Critique my ultrasonic translator circuit - Page 1
Forum section: electronics / projects

author: mark03
author_rank: Frequent Contributor
timestamp: November 06, 2017, 05:33:12 pm
content: Hopefully the schematic is visible below. I'm still learning how to attach images here.This is for listening to bats as they hunt using ultrasonic echolocation. I've experimented with fancier approaches, e.g. digitizing at several hundred kHz and performing downconversion in an MCU, but I wanted something simple and easy to build for a circuits lab I did with some high-school students. All of the passives are 1210 size and the chips are SOIC or SOT. I had the kids apply paste (no stencil) and tweezer the parts onto the board, then reflowed on a hot plate. And it works. The biggest challenge was the MEMS microphone, which only has pads on its underside. Most of the boards needed minor rework for various reasons, but at least half of those were due to problems with the microphone. This is partly my fault for using expired paste. I've had it in the fridge for five years!Anyway, I wanted to see if the experts here had suggestions for improving the design while preserving relative simplicity and low BOM cost (~ $10 including the PCB, in qty 20). I also do not have as deep an understanding of the mixer as I would like. I thought that this fully differential design would be "double balanced" with only the sum/difference signals at C8. In reality there is a large LO signal there, although it is common-mode, not differential. This is not my day job so it's quite possible I have made a blunder or two, and the circuit only works by luckThanks in advance.

author: orolo
author_rank: Frequent Contributor
timestamp: November 07, 2017, 07:22:41 pm
content: A wonderful project, I wish I had the time to try it myself. The bats near my house emit tiny audible chirps, but I wonder what's going on beyond hearing frequencies.The desing looks sound to me. I simulated a simplified version in LTSpice (attached below), with almost ideal switches and LT1006s instead of the audio amp, and the doubly balanced filter seems to work. For an input at 11kHz, and 10kHz switching frequency, only the product at 1kHz is promient at the output of the first amplifier, with much lesser peaks at 21kHz, 19kHz and 7kHz. Nothing at 10kHz or 11kHz. So, if you are getting the LO signal, it could be a matter of the switching circuit: perhaps poor sync between the switches, bad decoupling (don't know the layout, but a cap for each switch might help, instead of a cap from one to the other), or a bad clock signal (I'm sure you probed with an oscilloscope, but the oscillator components are a bit off from the suggested in theNXP datasheetp. 15; using their formula and dividing by 8 I get a frequency in the range 277kHz-40kHz). Other cause of distortion could be variations in resistance of the switch, but the signal level into the switch seems to be at absolut max. about 400mV peak to peak, so it's doubtful it can count too much.Sorry I coulnd't help more. Maybe the simulation can give you some hints. It's a very nice project, thanks for sharing it.

author: mark03
author_rank: Frequent Contributor
timestamp: November 07, 2017, 09:26:02 pm
content: Thanks Orolo! I've attached some photos of the finished project.Quote from: orolo on November 07, 2017, 07:22:41 pmI confess, I never did simulate the circuitI tested it on a breadboard using CMOS 555 LO instead of the 4046, and decided that any unexpected mixer products were probably due to LO duty cycle imbalance. So I swapped in the 4046 and went straight to "production". There is just the one decoupling capacitor for both switches, and it is off to one side, so you may be right about that being a factor.My LO tuning range is fairly consistent across ten copies, from about 20-75 kHz. I did have to play around with the formula and the log pot quite a bit to achieve quasi-linear tuning, and in the end, I had to change the tuning capacitor from 120 to 82 pF to bring the frequency up to where it was supposed to be. Then I screwed up the orientation so I had to cut a trace to have the pot facing outwards with log(exp()) tuning instead of exp(exp()).One question I had about the output stage was how to select the right op-amp. Very few of them specify THD at stiff loads, so I ended up going with the LM4808 which was advertised as a headphone amp. It was dirt cheap, $0.71 in qty 10 at Mouser, but I feel like that component selection could still be optimized, either for cost or performance. I'd like to hear how a more seasoned circuit designer would approach the question.