128Hz Bandpass Filter
128Hz Bandpass Filter
(OP)
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
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





RE: 128Hz Bandpass Filter
Gunnar Englund
www.gke.org
RE: 128Hz Bandpass Filter
We are injecting a 5A 128Hz signal into a remote earth stake and measuring the potential rise. We need to filter out any power system frequency voltages and associated harmonics. ie 50Hz, 100Hz, etc
This is why we use 128Hz as it is removed from any of the above frequencies.
The input voltage could be anywhere upto 10V in bad soil conditions. However, usual volatge inputs are around the 100mV range.
Load impedence is input to Digital Multimeter. Source impedance will vary with soil conditions, etc
Cheers!
RE: 128Hz Bandpass Filter
A very efficient filter topology is the opamp with a Twin-Tee in its feed-back.
Parallel the Twin-Tee with a resistor to get a well defined gain. Remember that band-width is the -3 dB point and that there may be enough unwanted signal left at 100 and 150 Hz to influence the measurements.
With the source (1 - 100 ohms, probably) and load impedances given, I think that a passive LC filter would work quite well. It has advantages like no battery needed, no saturation (within limits) and usually more robust.
If you want to have full control, go DSP or use one of the ready-made filters offered by several IC manufacturers.
Gunnar Englund
www.gke.org
RE: 128Hz Bandpass Filter
The switched capacitor filters are also quite effective at low frequencies.
A gyrator based filter is also a possibility. Gyrators are effectively synthesised inductors using op-amps and capacitors to produce 'inductors' with very high Q-factors which could not be built as wound components (except maybe using superconductors). These allow filters with very high Q and thus very steep cutoff to be constructed.
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I don't suffer from insanity. I enjoy it...
RE: 128Hz Bandpass Filter
How accurate are your frequencies? If the center frequency of your filter is a few hertz off from your signal, you might drop a few dB. If that's an issue, you need to design a filter with a flat pass band.
Check out http://www.linear.com/pc/categoryProducts.do
Linear makes a variety of switched capacitor filter building blocks that should do the job.
Jim
RE: 128Hz Bandpass Filter
You could simply A/D the whole thing at the receiving end and then use an FFT to ferret out your 128 Hz signal. It doesn't seem like you'd need too many bits for the sorts of signal levels you've mentioned: 16-bits 96dB would seem to be plenty. This approach might be extremely applicable if you're planning to use a laptop anyway to log the data. The solution becomes mostly SW with an off-the-shelf A/D card and a cable.
You could (should?) also modulate a code onto your 128 Hz probe signal to help the receiver pull it out of the noise. One example that springs to mind is a Barker Code. Using codes would allow your receiver to dig the signal out of the noise floor. There are coding techniques that allow endless integration. Modulating a code is an extremely powerful technique. The only downsides are maybe extra cost and the time to acquire. Might take several seconds to acquire the code when the carrier is only 128 Hz.
I could go on & on... In summary, if you look at this problem from the view of radio (think: 128 Hz radio link over a wired path), there are a brazillion techniques to extend the capabilities beyond what could be done using a plain old 'TRF' filter.
You should assume that there will be some unknown noise floor even at 128 Hz. Do NOT assume that the frequency is silent other than your signal. Sometimes the noise will be very high amplitude. Make sure your system can deal with the worst case, even if just an error message about noise.
If you don't use an advanced code with this capability built-in, then you should probably include an automatic system to help distinguish your signal from the noise floor in the same bandwidth. Maybe an on-off keying cycle. If you don't, then there would have to be two operators with walkie-talkies yelling back-and-forth, "Turn it off for a second. Is it off? Oh darn, it's still there. Are you sure that it is off?" etc.
Then again, simple filtering might do the job most of the time.
RE: 128Hz Bandpass Filter
in the 128 hz band that not created by your test.
You could build your source to cycle on/off at a 1 hz
rate. Set up the FFT to sum the energy while your signal
is on and subract the energy when your signal is off.
That way you can reject energy from other sources.
Many simply pc data aquisition devices come with FFT
analysis programs.
RE: 128Hz Bandpass Filter
Download a free copy of FilterPro (Texas Instruments) and play with it. You wil see that a Butterworth 6 pole filter still only has 10dB attenuation at 100Hz if you set the 3dB corner at 120Hz. You need to establish some facts about your system. How much 50Hz do you need to filter out? "All of it" is not an answer. Will 45dB do. If not then you need more than 6 poles.
RE: 128Hz Bandpass Filter
I like Logbook's idea about the assymetric filter.
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I don't suffer from insanity. I enjoy it...
RE: 128Hz Bandpass Filter
Any type of active filter, switched capacitance filter, or the DSP approach may create more problems than it solves. Very easy for the filter to be overloaded, or suffer evil effects from clipping, intermodulation, aliasing and so on. Unless you plan to monitor the signal with an oscilloscope, I would be very wary of anything fancy, even if it has some sort of auto ranging on the input.
My vote goes to a completely passive filter. High insertion loss can easily be made up by some additional gain at the output to bring the whole thing up to exactly unity at 128Hz. It would be immune to overload from powerful out of band signals.
What goes in at 128Hz comes out unmolested.
RE: 128Hz Bandpass Filter
TTFN
RE: 128Hz Bandpass Filter
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I don't suffer from insanity. I enjoy it...
RE: 128Hz Bandpass Filter
Higher, but not really that much higher that a well designed system should be fooled.
OP had stated: "...input voltage could be anywhere up to 10V in bad soil conditions. However, usual voltage inputs are around the 100mV range."
Those are very good healthy signal amplitudes. One could easily pull those out of the noise, even the FULL AC voltage of xxx volts riding the same wire (!), using a normal 16-bit A/D. Nothing difficult about the range or ratio of 100mV to xxx volts.
But he still needs a method to distinguish the signal from the noise floor. A simple filter simply won't distinguish 128 Hz signal from 128 Hz noise. Although 128 Hz may seem (mathematically) like it might be a quiet frequency, this ignores broadband noise. AC power wiring can be a VERY noisy place.
If the system doesn't distinguish (automatically), then the operator(s) will have to do so (manually).
RE: 128Hz Bandpass Filter
An all passive filter at 120Hz is going to use components that are far too large for comfort. The Sallen-Key filter should do nicely. I have simulated the response on SPICE with a 6 pole Chebyshev high pass filter and a single pole low pass on the output (1K-470nF). It takes 40ms for the 128Hz signal to ramp up to the required steady state level (6 cycles of the waveform). This is the sort of AC transient I was expecting, but which might not be obvious at first glance.
RE: 128Hz Bandpass Filter
RE: 128Hz Bandpass Filter
I'll bet that almost every noise source on the typical power line (or associated running-parallel ground wire) would NOT be characterized as 'narrow band'. Think about hash, static, contacts opening, switching power supplies, leaky insulators, motors, various equipment, etc. etc. etc.
There will be a broadband noise floor, even at 128 Hz or 130 Hz, or 147.7 Hz, or any other number you care to mention. You're not likely to find a reliable 'clear channel' anywhere on that band (radio-speak).
If everyone is thinking in terms of narrowband noises on the power and ground lines, then they'll be disappointed.
The noise spectrum will obviously vary from place to place, so even making the effort to test a few locations would be useless in so far as trying to prove a negative.
Therefore, the system should include a method to distinguish in-band noise from the test signal. Even 'God-like' <insert your favorite filter here> filtering alone will not do that.
System needs on-off keying at the very least. Can be done manually (just need a paragraph in the instruction manual).
RE: 128Hz Bandpass Filter
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I don't suffer from insanity. I enjoy it...
RE: 128Hz Bandpass Filter
RE: 128Hz Bandpass Filter
How about a fairly simple passive rough as guts front end to strip off all the really high amplitude impuse noise, followed by a narrow band active filter of some type ?
A commercial wave analyzer sounds like an excellent solution, provided there is power to run it where the measurements need to be made.
RE: 128Hz Bandpass Filter
TTFN
RE: 128Hz Bandpass Filter
I'll do some more research with this new information and get back if I need any more help.
Thanks again!
RE: 128Hz Bandpass Filter
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.
RE: 128Hz Bandpass Filter
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.
RE: 128Hz Bandpass Filter
RE: 128Hz Bandpass Filter
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
RE: 128Hz Bandpass Filter
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RE: 128Hz Bandpass Filter
RE: 128Hz Bandpass Filter
TTFN