Hello everyone.
I'm a rudimentary electronics enthusiast --i.e. I'm at all well versed in theory--. I've built a few utilitarian PCB's and circuits.

In the same utilitarian line, I came across a video on youtube (in Spanish) where someone demonstrates a picogram electrobalance based on a quartz crystal oscillator. This person does not go into details into how to build a proper crystal resonator (just suggests using a Hartley or Collpits oscillator) for exciting the crystal. The basic theory of operation is that as the mass of the crystal (crystal+ load) increases its resonance frequency is reduced, naturally. By measuring the resonant frequency of the unloaded crystal, and the resonant frequency of the crystal with a calibration mass, it is possible to deduce extremely tiny masses.

Googling for "Collpits" and "Heartley" oscillators brings up nothing related to quartz crystals, only LC circuits. I read that a crystal does behave like an LC resonator, but I'm in no way qualified to modify either oscillator circuit to use a crystal instead of an LC network. Furthermore, the resonant frequency of such oscillators is fixed, and determined by the components used. Whereas I'd need the circuit to "self-tune" to the new resonant frequency of the crystal after it's loaded by the sample. In the video, the person is plugging the crystal into a signal generator and using a frequency counter. As he loads the crystal with a thin layer of ink, the signal generator seems to self-tune to the new resonant frequency. I'm not in possession of a signal generator.

So, could anyone suggest a circuit that could perform the needed feature of oscillating at the resonant frequency of the crystal and self-tuning? On the Wikipedia article on crystal resonators it's explained such oscillators use positive feedback, but again, I'm in no way qualified to design such a circuit.

Adding to the above, I also wonder if there could be a digital, sort of "arduino-based" alternative for doing this?

Thanks.

 

Welcome to AAC!

So, could anyone suggest a circuit that could perform the needed feature of oscillating at the resonant frequency of the crystal and self-tuning?

You didn't mention frequency or voltage swing requirements, but I'd go with an inverter IC:

This came from Stack Exchange. The stuff I clipped on the right was an amplifier.

The 74AC family is faster than 74HC and has higher output drive current.

 

There is a good reason for calling You Tube the "CARTOON CHANNEL", which is that there is a whole lot of funny stuff presented as reality. Certainly the mass of a crystal will affect the frequency of oscillation. So will the temperature and the cleanliness and the attachment scheme, as well as the voltage and capacitance of the oscillation circuit.

 

Very precise crystal oscillators are designed to operate in a temperature-controlled oven. I'm not sure of your concept of self-tuning. It is possible to monitor the frequency output of an oscillator and modify component values by using a variable capacitance diode (varactor) to pull the crystal frequency over a fairly narrow range.

Phase locked loops in conjunction with a frequency divider chain are often used to "lock" a high frequency oscillator to a lower frequency reference.

We really need more information on exactly what you have in mind. Don't bother with any ambiguous statements like: "it has to be as accurate as possible". Those kinds of statements are helpful at all.

 

This claimed application requires the crystal to be oriented so that items to be weighed can be set on it in a repeatable position without being lost or damaging the crystal. so I am asking for description of that arrangement. I am quite familiar with crystals as used for frequency control in electronic circuits and I can not imagine one of them being used as a weigher platform. (scale platform).
In addition, how would such a scale be calibrated, given the many things that affect a crystal oscillator frequency.
So how was the demonstration scale as shown on the cartoon channel calibrated?? And how were the results displayed??
I understand the effect of changing mass on the frequency but how it could be even slightly useful at all is unclear.

 

Hello everyone.
I'm a rudimentary electronics enthusiast --i.e. I'm at all well versed in theory--. I've built a few utilitarian PCB's and circuits.

In the same utilitarian line, I came across a video on youtube (in Spanish) where someone demonstrates a picogram electrobalance based on a quartz crystal oscillator. This person does not go into details into how to build a proper crystal resonator (just suggests using a Hartley or Collpits oscillator) for exciting the crystal. The basic theory of operation is that as the mass of the crystal (crystal+ load) increases its resonance frequency is reduced, naturally. By measuring the resonant frequency of the unloaded crystal, and the resonant frequency of the crystal with a calibration mass, it is possible to deduce extremely tiny masses.

Googling for "Collpits" and "Heartley" oscillators brings up nothing related to quartz crystals, only LC circuits. I read that a crystal does behave like an LC resonator, but I'm in no way qualified to modify either oscillator circuit to use a crystal instead of an LC network. Furthermore, the resonant frequency of such oscillators is fixed, and determined by the components used. Whereas I'd need the circuit to "self-tune" to the new resonant frequency of the crystal after it's loaded by the sample. In the video, the person is plugging the crystal into a signal generator and using a frequency counter. As he loads the crystal with a thin layer of ink, the signal generator seems to self-tune to the new resonant frequency. I'm not in possession of a signal generator.

So, could anyone suggest a circuit that could perform the needed feature of oscillating at the resonant frequency of the crystal and self-tuning? On the Wikipedia article on crystal resonators it's explained such oscillators use positive feedback, but again, I'm in no way qualified to design such a circuit.

Adding to the above, I also wonder if there could be a digital, sort of "arduino-based" alternative for doing this?

Thanks.

Stack Exchange, IIRC, listed some replies for xtal Clapp oscillators.

 

Stack Exchange, IIRC, listed some replies for xtal Clapp oscillators.

Before spending time developing a circuit, it makes sense to understand exactly how the sample and the oscillating crystal will work out. There are miles of difference between the theory and any usable application of such a scheme.
The reason that the video on the "Cartoon Channel" did not go into detail is probably because it was all a fake!!
It is rather challenging to duplicate results when the original is a fake.

 

The video in the post #1 link apparently has a frequency counter showing the oscillator running at ~2998kHz, and a 'scope showing the frequency as ~4.2MHz. Seems a big discrepancy.