oscillator shielding - Page 1
Forum section: electronics / beginners

author: Cali-Sam
author_rank: Contributor
timestamp: December 20, 2013, 04:32:29 am
content: i built an oscillator based on a 74ac14 schmitt trigger, seealthough i only needed a couple kilohertz of oscillations, i realized that, with a trim pot, i can drive the sucker to an excess of 2mhz.the question is::::: how high can i drive the frequency of oscillations before shielding the circuit becomes needed?

author: w2aew
author_rank: Super Contributor
timestamp: December 20, 2013, 05:01:49 am
content: There's no firm answer, because it depends on a number of things, such as: how is the circuit laid out? How good is the decoupling? Are the outputs properly terminated? What's the sensitivity of nearby circuits? What are your trying to protect with the shielding? Etc. etc.Even at a few kilohertz, it may still need to be shielded. A square wave like this is made up of many high order harmonics. The faster the rise/fall times, the higher the harmonic frequency content. Did you see my video on the spectral content of square waves?

author: Cali-Sam
author_rank: Contributor
timestamp: December 20, 2013, 05:39:30 am
content: Quote from: w2aew on December 20, 2013, 05:01:49 ami follow your videos indeed. i have seen your video on the spectral content of square waves, very informative!!!i built it on a prototyping board... quite honestly, i know little about the traps of laying out such circuit, or any of the other factors for that matter.

author: Rerouter
author_rank: Super Contributor
timestamp: December 20, 2013, 06:55:05 am
content: what kind of prototyping board?for the traps, when you get into high frequency edges, the current wants to return directly under the signal trace, rather than the straightest line (this is the path of least impedance but you dont need to understand it fully to use the tricks) so a ground plane or a signal ground following along directly opposite allows for faster signals to pass over a given trace,equally high speed signals do not like going across gaps in the ground plane beneath it, (it causes a reflection as the distance between it and the ground plane / trace control the impedance of the trace, and a large changes in impedance cause reflections and other RF wizardry problems) this is why smd components are so popular, because by keeping very close to the board, there is almost no change.decouple everything that has a fast edge, and if your laying something out, do this first, you want your power to your devices to go [power supply section] - [decoupling cap] -  not something where current has to pass across the device pin to reach the cap, and bigger doesnt always mean better, generally lower impedance or esr helps moreand finally terminations, now this isn't necessary for short lengths with 74 series chips, but for longer stuff, you calculate the approximate impedance with one of the various online calculators, and terminate your signal with that resistance, this ensures part of the signal doesn't get reflected back down to the output and upset whatever is trying to read off that outputas for your circuit, applying these principles would help, but as for how much comes down to any number of factors.

author: Zero999
author_rank: Super Contributor
timestamp: December 21, 2013, 12:19:45 am
content: The oscillator itself should work with no shielding at all but it could interfere with other nearby circuits.Try replacing the resistor with a 100uH inductor and the capacitor with a 1k resistor and see what happens.

author: mrkev
author_rank: Regular Contributor
timestamp: December 21, 2013, 03:01:15 am
content: Well in fact high speed logic can be a source of various things. You should realise that when the state changes, the rise or fall time is inns. If you do fourier on signal like that, you will get some MHz (harmonics can go really high).So with a really bad PCB design, no coupling, etc. you could get quite noisy circuit even if you run just kHz through.That is also the reason why you should always use logic with appropriate speedMany people think that the higher the speed, the better it'll perform, but it's usually the other way around...

author: matkar
author_rank: Regular Contributor
timestamp: December 21, 2013, 12:52:55 pm
content: I achieved a stable oscillation near 4MHz with SMD components with no PCB and a small trimmer. I needed a 3.2768MHz source to feed a microcontroller. I didn't have the right quartz crystal and I didn't want to wait for entire weekend to get one.

author: Zero999
author_rank: Super Contributor
timestamp: December 22, 2013, 03:00:42 pm
content: Quote from: matkar on December 21, 2013, 12:52:55 pmThat will work fine in non-critical applications but note that an RC oscillator won't be as stable or as a crystal oscillator so it won't work where you need an accurate/stable clock.

author: matkar
author_rank: Regular Contributor
timestamp: December 22, 2013, 04:44:18 pm
content: Quote from: Hero999 on December 22, 2013, 03:00:42 pmYes, the application didn't need an accurate clock source so this method was fine to make it work. Eventually I changed the oscillator with a quartz.