what's the cheapest way I can generate a 100MHz sine wave?  - Page 1
Forum section: electronics / beginners

author: peps1
author_rank: Regular Contributor
timestamp: July 19, 2023, 05:18:27 pm
content: Hi, a bit of a strange one here.I need a 100MHz signal to prove that a pocket oscilloscope cant read up to 100MHz, but I really only work in audio electronics so have nothing that will generate a signal that high.Is there a kit project or something along those lines that can get up that high without spending too much money?

author: dobsonr741
author_rank: Frequent Contributor
timestamp: July 19, 2023, 05:37:16 pm
content: Get a SI5351A based module, hook it up to a loose Arduino or anything else can send I2C. You can generate any frequency in between 8KHz to 160MHz.

author: TimFox
author_rank: Super Contributor
timestamp: July 19, 2023, 05:44:57 pm
content: The same question was asked in 2016, with mhz incorrectly used for MHz:Useful reply to that question:https://www.eevblog.com/forum/testgear/cheapest-way-to-generate-100mhz-sine-wave/msg1002605/#msg1002605There are one-transistor Colpitts L-C oscillators posted on the web:e.g.https://www.eeeguide.com/colpitts-oscillator-using-transistor-circuit/

author: peps1
author_rank: Regular Contributor
timestamp: July 19, 2023, 05:54:57 pm
content: Thank you so much, just one of these little blighters?https://www.amazon.co.uk/Youmile-CJMCU-5351-Si5351A-generator-controller/dp/B09698D3K4/ref=sr_1_1_sspa?crid=28TUL12SESI1W&keywords=SI5351A&qid=1689789229&sprefix=si5351a+%2Caps%2C52&sr=8-1-spons&sp_csd=d2lkZ2V0TmFtZT1zcF9hdGY&psc=1

author: dobsonr741
author_rank: Frequent Contributor
timestamp: July 19, 2023, 07:54:40 pm
content: Yes. AliExpress has them too.

author: Vovk_Z
author_rank: Super Contributor
timestamp: July 19, 2023, 08:29:55 pm
content: Quote from: peps1 on July 19, 2023, 05:18:27 pmPossibly it can be much easier to use slew rate of some relatively low frequency square wave signal but not 100 MHz sine wave if you don't have SOMETHING to check or to measure that 100 MHz sine.

author: TimFox
author_rank: Super Contributor
timestamp: July 19, 2023, 08:35:31 pm
content: You can find cheap packaged crystal oscillators on eBay, for between $7.00 and $20.00, for examplehttps://www.ebay.com/itm/123714733192?hash=item1ccdf90488:g:IvYAAOSwTgJcnoHg&amdata=enc%3AAQAIAAAAwOzwoUx8stxsN6%2FaFMZMHGyNu935gQrPYXLC8r0QJq0D8PYQBo3WjzR%2Frts5c9Te7ArHb4YP3%2FcZPOcAjeoMkR%2FxUaZQbGrLCKqSm4s9V8Zw7TjJFTbXjXiyGP3vOBQY5GMWr6SkYE1W5nNm5gmXXSbM4TT8z5GW3AIH0%2Bsi7xJ2qnojGsH2OvWW%2F4KxNNr6VkVmWCEzU%2B2TBn%2FgNrAYQArqqq%2BkreA9x%2F0uCu9lyIJdp2ZjxcKAUrqFZh9dqlbgaw%3D%3D%7Ctkp%3ABk9SR4i93v2tYg

author: Zero999
author_rank: Super Contributor
timestamp: July 19, 2023, 09:06:24 pm
content: It can be done with a single NPN transistor, a few resistors, a couple of capacitors and a home made coil.https://www.circuits-diy.com/simple-fm-transmitter-by-using-one-transistor/Obviously skip the microphone and aerial. It's not very stable, but good enough just to produce a test signal, so see if the oscilloscope is working or not.

author: radiolistener
author_rank: Super Contributor
timestamp: July 20, 2023, 02:16:02 am
content: as told above, si5351 is the most cheapest solution. The module price is about 1 USD:https://www.aliexpress.com/item/32883385728.htmlIt also requires some MCU module to setup si5351 registers, you can use STM32 blue pill, black pill or arduino...si5351 produces square wave with about 1 ns rise time. If you want sine wave, you're needs to put LPF with cut off at about 120-150 MHz. But in your case LPF is not required, square wave is even better for oscilloscope testing.

author: BillyO
author_rank: Super Contributor
timestamp: July 20, 2023, 08:08:49 pm
content: You can make a fast rise time oscillator with a single 74AC14 and a handful of other parts. Maybe $5.This one was characterized at 1.03ns rise time. Plenty fast enough to test scopes up to 200MHz or so.

author: Infraviolet
author_rank: Super Contributor
timestamp: July 21, 2023, 06:20:14 pm
content: To my understanding most of the options so far listed will give square waves rather than sines. A square wave with a known rise time would be appropriate enough in practice for this use case of verifying a scope's performance, better than a sine perhaps (as imperfections in the scope's analog frontend would tend to distort waveforms too fast for it towards being more sine like). But for a close approximation of a sine wave, a square wave can be fed in to a series of low pass RC filters. The low pass filter's frequency would need to be set so as to not attenuate much at the frequency you want, but attenuate quite strongly by the time you reach the next harmonic to be found in the fourier transform of the square wave. You might need to amplify the sine wave after the filtering stages, depending on initial square wave voltage and how smalla wave you can tolerate at the scope's input, I think some specialised transistors could be able to do that at this frequency, but I think most transistors wouldn't be able to keep up.

author: magic
author_rank: Super Contributor
timestamp: July 21, 2023, 09:11:03 pm
content: The problem with those cheap solutions will be knowing what is the exact output they produce: is it really sine, what's the amplitude, what's the true frequency (if you care). Then you can verify if the scope under test shows the waveform properly.It would help if you have access to a know good oscilloscope for comparison. Then it doesn't even matter how shitty the generator is - you just want to see the same waveform as on the reference scope.

author: srb1954
author_rank: Super Contributor
timestamp: July 22, 2023, 04:13:53 am
content: Using a square wave with a fast enough edge is a much easier way to verify the performance of a scope. A square wave has lots of harmonics so you are effectively verifying the performance of the scope over a wide frequency range with a single signal. A slow rise time indicates insufficient bandwidth and any ripples in the top of the viewed waveform is indicative of undesirable frequency response or phase variations.A 100MHz scope should exhibit a 10%-90% rise time of less than 3.5ns provided the square wave generator has a fast enough edge, say < 1.1ns rise time. There are plenty of simple designs around for generating suitable fast rise square waves. You can also buy pulse generators specifically designed for scope verification, e.g Leo Bodnar Fast Rise time Pulse Generator, but these tend to be higher performance specified with <100ps rise times for verifying higher performance scopes.Verifying a scope's performance with a sine wave is more complicated as you need to measure at a number of different frequencies and be sure that your sine wave generator has a constant amplitude at all frequencies. There are specially designed leveled signal generators for this purpose, e.g. Tektronix SG5030, but these are fairly complicated and expensive devices.

author: TERRA Operative
author_rank: Super Contributor
timestamp: July 22, 2023, 05:36:50 am
content: If using a sinewave to figure out a scopes -3dB point, you'll need to know the actual P-P output of the sinewave generator.If another scope or a power meter is available to make that measurement you are set, but if not, you'll end up with a chicken and egg scenario.I would be inclined to use a fast rise-time pulse/squarewave generator too along with a sinewave to get a better idea of the bandwidth.Using the pulse/squarewave generator, Bandwidth = 0.35/rise time(in seconds)Then also use the sinewave generator to check when the displayed signal decreases by -3dB (or drops to 70.7% of it's original value).

author: MathWizard
author_rank: Super Contributor
timestamp: July 22, 2023, 07:01:51 am
content: I have 2-terminal quartz crystals up to 24MHz. What's the max frequency they make them for ? I made some common Quartz crystal oscillator circuits, they worked ok. I never tried feeding them into anything that would use them as a clock, and then make a set peak-to-peak voltage.One of these days I need to probe around on my old motherboards and video cards. Some of them must have very fast clocks, and probably a listed peak voltage somewhere.I haven't done any hands on EE this summer, I should dig up 1 of those motherboards, and see for myself.

author: connectTek
author_rank: Regular Contributor
timestamp: July 22, 2023, 08:07:32 am
content: https://a.aliexpress.com/_mNLkIS6

author: gf
author_rank: Super Contributor
timestamp: July 22, 2023, 09:02:43 am
content: Quote from: Infraviolet on July 21, 2023, 06:20:14 pmMy poor man's approach (due to lack of an RF sine wave generator) is to use e.g. NanoVNA (-> SI5351) as generator, not to care about the generator's exact waveform (which is not a sine wave), but to do a FFT to measure the amplitude of the fundamental frequency component. Reference measurement of the fundamental frequency's amplitude can be done with a spectrum analyzer then (e.g. tinySA).At low frequencies (say f < fs/100), the error due to aliasing is almost negligible. At higher frequencies, aliasing of the harmonics becomes significant, but can be addressed by choosing a frequency plan so that none of the first (say) 100 harmonics fold back to the fundamental frequency in the first Nyquist region, so that fundamental and (folded) harmonics can still be clearly separated in the frequency domain. This implies of course that the measured frequency points cannot be arbitrary (in particular they cannot be integral fractions of the sample rate).

author: Swake
author_rank: Frequent Contributor
timestamp: July 22, 2023, 04:36:08 pm
content: Wouldn't an old analog FM radio tuned to a 100MHz broadcast be able to help out? There is surely some oscillator signal that can be tapped.

author: BillyO
author_rank: Super Contributor
timestamp: July 22, 2023, 04:53:25 pm
content: Quote from: Swake on July 22, 2023, 04:36:08 pmYou'd need something that can accurately measure the output of the local oscillator so that you can compare that with what the scope is telling you. The easiest and least expensive way to do this is with a fast rise time oscillator.The following video get's interesting @ 5:59. There is also a previous video (linked at the end) that goes into the design. Warning, these are sponsored videos.

author: TimFox
author_rank: Super Contributor
timestamp: July 22, 2023, 06:08:12 pm
content: Quote from: MathWizard on July 22, 2023, 07:01:51 amSee my reply #6 above. Packaged crystal oscillators can be cheap.

author: BillyO
author_rank: Super Contributor
timestamp: July 22, 2023, 07:22:26 pm
content: Quote from: TimFox on July 22, 2023, 06:08:12 pmAgain, how does he determine the output level of the oscillator?

author: TimFox
author_rank: Super Contributor
timestamp: July 22, 2023, 07:45:28 pm
content: Quote from: BillyO on July 22, 2023, 07:22:26 pmThe original question stated: "I need a 100MHz signal to prove that a pocket oscilloscope cant read up to 100MHz"For that purpose, the spec on the packaged oscillator should suffice.To really determine bandwidth, a variable frequency oscillator is needed (with constant or calibrated output).

author: BillyO
author_rank: Super Contributor
timestamp: July 22, 2023, 08:01:13 pm
content: Quote from: TimFox on July 22, 2023, 07:45:28 pmIt can be done with a fast rise time square wave too. Single measurement and cheap to build.

author: TimFox
author_rank: Super Contributor
timestamp: July 22, 2023, 09:02:09 pm
content: Note that if you apply a good square wave with fast edges and a flat top to a 'scope with a bandwidth of approximately 100 MHz, at most only the fundamental sine wave will be displayed.The ratio of pk-pk values for the original square wave and the resulting sine wave is 4/(pi), plus another factor for how far down the 'scope is at 100 MHz (perhaps 3 dB).

author: BillyO
author_rank: Super Contributor
timestamp: July 22, 2023, 09:11:30 pm
content: I'm talking about the relationship between rise time and BW. Where BW=0.35/risetime. A 100MHz scope should have a risetime of close to 3.5ns. If you build a fast risetime oscillator at around 1ns and you put it on you supposedly 100MHz scope and you record a rise time of something like 10ns then you know it does not meet specs and would be awful at 100MHz.The one I have already posted about has a risetime of less than 1.1ns, is very cheap to make, is easy to make and handy to have around.

author: Konkedout
author_rank: Frequent Contributor
timestamp: July 23, 2023, 02:18:26 am
content: Cheapest? How about my favorite cheap and dirty oscillator:1) Take a 74HC1G04 inverter gate.2) Connect normal supply rail bypassing, such as 1 uF from Vcc to ground.3) Take maybe 10 turns of insulated wire, wrap around a pencil to make an air core inductor. Connect this from input to output.4) Connect maybe 100 pF from output to ground, and 330 pF from input to ground. The capacitance ratio should be inversely proportional to the voltage swing expected at these two nodes. So 3:1 to 5:1 sounds good.5) Power up with 5V and observe oscillation.6) Adjust the inductor (squeeze, stretch, add or remove turns) and/or capacitor values to get desired frequency.NOTE: This oscillator is believed HIGHLY ILLEGAL with regard operating conditions specified in the datasheet. I design good stuff but this is an experimental toy only. I think the output voltage swing will normally exceed the supply rails for example. But I think it should work for a while! If the IC gets hot, turn off power and let it cool for a few minutes. It has been many years but I made an oscillator like this once using....I think it was Fairchild high speed logic from the late 1970's.