Crystal oscillator output - Page 1
Forum section: electronics / beginners

author: crash2140
author_rank: Contributor
timestamp: April 14, 2015, 09:51:13 pm
content: For my project, I needed a 75MHz crystal oscillator, and I bought one. However, when I probed its output with my oscilloscope, it looked way more like a sine wave than a square wave. Is this normal for high frequency crystal oscillators, do I need any external circuitry to fix it or is it the component that is defective?Thank you.

author: edpalmer42
author_rank: Super Contributor
timestamp: April 14, 2015, 10:02:01 pm
content: More info required.What scope & probe? What bandwidth? If it's a 100 MHz scope & probe, it will only show you the 75 MHz fundamental. i.e. a sine wave.What oscillator model? Different models can have an output that's a sine wave, clipped sine wave, or square wave.Ed

author: ataradov
author_rank: Super Contributor
timestamp: April 14, 2015, 10:02:07 pm
content: This is normal. Have you taken into account the bandwidth limitations and capacitance of your scope?

author: crash2140
author_rank: Contributor
timestamp: April 15, 2015, 05:26:28 pm
content: I'm using a 60MHz probe, on a 50Mhz bandwith (analog) scope.

author: ataradov
author_rank: Super Contributor
timestamp: April 15, 2015, 05:27:59 pm
content: Then you are lucky to see even a sine wave. Don't worry, your oscillator is fine.

author: edpalmer42
author_rank: Super Contributor
timestamp: April 15, 2015, 07:53:22 pm
content: I agree with ataradov. I'm a bit surprised you saw anything.In order to make a measurement, your equipment has to be better than the device being measured. In this case, your scope & probe would have to have a frequency range high enough to include at least a few harmonics from the oscillator. You'd need something like a 350 MHz scope to get a reasonable idea of what the oscillator's output looks like.Ed

author: crash2140
author_rank: Contributor
timestamp: April 15, 2015, 08:56:40 pm
content: I didn't even think about the probe's bandwidth, as I'm not used to work with high frequencies. So there's probably nothing wrong with the oscillator.Thanks!

author: TSL
author_rank: Regular Contributor
timestamp: April 15, 2015, 11:17:36 pm
content: And... just to mention because no one else did, crystal oscillators are naturally sine. You need additional circuitry to change that waveform to something else.Also since its 75Mhz the crystal is either working at an overtone since fundamental resonances for crystals are usually no higher than 30Mhz or the 75Mhz is derived from some multiplication of a fundamental. i.e. for either overtone or post stage multipliers your output is 3 x 25MHz.regardsTim

author: ataradov
author_rank: Super Contributor
timestamp: April 16, 2015, 01:10:17 am
content: I'm 99% sure OP talks about crystal generator.

author: crash2140
author_rank: Contributor
timestamp: April 17, 2015, 08:22:01 pm
content: It's something like this:Quote from: TSL on April 15, 2015, 11:17:36 pmCan you be more specific, please?

author: ataradov
author_rank: Super Contributor
timestamp: April 17, 2015, 09:14:37 pm
content: Quote from: crash2140 on April 17, 2015, 08:22:01 pmIt is just a package. There can be anything on the inside.Quote from: crash2140 on April 17, 2015, 08:22:01 pmBasically, if you need to provide power to the device, then it is a generator and it is likely to have square logical level output.

author: TSL
author_rank: Regular Contributor
timestamp: April 18, 2015, 05:14:03 am
content: As ataradov has indicated, that's a self contained crystal based generator. i.e. there is a crystal plus additional circuitry to make it oscillate and most likely a squaring circuit and amp for output. Often this is just a PIC + crystal in a can.regardsTim

author: crash2140
author_rank: Contributor
timestamp: April 19, 2015, 08:10:21 pm
content: Then I will just assume that it outputs a square wave, which is the most likely, and hope that the project works.Thanks for your help!

author: Zero999
author_rank: Super Contributor
timestamp: April 20, 2015, 06:41:18 pm
content: Quote from: TSL on April 18, 2015, 05:14:03 amWhy on earth would they use a microcontroller?That would make no sense, when a couple of logic gates are much cheaper.

author: TSL
author_rank: Regular Contributor
timestamp: April 20, 2015, 08:58:38 pm
content: Quote from: Hero999 on April 20, 2015, 06:41:18 pmMy use of the word PIC is an over simplification of whats inside a can oscillator. Some are simple divider circuits and others more.I was just trying to convey the fact that can oscillators are not just crystals but have circuitry, sometimes complex, to produce the required frequency and waveform.4 pin can oscillators are generally fixed frequency devices. The 4th pin can sometimes be for tuning voltage input so that the oscillator can be part of a PLL etc.6 pin can oscillators, such as the si570 are much more complex and containing DSP/PLL as part of their construction.See attached pics...regardsTim

author: wraper
author_rank: Unknown
timestamp: April 20, 2015, 10:36:31 pm
content: Quote from: TSL on April 20, 2015, 08:58:38 pmHow anyone should understand you if you are talking in your own language? For anyone else PIC means a microcontroller made by Microchip.

author: TSL
author_rank: Regular Contributor
timestamp: April 21, 2015, 12:46:15 am
content: Quote from: wraper on April 20, 2015, 10:36:31 pmMy apologies, I was using the acronym PIC in one of its original forms "Programmable Integrated Circuit" to indicate the generic nature of what the circuit might be inside the can.Sorry for the confusion.regardsTim