True sine wave oscillator for RF - Page 1
Forum section: electronics / beginners

author: Vindhyachal.takniki
author_rank: Frequent Contributor
timestamp: October 23, 2020, 05:41:02 am
content: 1. Looking for a true/pure sine wave oscillator of fixed 40.68Mhz to drive this ckt:https://www.mouser.in/ProductDetail/NXP-Semiconductors/MRF101AN-40MHZ?qs=T8HDo%252BTk69V2c0bO8xVorg==2. Found one source :https://www.digikey.com/en/products/detail/crystek-corporation/CRBSCS-01-50.000/47411423. Most of other sources I found of oscillator are not sine wave, but this company seems to less popular. So any other brand which is more popular. Crystal should be sine wave though only at fixed 40.68Mhz only.

author: Mechatrommer
author_rank: Super Contributor
timestamp: October 23, 2020, 06:00:29 am
content: maybe you can look for DDS IC solution like AD9851, much cheaper, there's module option in ebay if this is one off, con is, it needs programming, you can make dedicated small mcu for it. pro is you dont need much filtering work (china module option) and its frequency adjustable. but if you think thats not suitable, too big, not pure enough or not pro looking, and you dont care about filtering work, then you can just snap any xco -> filter -> buffer or gain amp, should be still a lot cheaper than your CRBSCS-01 stuff.

author: radiolistener
author_rank: Super Contributor
timestamp: October 23, 2020, 06:40:39 am
content: Quote from: Mechatrommer on October 23, 2020, 06:00:29 amAny DDS produce very dirty sine, which consists much more spurs and phase noises than usual crystal oscillator.Quote from: Vindhyachal.takniki on October 23, 2020, 05:41:02 amCrystek is well known brand for ultra low phase noise oscillators.Please clarify - how pure the sine wave do you need?What is acceptable phase noise level?What is acceptable THD level?Basically you can use square wave oscillator with ultra low phase noise and remove harmonics with a good low pass filter. On the output you will have very clean sine. Harmonics rejection will depends on your LPF.

author: Mechatrommer
author_rank: Super Contributor
timestamp: October 23, 2020, 07:15:14 am
content: Quote from: radiolistener on October 23, 2020, 06:40:39 amBPF cant fix that? i was thinking 1 solution fix all in case he cant find XCO closest to the required frequency (luckily this case 40.68MHz XCO is available)

author: George Edmonds
author_rank: Frequent Contributor
timestamp: October 23, 2020, 08:54:48 am
content: HiA good low pass filter will clean up a sine wave, all good RF signal generators do exactly this. Basic rule any RF signals that contain no harmonics are sine waves.Phase noise is a different matter, but a OCXO is by far better in this respect than any DDS.George G6HIG Dover UK

author: radiolistener
author_rank: Super Contributor
timestamp: October 23, 2020, 09:11:36 am
content: Quote from: Mechatrommer on October 23, 2020, 07:15:14 amNo, BPF cannot fix that. Because noise and spurs scattered across the entire spectrum and sitting near the carrier. So, the part of these noises will be passed through BPF.

author: Vindhyachal.takniki
author_rank: Frequent Contributor
timestamp: October 24, 2020, 04:31:19 am
content: 1. Trying to make a blood bag sealer which use RF at 40.68Mhz to seal tubes.2. This ckt has RFin of 40.68Mhz, so need to select right sine wave clock source, then this ckt will beused as amplifer section:https://www.digikey.in/product-detail/en/nxp-usa-inc/MRF101AN-40MHZ/568-14755-ND/97491353. So trying to find right sine wave RF source for it , one I found here:https://www.digikey.com/en/products/detail/crystek-corporation/CRBSCS-01-50.000/4741142%C2%A0Sent mail to this company whether 40.68Mhz they can also make, waiting for their reply.4. ckt in (2), seems to class C amplier only.5. Any other cheaper RF source sine wave? ANother I found is company Rakon.

author: BrianHG
author_rank: Super Contributor
timestamp: October 24, 2020, 05:07:43 am
content: Note that using a 40.68 MHz square wave oscilator going through a tuned LC filter with 1 transistor amp will give you a true sine wave. Example:https://www.mouser.com/ProductDetail/Renesas-IDT/XLH730040680000X?qs=%2Fha2pyFadugdBN1KXC7pqmXkGzXFIIY%2F9j1dla8m4rhPHCFceHq2Aw%3D%3DOutput fed through a 0.2-1pf cap and a tunable inductor feeding the base of a simple npn transistor in common emitter mode will make a clean sine wave. Basically the oscillator is giving the inductor an absolute minimal stable tuned kick while the inductor/transistor is using the inductor/cap as a basic tunes oscillator circuit. Or, you can call this a very narrow bandwidth filter. I would add a second npn transistor in emitter-follower configuration. Another option is using 1 or 2 small signal JFets as they better isolate reflections from whatever your are driving if necessary.I found these small shielded RF tunable inductors are the best for aps between 5 & 60 MHz.https://www.coilcraft.com/en-us/products/rf/tunable-rf-inductors/vertical-mount/slot7/Also get a plastic/ceramic slot tuning tool.Newer third party RoHS versions exist.This is a solution which targets off the shelf in stock components, no custom orders.