128Hz Bandpass Filter 2

[JBirch]

Need to build / buy a 128Hz band pass filter. Having trouble finding any information regarding design, components required etc.

Needs to have a gain of 1, and a bandwidth of 10Hz. Needs to be an active filter to keep losses to a minimum.

Thanks heaps in advance,
Jamieson

 

[IRstuff]

Since you are doing synchronous detection, a PLL should be ideal. It essentially ignores out-of-band signals and noise, and will work well at averaging out in-band and even on-frequency noise.

TTFN

 

[JBirch]

Thankyou very much to everyone for their help. There are certainly a few options to explore here.

I'll do some more research with this new information and get back if I need any more help.

Thanks again!

 

[jimkirk]

Also try doing a search for "lock-in amplifier".

If you can use your signal generator as a reference, use that as the carrier input to a balanced (de)modulator (look up the MC1496 for reference information). You can also build one with some CMOS switches (CD4066 for example). Amplify the signal you're examining and apply it to the demodulator input. Low pass the output appropriately, adjust the phase of the carrier appropriately to maximize the output (at 128 hertz, phase probably won't be a problem) and the DC component is proportional to your 128 Hz signal.

Looking up "homodyne" may also give you some leads. This is pretty much the same as above, but with a local oscillator sycnhronized to the carrier, where the lock-in amplifier uses the carrier itself.

 

[bogeyman]

Go for the wave analizer mine cost £15, its old, obsolete, filled with germanium transistors like grandad used but with a 3Hz passband you will have no trouble getting good results.
Get one from ebay, the TF2330 can run of 24-30 volts of external battery (pos earth!) with about 60mA drain.
The TF2330 has a superb quartz crystal IF strip that you could not build for less than the price of a set of 3 matched 100Khz crystals, probably several hundred pounds for these alone.
IF you get one dont fiddle with the IF alignment it takes weeks to get it just right.

 

[cbarn24050]

I paid £10 for mine, including manual. I had to replace some of those transistors but silicon ones seem to work just fine.

 

[Cappels]

Here are a couple of lock-in amplifier experiments.

I have also used a 7 bit Barker code, with some success, in place of the 1 kHz square wave shown in the experiments.

http://72.41.86.92/dproj/dlmom/dlmom.html

http://72.41.86.92/dproj/syncdet/syncdet.html

 

[Cappels]

For the record, here is the experiment using the 7 bit Barker code, which also includes a phase-locked loop to phase lock to the incoming code.

http://72.41.86.92/dproj/selflocksyncdet/Synchronous_Detector.html

 

[VEBill]

Extremely cool.

 

[IRstuff]

Great writeups as well.

TTFN