How to make a 50MHz or faster square wave - Page 1
Forum section: electronics / projects

author: hobbyist
author_rank: Newbie
timestamp: August 07, 2013, 02:32:21 am
content: Hello, first post ever. I want to build a 50 MHz or 100 MHz square wave oscillator for testing an oscilloscope's performance. It will be a small battery powered device that I could directly connect to the oscilloscope's channel 1 input.This circuit using a crystal and a 74F04 TTL inverter certainly works well below 50 MHz:http://members.iinet.net.au/~davem2/overclock/osc.htmlWhat would it take to generate a reasonably square 100 MHz waveform? Is the above circuit with its TTL inverter likely to work at 100 MHz? If not, and I have to use a 4 pin oscillator like the CTS MX045 series, what kind of transistor or other device should I use to get around the oscillator's 15 pF maximum load?Once the device is built, I plan to try it on a friend's 1 GHz Tektronix 7104, and on my 60MHz Rigol DS1064B (which does 2 gigasamples per second).

author: Rerouter
author_rank: Super Contributor
timestamp: August 07, 2013, 03:02:29 am
content: does the exact frequency matter to you, or is it more that it is a clean square wave?

author: ejeffrey
author_rank: Super Contributor
timestamp: August 07, 2013, 04:43:08 am
content: For measuring the response of a scope it is normally more illuminating to use a lower frequency square wave but still with a fast rise time. Then you can see the full step response before it gets muddled by a second edge.Anyway, the silicon labs Si510 can generate a clock at 100 MHz or higher:http://www.digikey.com/product-detail/en/510DBA100M000AAG/336-2427-ND/3985746

author: free_electron
author_rank: Super Contributor
timestamp: August 07, 2013, 04:52:25 am
content: rise time is what you need. Jim williams pulse generator... averages 182 picoseconds risetime for an 8 volt signal... you need a VERY fast machine to be able to actually measure it... ( scope in the 250.000$ range ...)

author: hobbyist
author_rank: Newbie
timestamp: August 07, 2013, 06:03:21 am
content: Thank you all for the replies. I would like to have 100 MHz just because the oscillations would be exactly two divisions wide on my scope. Also my scope has a counter that displays the triggering rate and it would be fun to see it read 100.000 MHz.The Jim Williams pulse generator certainly looks impressive. I agree that nothing else is likely to have a better rise time, but for this project I still want a square wave.Oscillators like the SI510 look great but can only drive a 15 pF load. My scope's input is somewhat higher than that, and I would like the device to work when connected to a 1x probe, a 10x probe, a length of coax, or right to the scope's input channel. What would I use to make the oscillator output handle a greater capacitive load? If a TTL chip won't handle 100 MHz, then I suppose that I would have to make some kind of VHF voltage follower. What kind of design would work well?

author: Frant
author_rank: Regular Contributor
timestamp: August 07, 2013, 08:14:43 pm
content: One simple solution would be an integrated oscillator (e.g. Abracon ASEMB series, 50 MHz) driving a fast clock buffer (e.g. ON Semiconductor NB3L553, 1 ns rise time @ 3.3 V). In order to drive a higher capacitance load, you can connect the buffer's outputs together (output-to-output skew is only 50 ps). An active probe (< 1 pF) is also a good thing to have.

author: PA4TIM
author_rank: Super Contributor
timestamp: August 07, 2013, 08:41:39 pm
content: http://www.pa4tim.nl/?p=2677about 500ps risetime and more as 100MHz if you want. I have it now fixed at 40MHz.I have also build some williams pulsers but they were not stable enough for testing and adjusting my 1S2 5 GHz plugin.

author: woodchips
author_rank: Frequent Contributor
timestamp: August 07, 2013, 09:38:07 pm
content: At 1ns rise time you have frequencies in the Ghz range, and at that point a matched 50 ohm system is needed, otherwise you won't see anything very much.For fast rise times still nothing seems to beat a tunnel diode, at least for simplicity. Have a look at circuits from Tek or HP for TDR pulsers.

author: edavid
author_rank: Super Contributor
timestamp: August 07, 2013, 09:43:40 pm
content: Fox does spec their 100MHz can oscillators to work into a 30pF load. I suspect most other brands will work also, they just won't meet their risetime spec.Forget 1X probes though...

author: JoeyP
author_rank: Frequent Contributor
timestamp: August 07, 2013, 10:31:15 pm
content: Quote from: hobbyist on August 07, 2013, 02:32:21 amEven 50MHz will be too fast for the 60MHz rigol. Keep in mind that for a 50MHz square wave, the pulse width would be only 10ns (minus generator's rise+fall times). The risetime of a typical 60MHz scope will be around 5.8ns. A 10ns wide pulse wouldn't even allow enough of a flat top to determine the true peak-peak amplitude of the pulse, so the 10% and 90% points will be calculated wrong and give you an error in the resulting measurement. Scope risetime measurements are typically done using repetition rates of 1MHz or less.