Fixed frequency RF Sine Wave - Page 1
Forum section: electronics / beginners

author: IIOSOH
author_rank: Newbie
timestamp: November 07, 2022, 10:35:49 pm
content: Hi everyone,I am trying to generate a sine wave with a fixed frequency of 100 MHz. I've been doing some research the past few days, but the more I read the more confuesed I get.From what I understood the best approach would be to use a DDS, especially if I want a variable output frequencies and waveforms but DDS ICs, capable of outputting a high quality signal are quite expensive or out of stock. Another approach would be to generate the signal using a PPL, but I couldn't find such an IC that naturally produces a sine wave so I would have to use one that generates a square wave from which I will then have to extract the fundamental frequency using a filter, but I am not sure if that approach is correct and will produce a clean sine wave. Of course I want to the signal to be as clean as possible but also keep the overall cost below ~10 Dollars.

author: radiolistener
author_rank: Super Contributor
timestamp: November 07, 2022, 10:37:52 pm
content: I think the most easy and cheap way is to buy Chinese TCXO for 100.000 MHz, like this:https://www.aliexpress.com/item/1005002412459200.htmlSince it has square wave you're needs to put low pass or bandpass filter on it's output to eliminate harmonics and get sine wave.If you want to deal with synthesizer PLL, you can usesi5351IC. You can buy module on aliexpress for about 2 USD:https://www.aliexpress.com/item/4000122200596.htmlIt produce square wave with 1 ns rise time, so it also needs to put low pass or bandpass filter on it's output in order to get sine waveBut it needs microcontroller with code in order to configure it at power on.TCXO doesn't needs microcontroller and has a little bit more clean output (less jitter)

author: Benta
author_rank: Super Contributor
timestamp: November 07, 2022, 10:56:11 pm
content: Please make your mind up: do you need a fixed frequency (100 MHz) or do you need a variable frequency?

author: David Hess
author_rank: Super Contributor
timestamp: November 08, 2022, 02:57:33 am
content: You could buy a can 100 MHz oscillator. Some even have a sine wave output.What I might do is start with a lower frequency crystal oscillator, say 25 MHz, and then use a pair of tuned doubling circuits to get 100 MHz.

author: pdenisowski
author_rank: Super Contributor
timestamp: November 08, 2022, 11:22:47 am
content: You could also buy a generic VCO and set the tune/control voltage such that it outputs the frequency you want. This gives you a lot of flexibility but would require two separate DC sources (Vtune and Vsupply)[Edit - just noticed the “under $10” part, so maybe not a VCO]

author: IIOSOH
author_rank: Newbie
timestamp: November 08, 2022, 07:24:27 pm
content: I only need a fixed frequency but have the possibilty to change it would be great, even though it's not a necessity.

author: IIOSOH
author_rank: Newbie
timestamp: November 08, 2022, 07:27:47 pm
content: So, it seems like the right approach for this price range would be to filter out the fundamental harmonic out of a square wave as that is what most of you suggested.Thank you very much.

author: jonpaul
author_rank: Super Contributor
timestamp: November 08, 2022, 07:35:52 pm
content: leo bodnar GPDSOhttp://www.leobodnar.com/

author: ogden
author_rank: Super Contributor
timestamp: November 08, 2022, 08:17:35 pm
content: How clean (SNR) sine you are looking for? Tuning range? Anyway - if you are looking for low cost then DDS for 100MHz is not the best choice. PLL+VCO is.  below 10$? - pick best TCXO you can get for < 10$ and consider you did the best.

author: IIOSOH
author_rank: Newbie
timestamp: November 08, 2022, 08:35:36 pm
content: Do you by any chance know what a PLL + VCO solution that produces a signal with reasonable quality will cost me approximately and won't the output still be a square wave? I couldn't find any PLL ICs that produce a sine wave.

author: ogden
author_rank: Super Contributor
timestamp: November 08, 2022, 08:42:04 pm
content: Quote from: IIOSOH on November 08, 2022, 08:35:36 pmNo wonder you can't find PLL IC's producing sine wave because they do not produce any kind of wave, just tuning (voltage) for VCO.

author: Mechatrommer
author_rank: Super Contributor
timestamp: November 08, 2022, 09:54:03 pm
content: Quote from: jonpaul on November 08, 2022, 07:35:52 pmgood info but not within OP's spec. and afaik is still outputting square wave, not pure sine.

author: IIOSOH
author_rank: Newbie
timestamp: November 08, 2022, 11:08:12 pm
content: How do I find out what the output waveform of these two PLL + VCO ICs ishttps://www.analog.com/media/en/technical-documentation/data-sheets/ADF4360-8.pdfhttps://www.analog.com/media/en/technical-documentation/data-sheets/ADF4351.pdfI guess that the answer is hidden somewhere here -https://imgur.com/a/905MRGubut I am very new to RF and analog and don't really know what to make out of this information.

author: radiolistener
author_rank: Super Contributor
timestamp: November 08, 2022, 11:22:05 pm
content: Quote from: IIOSOH on November 08, 2022, 11:08:12 pmsquare wave with rise time about 70 ps

author: ogden
author_rank: Super Contributor
timestamp: November 10, 2022, 09:49:52 pm
content: Quote from: radiolistener on November 08, 2022, 11:22:05 pmRight. You would not want to pick those to generate *sine* wave of single 100MHz frequency. Not sure to meet your 10$ budget, yet I would take 100MHz TCXO, add LC filter. Maybe amplifier if needed. Then consider it done.

author: IIOSOH
author_rank: Newbie
timestamp: November 10, 2022, 10:01:28 pm
content: Alright, thank you very much. I will probably build every single suggested solution, do some tests and share the results but I also have a few more questions - what ist the most common way to generate a high quality RF sine wave excluding the ridiculously expensive DDS ICs and what should I look for in the datasheets to determine if the output of an IC is a square or sine wave. I am not new to electronics but very new to analog design and don't really know how to read such datasheets yet or what to make out from most of the specifications and parameters. I good book suggestion about analog design for beginners would also be greatly appreciated.

author: ogden
author_rank: Super Contributor
timestamp: November 10, 2022, 10:23:00 pm
content: Quote from: IIOSOH on November 10, 2022, 10:01:28 pm1. LC VCO (w/o output divider!) + PLL - if you want frequency to be synced to reference clock which is < 100MHz, let's say 10MHz. 2. 100 MHz crystal (XO, TCXO) filtered by LC filter and amplified if needed 3. In case frequency accuracy not needed - then just stupid simple LC oscillator.I did not notice any harmonics/spurs or frequency stability specs provided, so you have to pick yourself.  Pick most expensive PLL only when you need frequency tuning.

author: Mechatrommer
author_rank: Super Contributor
timestamp: November 11, 2022, 04:44:23 am
content: Quote from: IIOSOH on November 10, 2022, 10:01:28 pmif you want fixed frequency at cheap arse price, the mentioned TCXO/OCXO whatever crystal clock is your best bet, but then for purer sine, you need to filter RLC and then filter again few stages to get purer sine, since signal got attenuated badly, you will want to reamplify if need to, this call for active opamp gain or transistor based amplifier, and then RLC filter again and again and amplifier again since its not quite there yet until you meet your spec. same applied to your suggested PLL, its just its more expensive option and can output many frequencies, in the middle option between DDS and crystal based clock. is a bit of wormhole if you think DDS synthesizer is an expensive option, free lunch is quite rare this days. ymmv.

author: fourfathom
author_rank: Super Contributor
timestamp: November 11, 2022, 06:27:20 am
content: You probably can't get much cheaper than a simple LC oscillator, but if you're not afraid of a little programming you might consider one of the Si5351 breakout boards (as radiolistener has suggested) and a small controller board, followed by a low-pass filter to convert the squarewave output to a decent sinewave. Using this technique you get frequency flexibility, reasonable stability (the Si5351 board uses a cheap xtal oscillator), and three available outputs. I have this kind of setup stuffed into an Altoids tin (without the lowpass filter) and it's actually pretty handy.Here are Amazon links to likely components:https://www.amazon.com/dp/B09NNVNW7M/https://www.amazon.com/Teyleten-Robot-Frequency-Generator-Breakout/dp/B09DM96KGS/Obviously the lowpass filter has to be designed for a particular frequency, or at least frequency range.