Question regarding function generators - Page 1
Forum section: electronics / rf-microwave

author: jpheim
author_rank: Newbie
timestamp: September 17, 2020, 09:30:49 pm
content: Hello all,First of all I'm a relative noob to RF topics in general and am quite low on the learning curve, so I hope to avoid making any incorrect assumptions. That being said I part of a team hoping to design an FM transmitter for a low earth orbit application at 420MHz. We are in the initial stages of design and obviously we need some way to generate our carrier frequency to modulate in the lab, hence the need for a function generator. My question is can I buy a generator such as the SDG1032X and simply multiply up to my desired frequency with PLL's IC's off digikey, or do I need to spend more to get a function generator that can do that off the shelf?Thank you!

author: bdunham7
author_rank: Super Contributor
timestamp: September 17, 2020, 10:01:17 pm
content: Are you using simple FM modulation or something else like FSK? You can get an old RF generator like my HP8656A or any of its relatives that will both generate your carrier and modulate it for you.

author: fourfathom
author_rank: Super Contributor
timestamp: September 17, 2020, 10:20:34 pm
content: Look at an RF Signal generator. If you want a new one, consider the Rigol DSG800. I have one. It generates a signal up to 1.5 GHz, and has flexible modulation capabilities. Used, that HP8656A that bdunham7 suggested is a good choice.You are also going to want to get a spectrum analyzer to verify the output of your transmitter.

author: jpheim
author_rank: Newbie
timestamp: September 18, 2020, 01:31:37 am
content: Quote from: bdunham7 on September 17, 2020, 10:01:17 pmCurrently it's just simple FM modulation. The HP8656A looks excellent thank you.Quote from: fourfathom on September 17, 2020, 10:20:34 pmI currently have a HP8560A in the lab, so hopefully I'm covered there. Thank you very much for the suggestions!

author: bdunham7
author_rank: Super Contributor
timestamp: September 18, 2020, 02:58:06 am
content: Quote from: jpheim on September 18, 2020, 01:31:37 amI don't know about 'excellent', I only listed it because I have one. You need to be careful buying those because a lot of them don't work and need fiddly repairs. I paid less than $2 per pound for mine, so overhauling the attenuators was just part of the acquisition. Do read the specifications of whatever you get to see if they will actually do what you need them to. I don't know what you are doing exactly, but it seems likely you will want to upgrade your modulation system at some point unless your needs are very basic.

author: tkamiya
author_rank: Super Contributor
timestamp: September 22, 2020, 01:28:42 am
content: Your problem will be stability. SDG1032X has the maximum frequency of 30MHz. Say you multiply that by 20 times.... your carrier frequency will move whatever 1032X drift is times 20. Also, you won't have fine tuning capability. Also, power output will not be adjustable reliably.In a pinch, you can use it but in reality, in any meaningful way, I believe you'll find it insufficient for your purpose. You'll need a proper signal generator. There are many older ones from HP that suits your needs. I'll be more inclined to direct you that way than trying to use a function generator.

author: coppercone2
author_rank: Super Contributor
timestamp: September 22, 2020, 02:41:55 am
content: the mixers are also dirty and require filtering. It is alot of design work to step up a signal, even if you have a stable one to begin withamplifier is easier to design then trying to step something up, because then you need AGC or something, in addition to everything elseupper frequency limits of cheap equipment is a cat and mouse game between using mixers and getting better designed equipment. It is appealing to do for reasonable hobbyists to get into the regime around 50GHz +. I believe it would be considered a hassle to a serious design team to have to use a mixer unless its some impressively high frequency (getting into decimal THz)

author: vk6zgo
author_rank: Super Contributor
timestamp: September 22, 2020, 04:06:06 am
content: Quote from: jpheim on September 17, 2020, 09:30:49 pmProducing a simple low deviation FM signal, then multiplying it up to the required frequency was, in the past, done by simple frequency multiplier stages, where the output of each stage was tuned to a harmonic of the input frequency.This was simple, but as the deviation increased with each multiplication, the initial low deviation was important, & seemingly non intuitively, not always easy.The later, & much more common method, used the "Superheterodyne" principle, where the FM was produced at a lower IF frequency, at the full required deviation then "upconverted" using a mixer & local oscillator, to the required frequency.I guess you could use a chain of PLL's up to 420MHz (or one PLL with a large division number) but the last VCO will have fairly stiff requirements as to it's stability.Finally, this isn't "bleeding edge" stuff----- AMSAT & other organisations have been designing & using satellite transceiver designs in the Amateur Radio 70cm band (just above your target frequency) for many decades.You may well find exactly what you need by googling for "Amateur Satellites".

author: CaptDon
author_rank: Super Contributor
timestamp: September 25, 2020, 05:28:58 pm
content: If you are that inexperienced and don't know you should belooking for a signal generator and not a function generatorwhy are you part of a design team? Also, there exist so manyoff the shelf low power transmitters in that frequency rangewhy do you have to design one? Low earth orbit, so shortlife expectancy I guess as it slows down and crashes back toearth? I think the ham radio community has UHF and earthorbiting items pretty much nailed down. Look in to someARRL books. Hams have had orbiting packages since the 1970's.