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Basic Technical Questions

I'm doing my master's thesis in QCM development for bio molecules detection. Its inspired by OpenQCM design (Thanks a lot for making your design and knowledge public).

I've a few questions to move forward with my design.

FreqCount documentation says it works on Arduino upto 8MHz. And for teensy around 60MHz. And in this forum I read somewher that OpenQCM uses undersampling to read up to 10MHz on Arduino Micro using FreqCount. But at which step is this undersampling done? Has OpenQCM modified FreqCount Library ? If so, where can I get that modified version of FreqCount?

What is the exact role of Quartz Crystal driver on Arduino Shield? Is it just converting frequency output of crystal to nice square shape wave with an Amplitude understandable by Arduino?


  • Hi Shabana,
    thank you for your interesting question and sorry for my reply delay.
    We use the original freq_count library without any changes. Unsampling is automatically processed by Arduino. To explain me better, I will give you an example. Arduino has a 16 MHz clock, so it samples correctly up to 8 MHz on a quartz signal that is 10 MHz. This means that by aliasing effect the last 2 MHz count subtracts them to 8 MHz and therefore Arduino believes to measure a frequency of 6 MHz. Therefore the system makes a mistake in measuring the absolute frequency of the sensor, but the frequency variations (those that are important to us) are always appropriate.
    In short, the clock of the Arduino loses the count of some maximums being "slower" than the sensor frequency. It's the same effect you would have if you watched a car wheel spin at high speed. Your eye being slower has the impression that the wheel turns in the opposite direction.
    Regarding your last question, the oscillator driver is a Pierce oscillator that detects the resonance frequency of the quartz, completing an internal phase lock. The output signal is a "sinusoidal" signal. We preferred not to use transistors or other electronic components to square the wave, as we preferred to keep the circuit as simple as possible.
    I hope I have been clear and if you need more information please do not hesitate to contact us again.
  • Hi
    thanks for your response but still i am little bit confused. As you said the output is sinusoidal and didn't use any electronic component to make it square wave. But it is given in the data sheet of Oscillator Driver IC (SN74LVC1GX04) that the output at pin 6 (Y) is fast edge square wave.. So please can you tell me what type of output we will get through Oscillator Driver.
  • You are welcome. In any case for completeness see the attached photo taken with the oscilloscope. As you can verify the signal output from the shield of openQCM, despite the datasheet of SN74LVC1GX04 declares a square wave is really similar to a "sinusoid". But consider that in order to improve performances in liquid environment we changed C1 and C2 with 100pF capacitors.
  • Ok thanks again. I understand the oscillator driver part now.

    And just a note for others reading this. Aliasing is handled in Java code at this line onwards

  • Hi..
    please tell me why are you specifically using quartz crystal of 10MHz?
  • Hi Shabana,
    we decided to use 10 MHz due to their higher sensitivity. Also, undersampling works very well right at those frequencies. The new system that we will launch within few days (openQCM Wi2) will use a Teensy 3.2 board as well as the integrated wireless so we will no longer need to use undersampling and it will be possible to use the device in the whole frequency range from 1 MHz to 20-25 MHz
  • thanku so much
  • Hi
    attached file is the schematics of QCM that i have download from the website. Here you it is mentioned that output is taken from the pin 'Y' of the oscillator driver IC. The output signal is square wave from this pin as mentioned in the data sheet. but you said at above that the output signal is sin wave.. If it is sine wave then output signal should be taken from pin X2.. so is this schematic have some problem? Please clear me this thing..
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