Electronics > RF, Microwave, Ham Radio

Drive a piezo at 6.78mhz

(1/6) > >>

ryba7:
Greetings all !

I want to use a 15mm , 6.78mhz piezo.
I have a class E amplifier ( 400 w)  , output at 50 ohms.
But the capacitance of the piezo is very high ( 10 n F), so when i try to send power, the current is too high and no voltage act to make the piezo move. Usually there is a coil parallel to the piezo to raise the impedance , but here a calculation give a way too small value for it (=-= 50nH), and the cable (1m)  has has a capacity and inductance that act.

How can I make my crystal oscillate?

Thanks.

Whales:
Size/position of piezo = voltage across piezo

Driving AC at high frequencies into capacitors -> very low Z (dead short), ridiculous amounts of current needed to get a target V waveform.

Piezos are hard.  I made a high-voltage amplifier (180V?) to run one a while back for optics positioning, but I could only do a fraction of the full voltage range peak-peak once above a few kHz due to the currents needed & difficulty keeping it stable in a closed feedback loop.  Albeit this was a massive piezo (~1-2uF?)

EDIT: Adding a coil in parallel might allow you to do some clever resonant stuff to avoid having to push/pull all of the current yourself?  But you'll need a really low R coil, a really low R piezo and really low R wires between them.  Your tiny 50nH might make sense, not sure about the feeding cable (maybe worth simulating the piezo C, parallel L and cable L in a spice sim).

ryba7:
I tried LT, only with the cable inductance ;it works theoretically but it is how to make such a small inductance that bother me.
I have a small VNA and tried measuring what should be a theoretical infinite impedance piezo/one turn coil joined to the cable but this is not the case.

What bother me is that you can find 2.4mhz piezo driver by the score on alibaba, so 6.78 should not be so hard...

Whales:

--- Quote from: ryba7 on July 23, 2021, 02:06:51 am ---I tried LT, only with the cable inductance ;it works theoretically but it is how to make such a small inductance that bother me.

--- End quote ---

Indeed it doesn't seem very practical.  You would probably have to build the amplifier ontop of the piezo, then make the inductor using a single turn of thick wire in free air.  Then don't put it near anything other than air and wood.

--- Quote ---What bother me is that you can find 2.4mhz piezo driver by the score on alibaba, so 6.78 should not be so hard...

--- End quote ---

What search terms do you use?  I can only find KHz drivers for ultrasonic cleaners.

Putting MHz into a piezo is not easy unless you can live with very little travel distance.

dmills:
15mm diameter? That is fucking massive for that frequency, not surprised that you are finding it basically undrivable.

When I was doing sonar stuff the usual approach was a fifth order network using a parallel inductor to tune out the fixed capacitance, and often using the transformer leakage inductance to form one of the inductive elements, generally we needed series resistance to kill some of the Q and get a reasonable bandwidth.

Can you get a more reasonably sized element?