Help diagnosing sonic anemometer! First complex(ish) project... - Page 1
Forum section: electronics / beginners

author: Bfreak
author_rank: Newbie
timestamp: September 21, 2024, 09:44:12 pm
content: HI all, first post, go easy on me! Long time fan of the YT channel, but an almost complete novice to electronics and circuit design.As a first 'more advanced' project, I am trying to make a sonic anemometerhttps://www.instructables.com/Sonic-Anemometer/. For now, I'm just prototyping and following this basic design, with the intention of moving it to PCB eventually.Now, everything appears to be set up correctly, and I have correctly managed to get the phase lock loop output to 40khz. Verified with a cheap-o oscillo, and also I can see it on spectroid if I crank it down to under 10k (and hear it, still young I guess)There are two things to note here. First of all, the guide suggests a 10k, and a variable resistor to achieve this. However, the variable resistor I'm using gets to 40khz fine, so the first 10k resistor in series with the variable one is not present on my board.Secondly, and this may be an important clue to whatever error I'm making, pin 4 on the phase lock loop IC (N) still reads 40khz, even when I'm not using the half voltage measuring point connected to pin 9 as outlined here:https://www.instructables.com/Sonic-Anemometer/#:~:text=Temporarily%20wire%20two%2010k%20resistors%20in%20series%20across%20the%20voltage%20supply%20and%20connect%20pin%209%20of%20the%20phase%20lock%20loop%20IC%20to%20this%20half%20voltage%20pointSo, that's as far as I've gotten. When I now measure the phase detector outputs on both the W and E ICs, I just get a constant 1.79v.One thing to note that is VERY frustrating here: I actually was getting good readings off of those pins, changing with airspeed over the sensor array a few days ago. I came back to it the next day, and something mysteriously changed... have now re-built the circuit several times, and cannot figure it out for the life of me.I'm using a *fairly* good benchtop power supply (Maplin N93CX).Any and all suggestions are appreciated. It's 11pm here though so replies will probably come in the morning.Thanks all!

author: jwet
author_rank: Frequent Contributor
timestamp: September 22, 2024, 03:45:34 pm
content: Congrats on getting as far as you have. I played with a kit version of this many years ago- I got it sort of working IIRC.The business with the 10k pot and fixed resistor might matter even though you think you have things aligned- install that R. The VCO output at pin 4 should be at 40 Khz when the VCO input (pin 9) is half scale. What the pin 4, pin 9 test is trying to do is break the loop and get the oscillator "centered" at 40 Khz with V/2 at the input. The oscillator needs to be centered so it can track input frequency or phase changes- its likely railed to one side. As you change the input to the pin 9, the oscillator should change frequency and be 40Khz at mid rail input.The first PLL drives the center driver with its VCO nominally set to 40k. It outputs at the center xdcr and is receives at the N RX. This PLL maintains the VCO frequency by phase locking to the transmitted output. The other two PLL's don't even use their VCO's only their phase detector. The VCO from PLL one is routed to all. They take the PLL 1 coherent oscillator and develop phase signals from the E and W signals.Its likely the first PLL is railed rather than centered. Once it gets running and locked up, the E/W Rx will give phase differences with wind speed. I hope this helps.

author: Bfreak
author_rank: Newbie
timestamp: September 22, 2024, 04:01:54 pm
content: Hi. thanks for the reply!QuoteGave this a go, and was only able to reach 20khz with that extra resistor in place. I'm absolutely not an expert, but is it safe to suggest that 10k is only there because thats just what the designer had on hand to reach 40khz with the variable R they had on hand?QuoteWhen doing this test, do you reckon I should be disconnecting it from pin 2 via the 10k resistor and the 0.1uf cap to ground? Or leave either, or both, in place?QuoteShould I be reading 40khz on on pins 14 of the receiver PLLs? Both are reading a flat 0.37vQuoteThat makes sense, but I can't see how to resolve it.Thanks again for your help!

author: jwet
author_rank: Frequent Contributor
timestamp: September 22, 2024, 04:56:27 pm
content: QuoteThe business with the 10k pot and fixed resistor might matter even though you think you have things aligned- install that R.Gave this a go, and was only able to reach 20khz with that extra resistor in place. I'm absolutely not an expert, but is it safe to suggest that 10k is only there because thats just what the designer had on hand to reach 40khz with the variable R they had on hand?There are formulas for frequency of the VCO vs RC, for R1 and R2. C1 is 2200 pF. These are the pin 11 and 12 R's. Its basicially its1/RC with the C padded by 30 pf, don't care here. The values given in the schematic would give 1/10k(pot at bottom) 2200pF = upper highest, 1/20k 2200pf upper low and 1/10M 2200pF for lower. The 10M is a non sense value- typo? We'll recalculate it. I would make the freq about 40K with the 10K pot centered at 5K and pin 9 at v/2 with the with the pin 2 stuff disconnected. At 5k, you'll get something like 1000 pF for C1. If you don't have these, you can add a 2200 pF in series with what's there. Make the other R 47k or something sensible- this will push it down to about 10khz.Note- the original design used the MC14046B, there are bunch of pin compatible versions of these parts including the CD4046, 74C4046, 74HC4046 etc.. The RC formulas are different- look at your data sheet or just cut and try to get the pin 4 pin 9 test working with the pot somewhere near center.QuoteThe VCO output at pin 4 should be at 40 Khz when the VCO input (pin 9) is half scale.When doing this test, do you reckon I should be disconnecting it from pin 2 via the 10k resistor and the 0.1uf cap to ground? Or leave either, or both, in place?YES- you want to open the loopQuoteThe other two PLL's don't even use their VCO's only their phase detector. The VCO from PLL one is routed to all.Should I be reading 40khz on on pins 14 of the receiver PLLs? Both are reading a flat 0.37vYes when everything is aligned, there listening to the sound but you not making 40K constant yet. Tackle this after the 4/9 test.QuoteIts likely the first PLL is railed rather than centered.That makes sense, but I can't see how to resolve it.Thanks again for your help!

author: SmallCog
author_rank: Frequent Contributor
timestamp: September 23, 2024, 11:13:22 pm
content: You may find this article detailing methods to avoid bird damage to ultrasonic wind sensors interestinghttps://web.archive.org/web/20230417173751/https://library.wmo.int/pmb_ged/iom_116_en/Session%203/P3_21_Schubotz_Intercomp_birdrepellingtechniques_ultrasonic_anemometers.pdfMy personal favorite is the automotive antenna that periodically pops up to poke the birds