AM modulation at 120 kHz using 6218 BNC (250 kS/s)

[M.GH]

Hello!

I am going to drive a coil (about 100 uH and 5 ohm) to transmit sound signal (ex. swept) to a vibrator. The frequency should be around 100 kHz. The first issue was that the card does provide 250 kS/s which is only twice the sampling range I need. As a result, I don't have a nice Sine wave as a carrier so I decided to use a triangular wave which is read from an excell sheet with freq. 120KHz and sample it with 240 k. So sampling is one of the issue I have had!

Second, with a serie tuning circiut (RC) on the transmitter side before the coil, I don't get more than 40mV at the vibrator which is still low and I need at least 100mV. Then I decided to use a parallel circuit to provide more current but I ended up a clipped voltage in transmitter side and as a consequence a distorted sine at the output. I don't know if it is practically possible to use parallel RC for tuining!

DAQ I am using is 6218 BNC!

I appreciate any help in this issue!

All the Best,

MG 

[Henrik_Volkers]

Here is the thing I would give a try:

You get 5V 50mA from the +5V output (or use a external power supply)

You use a digital output in a class C scheme to drive a transistor that run the coil /capacitor circuit.

With the digital output you can use a PWM to AM modulate the 120kHz (or 180kHz 🙂 ) The DO is much faster than AO, and at 120kHz you are close class C anyway 😉

You say AM modulation but whant to sweep the frequency (FM) ??

Is the frequency of high importance? If you want to transmit AC POWER  at resonance : Build a Royer converter with your sending coil and use the AO to control the DC power supply....

And you should try LC in parallel with a 5k in series (to protect the AO)  to the AO.

[M.GH]

Thank you Henrik! 

If I understand well, your idea is using the PWM for modulation and  a transistor outside the card as class C amp. top drive the coil. I have not used PWM so I need to read about that! But what I did is simple multiplication in LV for the modulation (attached pic) and the best is to not have an external module unless I have to. Becasue the card is an HV isolated and if I  use anything else then I'll have go under so much regulation stuff.

About Sweep, my modulating signal is going to be swept sine ( up to 10 kHz) but now , I just use a pure tone of 2 kHz! 

Best,

MG

[Henrik_Volkers]

Sieht aus als wenn Du LV in Deutsch verwendest 😉 .....however...

have you tried this ?

I you run it at 100kHz measure the voltage at the coil .... And run a sweep over the exitation frequency to find the resonance.

(If you have a sine genrator and a scope... even better.)

If you are a student or/and have the sound and vibration toolkit available. just give the Frequency Response with Swept sine a try 😉

Still didn't get the HV part. is your hole usb device isolated?

It will still need power, so you can't use that 5V powersupply to?

(the spec say that the USB621x have a fused 400 mA maximum load , standard USB isolators should provide 500mA (or more) so still 100mA  for a decend coil driver 🙂

what is the difference between a coil hooked up to the 6218 and a coil with a small circuit?

Seems to you want to send power and information (modulated) to another coil.... 

Provide more information and I migth give you some more hints... 

[johnsold]

The reactance of the coil at 120 kHz is about 75 ohms. The total voltage across  the coil including the internal resistance at 2 mA is about 151 mV. Most likely you are hitting the current limit of the AO circuit. You really are going to need a buffer amplifier.  

It is not clear what you are trying to do with the AO. You cannot get any kind of "waveform" when generating a signal at 1/2 the sampling frequency. You get exactly two samples per cycle. What two samples do you want? If you sample at the zero crossings for example, all the samples are zero! At any other point you get samples which can only be meaningfully interpreted as peak values because you have no phase information.

Why do you want a carrier frequency of 120 kHz? If you are trying to drive the device at resonance, what happens if the resonant frequency is above the Nyquist frequency for your DAQ device? How will you sense and adjust for resonance?

I think that before we try to solve the drive and tuning issues, we need to clarify what the real requirements for the system are. What determines the carrier frequency or range of acceptable frequencies?  How much drive current, voltage, or power is required? What determines the modulation frequency, amplitude, and waveform?

Lynn

[Henrik_Volkers]

hi Lynn, it's again the cristal ball issue 😄