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Converting a 50kHz high pass filter to a 5.3kHz HP filter

  1. Aug 19, 2015 #1
    1. The problem statement, all variables and given/known data
    Converting a 50kHz high pass filter to a 5.3kHz high pass filter.

    The circuit:

    2. Relevant equations
    Working out the Capacitance, C, and Resistance, R, of the circuit in order to work out the cut-off frequency, Cf, using:
    Cf = 1/(2*pi*R*C)

    3. The attempt at a solution
    I've managed to work out how to get to the 50kHz Cf, (R=63.63 ohm, and C=5e-8 F)

    But I don't know what to do in order to get the 5.3kHz Cf, should I adjust just the value of C, or R or both ? What is the most sensible thing to do to preserve the signal quality ? and circuit safety.

    NOTE: this is not a homework question but more a hobby exercise.
  2. jcsd
  3. Aug 19, 2015 #2


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    Staff: Mentor

    The resistors have been chosen so that the circuit this feeds "sees" an impedance here of around 50 Ω, so you probably won't want to change the resistors if that impedance is important. The resistors are in a resistive divider arrangement so provide some attenuation. Just change the C proportionally for the new frequency. It's not a very sophisticated filter. What signal are you trying to retrieve from the mains wires?

    This arrangement is safe only until there's a capacitor breakdown, at which time this connects the HF OUT terminal directly to the mains. It should be arranged so that no part of this circuit, nor of the entire circuit it feeds, can come into contact with anyone or anything metal. That means fully enclosing it all in a proper insulating box with no wires out and no exposed conductive material or metal.

    If these safety measures can't be provided, the circuit should not be constructed.
  4. Aug 19, 2015 #3
    Thank you for the reply.

    I am looking to monitor EMI from LCD screens on the power line (I expect to detect a signal around 60 kHz).

    Concerning safety, I am really aware of it. I was thinking of using a isolating transformer but I was told it would greatly damage the signal quality. Do you have any suggestions ? Since HF OUT is going to be connected to a microcontroller and the microcontroller to a PC I would like to prevent any contact with the mains in case of a component/circuit failure.

    For more info on the project : https://www.google.co.uk/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&ved=0CCcQFjABahUKEwjL9dfEoLXHAhWBtBQKHd4yB7Q&url=https://dub.washington.edu/djangosite/media/papers/uTouch_CHI2013.pdf&ei=4YTUVYuBD4HpUt7lnKAL&usg=AFQjCNEGxbuP4_WeL03Gq35y1kAyTbvdpQ&sig2=kGfBz7Orua7U-H8pQYCuoA&bvm=bv.99804247,d.d24&cad=rja
    Last edited: Aug 19, 2015
  5. Aug 19, 2015 #4


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    Staff: Mentor

    Transformer isolation would be the ideal. What upper frequency are you interested in?
  6. Aug 19, 2015 #5
    The paper I'm basing my work on doesn't mention any. Most of the LCD row rate frequencies will be around 45 to 70 kHz I suspect.

    Here is the protocol as described in the paper:

    "To measure the EMI on the power line, we used the same hardware used in LightWave [4]. An analog high-pass filter (HPF) with a 3 dB corner frequency of 5.3 kHz is used to reject the strong 60 Hz component. The output of the HPF is sampled at 1 MS/s using a 12-bit analog-to-digital (ADC) converter in the USRP (Universal Software Radio Peripheral) followed by transforming the signal into frequency domain using a 32,768-point fast Fourier transform (FFT), yielding a frequency resolution (or bin size) of 30.5 Hz. The signal from the USRP is then fed into the computer for data analysis. It should be noted that a USRP was used in this prototype simply for convenience."
  7. Aug 19, 2015 #6


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    Staff: Mentor

    So you are interested in the 5kHz...100kHz range. That sounds like the audio range, (no pun intended) meaning you have frequencies that a good quality audio transformer will handle. So I'd be looking for an audio transfomer rated to the same voltage and isolation standards as is a mains isolation transformer. Does such exist? I have no idea, but I've never needed to find one.

    Others may be able to suggest a solid-state device which can achieve this isolation. You could build something incorporating optoisolation of the output from the mains side: after the filter would be a LED-photodiode combination with the mains side powering nothing more than the filter and LED, meaning very little of the circuit need be on the hot side.
  8. Aug 21, 2015 #7
    Thanks for the information. Could you tell me what voltage would go through the microprocessor with the circuit from the diagram ? I am not clear at how to reduce the voltage enough not to fry everything.

    Also, I thought the audio range handled by audio transformers was 20Hz to 20 kHz only ?
  9. Aug 21, 2015 #8


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    Staff: Mentor

    Reviews of high quality audio transformers speak of finding them being flat to within 1dB out beyond 100kHz., e.g., http://sound.westhost.com/articles/audio-xfmrs.htm

    You may need to approach manufacturers to discover whether they make (or can make) these with ratings compliant with mains operation.

    The filter you present is a single pole HP, so the attenuation it gives follows that standard graph.

    It would be unwise, with whatever method you use, to not include a voltage limiter for the recovered signal. A couple of diodes in series will clamp the voltage to around 2v, yet have practically no effect on levels under, say, 1.2v, and you do need to accommodate peaks of both polarities.
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