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I’m trying to make an ultrasonic distance sensor and got the 40kHz emitter working but am struggling with getting the receiver circuit working properly. At first I just amplified the output of the receiver without filtering and it picked up a lot of noise (mostly around 25kHz), which was screwing up my distance measurements so now I’m trying to implement a bandpass filter. I used a design for a narrow (single frequency) band pass filter #4 on this page and used this calculator to get part values for a filter with a gain of 10 centered at 40kHz and a Q of 10. I simulated the resulting circuit in Multisim and it worked as expected but when I tried breadboarding it the gain was between 2 and 3 instead of 10 (I used OPA2134 op amps for both simulation and testing). Other than the decreased gain it seemed to be working and I wasn’t sure what the problem was so I just connected four stages back to back to get the gain high enough and got a barebones PCB made of that design. On the PCB the gain of each stage actually increased to between 5 and 6 (still short of the expected 10) and they worked independently but when connected together I started seeing a 36kHz oscillation at the output of the filter chain even when there wasn’t any input signal. I read that if a high gain amplifier’s output couples to its input that can make it act as an oscillator so I’m not sure if this is happening somehow (maybe by the output traces crossing the input traces?).

Basically, what I’m trying to figure out is why the gain of my band pass filter is lower when implemented than simulated and why four stages cascaded together becomes unstable?

Current schematic of one stage: enter image description here

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    \$\begingroup\$ Component values are? (better - use the schematic editor for one stage) \$\endgroup\$ – Brian Drummond Feb 9 '15 at 19:52
  • \$\begingroup\$ As Brain says a schematic with values would help. (single supply or bipolar?) (Are you using smallish R's?) The 25 kHz is most likely the fluorescent lights in the room. try turning them off. (Sometimes fluorescent light "go" at closer to 40 kHz.. so look out if you plan on using this somewhere else.) \$\endgroup\$ – George Herold Feb 9 '15 at 20:44
  • \$\begingroup\$ Added the schematic of one stage to the above post. Its using a single supply at 3.3V (which might be a bit low for the OPA2134 since they're rated at +-2.5V but have tried some other op-amps and they work similarly). Will look more into the fluorescent lights, would they be producing sound at 25kHz that the mic would pick up or would it be electrical interference adding noise to the circuit afterwards? \$\endgroup\$ – 23527934593 Feb 9 '15 at 22:01
  • \$\begingroup\$ What about the Q factor (in case of gain=2...3)? \$\endgroup\$ – LvW Feb 10 '15 at 9:36
  • \$\begingroup\$ More than that - the opamp requires at least (+-) 2.5 volts; don`t you think that a single source of 3.3V is a bit too small? Perhaps this limit cannot be seen in simulation? \$\endgroup\$ – LvW Feb 10 '15 at 9:43
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I'm not quite sure what issues you're having with your specific driver circuit, but I do have a question for you: why use an opamp ($4?!) for this? Murata has an application paper out on piezo devices (I assume you're driving a piezo) and they recommend a simple BJT driving a resistor in parallel with the piezo. See page 4:

http://www.murata.com/~/media/webrenewal/support/library/catalog/products/sound/p15e.ashx

Separate your project into pieces. Get your driver in order first, working like you would expect, and then focus on receiving and filtering.

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  • \$\begingroup\$ Sorry if it wasn't clear but the driver and emitter are working fine, this post was only in regards to the filter on the receiver. \$\endgroup\$ – 23527934593 Feb 9 '15 at 21:48

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