Hero999 on September 03, 2016, 08:32:22 amYou never tried the gold plated audio cables made by finnish virgins on full moon, right?Quote from: Hero999 on September 03, 2016, 08:32:22 amWhich is not made better by using quite large caps.I used to build the TDR variant using a 74HC14 and the jitter was quite large.Quote from: Hero999 on September 03, 2016, 08:32:22 amIn this case fame isn't the issue, just pure number.https://en.wikipedia.org/wiki/List_of_the_most_common_surnames_in_GermanySCNRCheers

author: nonsuchpro
author_rank: Contributor
timestamp: September 03, 2016, 11:05:08 am
content: Yeah, I had to add "Schmitt" (which was red squiggly lined) to my dictionary.There's gotta be a way to keep them stable. It's not the 555 that I like. CD40106 NOT Gates sound FAT!

author: nonsuchpro
author_rank: Contributor
timestamp: September 03, 2016, 11:09:20 am
content: Here's the basic oscillator

author: edavid
author_rank: Super Contributor
timestamp: September 03, 2016, 11:20:48 pm
content: If you adjust the 555 oscillator and the 40106 oscillator to have the same duty cycle, they will sound exactly the same.

author: zlymex
author_rank: Frequent Contributor
timestamp: September 04, 2016, 01:20:11 am
content: One way to stabilize the frequency against temperature is to use compensation.However, because the variation of threthold voltage is probably very large, you need something like NTC or PTC which have non-linear response that can only be use in a narrow range of temperature(about 10 to 20 degree C)

author: nonsuchpro
author_rank: Contributor
timestamp: September 04, 2016, 10:46:54 am
content: Quote from: edavid on September 03, 2016, 11:20:48 pmYou would think... but they don't. Not to my ears. I've made many done synths like this one...https://youtu.be/JtE7MM7Q33E.I'm telling ya, there's something about the Schmitt Trigger.

author: nonsuchpro
author_rank: Contributor
timestamp: September 04, 2016, 10:50:18 am
content: Quote from: nonsuchpro on September 04, 2016, 10:46:54 am

author: tatus1969
author_rank: Super Contributor
timestamp: September 04, 2016, 11:23:29 am
content: Quote from: nonsuchpro on September 04, 2016, 10:50:18 amUse a comparator with rail-to-rail output capability instead of a logic gate, add hysteresis. Logic gates have very wide spread in thresholds. If you use NP0 or C0G capacitors you'll get very stable results.https://en.wikipedia.org/wiki/Relaxation_oscillator#Comparator.E2.80.93based_relaxation_oscillator

author: nonsuchpro
author_rank: Contributor
timestamp: September 04, 2016, 02:39:11 pm
content: QuoteDogs and Cats, living together,.... MASS HYSTERESIS!!!Any particular chip(s) in mind?

author: nonsuchpro
author_rank: Contributor
timestamp: September 04, 2016, 02:54:13 pm
content: I was just looking through my IC's and I came across 8 NOS Intel 8253 Programmable Timers. So for the hell of it I downloaded the datasheet and searched for usage. I was happy to see the info below. Might be fun to try out.The Intel 8253 is capable of operating in 6 different modes:MODE 0 - Interrupt on Terminal Count------------------------------------When this mode is set the output will be low. Loading the count registerwith a value will cause the output to remain low and the counter will startcounting down. When the counter reaches 0 the output will go high and remainhigh until the counter is reprogrammed. The counter will continue to countdown after terminal count is reached. Writing a value to the count registerduring counting will stop the counter, writing a second byte starts thenew count.MODE 1 - Programmable One-Shot------------------------------The output will go low once the counter has been loaded, and will go highonce terminal count has been reached. Once terminal count has been reachedit can be triggered again.MODE 2 - Rate Generator-----------------------A standard divide-by-N counter. The output will be low for one period of theinput clock then it will remain high for the time in the counter. This cyclewill keep repeating.MODE 3 - Square Wave Rate Generator-----------------------------------Similar to mode 2, except the ouput will remain high until one half of thecount has been completed and then low for the other half.MODE 4 - Software Triggered Strobe----------------------------------After the mode is set the output will be high. Once the count is loaded itwill start counting, and will go low once terminal count is reached.MODE 5 - Hardware Triggered Strobe----------------------------------Hardware triggered strobe. Similar to mode 5, but it waits for a hardwaretrigger signal before starting to count.

author: zapta
author_rank: Super Contributor
timestamp: September 04, 2016, 04:31:56 pm
content: What frequency and stability do you need?You can use for example an IC like this one that that has an oscillator (RC and Crystal) and a divider.http://www.nxp.com/documents/data_sheet/HEF4060B.pdf

author: tatus1969
author_rank: Super Contributor
timestamp: September 04, 2016, 06:54:27 pm
content: Quote from: nonsuchpro on September 04, 2016, 02:39:11 pmnot necessarilyfor example TLV3201. Make sure the output does not have to drive too much, << 1mA. Otherwise the output impedance would be another source of temperature dependency. You should be able to get < 2% accuracy with this one, plus a 1% NP0 capacitor.

author: tatus1969
author_rank: Super Contributor
timestamp: September 04, 2016, 06:57:35 pm
content: Quote from: zapta on September 04, 2016, 04:31:56 pmThat one uses a standard CMOS schmitt trigger and is in no way better in terms of frequency stability. Have a look at fig.11 in the datasheet.

author: zapta
author_rank: Super Contributor
timestamp: September 04, 2016, 07:23:41 pm
content: Quote from: tatus1969 on September 04, 2016, 06:57:35 pmYes, I looked at it, and also at figure 7.

author: tatus1969
author_rank: Super Contributor
timestamp: September 04, 2016, 08:32:29 pm
content: Quote from: zapta on September 04, 2016, 07:23:41 pmfig7 shows a quartz oscillator, whereas this thread is about RC oscillators.

author: zapta
author_rank: Super Contributor
timestamp: September 04, 2016, 08:49:29 pm
content: Quote from: tatus1969 on September 04, 2016, 08:32:29 pmThe way see it, this thread is about helping the OP designing a stable oscillator that meets his requirements. That's why asked for the required frequency and stability.