SI5351 as VFO up in to microwaves? - Page 1
Forum section: electronics / rf-microwave

author: rwgast_lowlevellogicdesin
author_rank: Frequent Contributor
timestamp: August 02, 2016, 08:42:25 pm
content: Im currently using the SI5351 in a project where it produces the carrier wave which is then mixed togather with a signal output in GNURadio, to make a transmitter. Is there any reasonable way to push the SI5351 up to a few ghz, maybe with some sort of PLL or something?

author: rwgast_lowlevellogicdesin
author_rank: Frequent Contributor
timestamp: August 03, 2016, 08:42:58 pm
content: Is this just kind of a stupid question? There are a lot of clock generation chips out there but none quite as neat as the SI5351 becuase it can create any frequency it doesnt appear to be locked by a divider.Now I have never used a PLL chip before but Ive always assumed its purpose is to lock on to the incoming signal and then multiply it to create a higher value clock? If a PLL is the answer Im not even sure where to start.... I have some jelly bean pll chips and CB pll's laying around but I know those arent the answer obviously I would be looking for somewhat of a more "designer chip". Im not even sure if a PLL would work at all.... I would like to be able to generate any frequency up to at least 2.6ghz. I guess I basically want a DC-2.6ghz or higher VFO. Am I totally going about this all wrong?

author: hendorog
author_rank: Super Contributor
timestamp: August 03, 2016, 10:00:41 pm
content: I've been working on a 6GHz synthesizer, bit of a learning curve but that's why I do it:) It uses the MAX2871 chip.That chip has dividers which allow output down to 23.5MHz, so IMHO you would take two of these PLL's and mix the outputs together to get down to DC.The spurs and mixing products would be an issue though and so filtering and carefully choosing frequencies to simplify the filtering are going to be important.Or you could take one PLL and mix it with your SI5351 to do the same thing. The filtering would be more difficult as the difference in frequency between the wanted and unwanted outputs of the filter would be much closer.

author: rwgast_lowlevellogicdesin
author_rank: Frequent Contributor
timestamp: August 04, 2016, 05:02:33 am
content: Well the SI is so cool becuase it will drive 8khz to 200mhz any old frequency, it is square out which is perfect for me all mixers I use do better with square switching. I guess I said DC to 2.6ghz just to say it... DC is not that important lol. I just want a wide band (usp to 2.6ghz) square wave VFO that can be stepped 1khz and also has adjustable drive current. I do realize what I am asking for may not just be in a chip like the si5351... I could look around and figure out how to do mixing of the si5351 with a hi frequency PLL, but maybe there is better ways... I have a few VFOs around for different bands, although these are sin output they give me an idea. Instead of looking at one chip or two chips mixed to give me the frequency I want maybe I I could find square wave synthesizers that act like the 5351 but cover different bands and then use software to stitch them all together making one wide VFO, or does this sound more ridiculous? Im sorry if this sounds tot?lly stupid, I have never given frequency generation a thought past what my micro could do until recently when I started with RF.Right now the SI will work well for a DC-200mhz VFO, getting to 2.6 ghz would be the ultimate goal and may take a bit more study and smaller steps to get there. What could I do to get to at least 500mhz? It would be nice to get at least a dual band VHF/UHF set up going. Is there some way I could much more easily do this using my si chip, run two VFO's at one 1-200 and 201-500?If any of you gurus were to make a digital VFO from 50~100mhz up to 2.6ghz that would allow 1khz steps and current control what chips and methods would you most likely start with?Im going to be using a Pi3 to host my SDRs remotely after I get rtl_tcp running. I know there is a program called PI_TX now which allows CW,WFM,NFM,AM,OOK,PSK and a few others modulation modes to be broadcast very dirty using square waves and it says as long as you filter your transmissions quite a bit then amplify them the pie will work well to transmit up to 700mhz!!! Im stumped and cant wait to play with it, I mean the GPIO pins cant even flip at 700mhz so I have no idea what kind of software majick they may be using to get 700mhz square carriers out of that board!

author: hendorog
author_rank: Super Contributor
timestamp: August 04, 2016, 06:06:48 am
content: There are also DDS's which go close to 500MHz.http://www.ebay.com/itm/new-20-AD9910-Module-DDS-Development-Board-RF-signal-source-software-/131677991730Also you could use a doubler on your existing device, or just filter out and optionally amplify one of the existing harmonics. Since it's square wave output that implies odd numbered harmonics will be high.https://www.google.co.nz/search?q=square+wave+harmonics&oq=square+wave+harmonics&aqs=chrome..69i57.4374j0j7&sourceid=chrome&ie=UTF-8

author: rwgast_lowlevellogicdesin
author_rank: Frequent Contributor
timestamp: August 04, 2016, 07:10:12 am
content: Awesome chip suggestions, and awesome advice/tips.Is there maybe a a good solid chapter in one of the theory books or even something like the art of electronics that can get me a little deeper in to understanding the problems which arise with different solutions and the theory to help midi gate them? Im sure I could try my hand at a few techniques listed here and read some tidbits but I have no way to measure phase noise etc... so this is one of those times where reading theory and tried practical solutions with examples may be in my better interest than running down a certain path and running in to problems I cant see!

author: CJay
author_rank: Super Contributor
timestamp: August 04, 2016, 05:18:31 pm
content: Worth having a look at the service manuals and schematics of a 'shack in a box' Ham rig like the FT817, the PLL 'block' in that radio is a nice, easy to follow example of how it's done, three or four VCOs, a PLL and a DDS chip all working together to make a band switched signal source.

author: rwgast_lowlevellogicdesin
author_rank: Frequent Contributor
timestamp: August 05, 2016, 05:46:13 am
content: Thanks I will look in to that, I know nothing about commercial radios for the most part so without that tip id never even think to check how an FT817 works!I just remembered Mike Ossmans HackRf SDR is open source and it also has TX from 1mhz to 6ghz, probably wouldn't hurt to figure out what he does too.