White noise function of Chinese function generators - Page 1
Forum section: electronics / testgear

author: Dodecahedron
author_rank: Newbie
timestamp: September 08, 2018, 06:26:49 pm
content: Good day, gentlemen!I've been looking to buy a function generator and since I don't have much money to spend, being a student, I am being tempted by the cheap Chinese function generators like FY6800 that most are probably familiar with.I want a function generator to play around with filters and other RF stuff at 27 MHz, so a 30-60 MHz Chinese generators should in theory suffice. I also want to use the white noise functionality for the filter testing and so forth. However I can't seem to find any information on white noise specification of these generators. Only on one place on ebay have I found the mention of frequency characteristic of white noise, where it said it was up to 10 MHz for FY6600(which I understood as it being flat up to 10MHz), which kinda put me off.Has anyone had experience with this functionality of such generators, is it really flat only up to 10MHz?

author: JS
author_rank: Frequent Contributor
timestamp: September 08, 2018, 07:45:38 pm
content: Noise in AWG is not the best, being so easy to build a noise generator you shouldn't be too fuzzy about it, use the ARB for what it is but when you need a good source of noise use a BJT be junction polarized as a zener (about 6V) and amplify it to the level you need. That takes two or three transistors, a bunch of resistors and you are good to go, and the generated noise has much better characteristics than the ARB will do, no matter the BW or quality of it.If you are going to do so, many projects out there, check them and pick one. For 30MHz BW there was one particular project around where they build a few in a meeting, and they found all will go over 30MHz, mostly around 50MHz and some much higher. For noise testing you always need to have the noise well characterized, or capture the noise going in and the filtered signal, so you do the math with both references.JS

author: Dodecahedron
author_rank: Newbie
timestamp: September 08, 2018, 08:33:09 pm
content: Thanks for the advice! I'm actually building a noise generator using a small RF amp IC and a zener, I just worry about the flatness up in the 30 MHz range, since it is essential for filter testing, and thought I wouldn't be able to achieve a "factory" quality.

author: JS
author_rank: Frequent Contributor
timestamp: September 09, 2018, 03:11:08 am
content: The amplifier should be properly selected, but the construction will limit the behavior in the end. Dead bug doesn't look nice but usually works the treat for that sort of frequency. In any case, you will be testing this with an instrument, so you should calibrate that instrument with the same source, and characterize it. If you are capturing data with an oscilloscope you should leave a channel to capture data directly from the source so you can use it while processing the data, with the correlated noise used for the measurement. Hence, having a replicated output from the noise source could be optimal.JS

author: Dodecahedron
author_rank: Newbie
timestamp: September 09, 2018, 11:10:20 am
content: The amp I have is BGA2867, I have made a pcb for it some time ago, but without much knowledge of RF layout and what do you expect, it started oscillating at 140 MHz. I guess I could deadbug it, but it being SOT363, it'll be a bit of a challenge with my soldering skills. Although I do not know, which construction would yield better results, a deadbug or a PCB with a bit more thought put into it.

author: iMo
author_rank: Super Contributor
timestamp: September 09, 2018, 12:27:41 pm
content: Quote from: Dodecahedron on September 09, 2018, 11:10:20 amWhile I had messed with MMICs in past I looked at the datasheet recommendation re PCB layout. Frankly, quite surprised how decoupling C position is important here..

author: rhb
author_rank: Super Contributor
timestamp: September 09, 2018, 05:24:55 pm
content: You might take a look at this thread:https://www.eevblog.com/forum/testgear/_nose-source-bg7tbl-2016-03-06_/msg1320375/#msg1320375It's usable with an SDR like the SDRplay RSP2 over an 8-10 MHz span once you get above 4 MHz. Below that there are issues with the pulsed operation. And it definitely not flat over a wide range.There are some other threads about noise sources using transistors within the last few months.You should also read Franco Rota's work.https://www.rf-microwave.com/app/resources/uploads/diodes/VHFComm_NW303.pdfwww.eme2008.org/papers/Lab%20equipment/i5sxn_Noise%20generator.pdf

author: Wolfgang
author_rank: Super Contributor
timestamp: September 09, 2018, 06:48:23 pm
content: Hi, I did some research regarding ARB generated noise generators:https://electronicprojectsforfun.wordpress.com/test-equipment/test-equipment-measurements/commercial-arb-noise-generators/and I also tried some LFSR noise sources:https://electronicprojectsforfun.wordpress.com/making-noise/digital-noise-sources-i-have-built/Results:- Even high quality ARBs need not be perfect white noise sources- Homebrew LFSRs work, but only to about 1/10th of the clock frequency their spectrum is reasonably flat.- If it needs to be very flat, other types of noise sources are a lot betterHope that helpsWolfgang

author: Dodecahedron
author_rank: Newbie
timestamp: September 11, 2018, 06:48:31 pm
content: Since general consensus seems to be that building noise source is the way to go I did build up a diode noise source on a custom pcb with the said RF ICs. First stage looks great noise jumps from -100 to -80 db at 0hz and somewhere -85db at 40 MHz so that's plenty flat for my taste. But when connecting to the second stage with the same amp, suddenly all kinds of frequencies appear (30Mhz, 100Mhz and higher). I guess I should start thinking about shieldingThough I do have a suspicion it might not be the only problem.

author: Wolfgang
author_rank: Super Contributor
timestamp: September 11, 2018, 08:14:21 pm
content: I dont know your amp, but if you already had stability problems this is an indication that some other chip might be more suitable:Some unspecific advice:- Use amps that provide good isolation and that have a good matching to prevent signal reflections and subsequent instability- Not too much gain in one stage.- Perfect decoupling of the power supply rails of the amps from one another. If you dont, this is your feedback path- Proper RF-style PCB layout.- Maybe some shielding- Dont overdrive your amps. Otherwise harmonics and mixing will ruin spectral flatness.- Regulated on-board power supply.Good luck !Wolfgang