Low Pass Filter: Round One

Low Pass Filter: Round One

First attempt at building a low pass filter

Borrowing an idea from one of the low pass filters I had seen previously I eventually designed this circuit.

 

It didn’t look as nice in real life….

The original circuit

 

The long part dangling out the bottom of the photo is the variable capacitor made from printed circuit board and covered in duct tape to make it water proof.

A quick check at a lower frequency confirmed that the oscillator and buffer were working as planned. I had to test it at a lower frequency because the frequency counter of my oscillator does not work above 50MHz. After setting the oscillator to its maximum frequency (about 170MHz) it was time to test the low pass filter and diode detector.

Test 1:

The multimeter was set to read the voltage between the diode detector and ground (across the resistor and capacitor in parallel).

Test 1: Multimeter read 0.3mV when the probe was dry
Test 1: Dry

The multimeter read 0.3mV across the diode detector when the capacitor was in air…

Test 1: 0.1mV dry
Test 1: Dry

But read -0.1mV when I placed the capacitor in water!

Results of Test 1:

I am not able to detect the effect of water on the low pass filter.

Test 2:

Perhaps something will happen if I swap the positions of the capacitor and resistor. This will turn the filter from being a low pass filter into a high pass filter.

Multimeter read 1.5mV when the probe was dry
Test 2: Dry

With the filter components reversed, the multimeter read 1.5mV when the probe was dry.

The multimeter read 4.4mV when the probe was wet
Test 2: Wet

But the multimeter only read 4.4mV when the probe was immersed in water.

Test 2 results:

A small result is still a result. Even if it is not a useful result.

Test 2a:

Thinking that I needed a larger resistor in the low pass filter I swapped the 100Ω resistor for a 1200Ω resistor.

The multimeter read 46mV when the probe was dry
Test 2a: Dry

The multimeter read 46mV across the diode detector when the probe was dry.

The multimeter read 47.8mV when the probe was wet
Test 2a: Wet

Alas, there was only a very small change from dry to wet with the multimeter reading 47.8mV when the probe was immersed in water.

Test 2a results:

Well, something happened.

Overall

The change in the voltage across the diode detector when the probe was dry compared to wet was small, only a few millivolts. In contrast, the multimeter read a voltage of almost 200mV when I held a probe in each hand. (Just in case you are wondering, I feel fine. If it isn’t normal to detect a voltage difference between your limbs, please let me know).

This change is not large enough for me to confidently say that it was due to the presence or absence of water around the probe.

Possible sources of problems:

  1. The probes: Perhaps they are not sufficiently affected by material around them. This could be compounded by the tape which is keeping the water too far from the surface of the probe. It is possible that most of the capacitance of the probe comes from the fibreglass of the probe itself and very little comes from the material that is surrounding the probe.
  2. The diode detector: The diode detector may not be sensitive enough to detect the alternating current running through the low pass filter. Diodes only let current through once the voltage reaches a certain voltage threshold. It is possible that the alternating current does not reach this threshold.
  3. The low pass filter: The filter itself may need to be adjusted to work optimally at 170MHz. Resistor and capacitor combinations may not work at high frequencies.
  4. The oscillator and buffer: Maybe there is not enough power coming from the circuit itself. If there is not enough power coming from the circuit, then there is very little to filter, which means that there would be even less for the diode detector to detect.
  5. The cat: No explanation needed.

Until the next experiments.

 

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