Square Wave using 25Mhz Crystal oscillator

[MarkE]

If you want to try out different frequencies, then you might be better off buying a low-cost pulse generator that can go up to 25MHz. 

BTW, unless you are using really small coils, 25MHz will basically be lost on a MEG.  If you really want to play around with MEGs then what you probably want is a signal generator and a power amplifier.  If you are after square waves, or square wave approximations, then a dual MOSFET driver that has some dead time built-in like the TI LM5102, or LM5105 paired to a couple of N channel power MOSFETs would do for the power amplifier.

[TheOne]

If you want to try out different frequencies, then you might be better off buying a low-cost pulse generator that can go up to 25MHz. 

BTW, unless you are using really small coils, 25MHz will basically be lost on a MEG.  If you really want to play around with MEGs then what you probably want is a signal generator and a power amplifier.  If you are after square waves, or square wave approximations, then a dual MOSFET driver that has some dead time built-in like the TI LM5102, or LM5105 paired to a couple of N channel power MOSFETs would do for the power amplifier.

When I talk about Square wave, I will have 2 inputs coil, if the wave is on, the coil1 is energized, when wave is off, the second coil is energized, I will use a NOR gate for that, I dont really need a dead time, unless I don't understand what you try to explain


Thank you for your advise about the 25 Mhz,  so far my 555 timer circuit have 3 setup, one with 1/2 hz, 60hz and 260khz all 50% duty cycle, i wanted to try later with higher value.


What value do you think I should try instead of 25 Mhz?


Do you have link about your "signal generator and a power amplifier"? that can go in the Mhz[/size]

[MarkE]

When I talk about Square wave, I will have 2 inputs coil, if the wave is on, the coil1 is energized, when wave is off, the second coil is energized, I will use a NOR gate for that, I dont really need a dead time, unless I don't understand what you try to explain


Thank you for your advise about the 25 Mhz,  so far my 555 timer circuit have 3 setup, one with 1/2 hz, 60hz and 260khz all 50% duty cycle, i wanted to try later with higher value.


What value do you think I should try instead of 25 Mhz?


Do you have link about your "signal generator and a power amplifier"? that can go in the Mhz[/size]

To make a square wave you need a transistor that when it turns on will pull the output up to a voltage, and for the falling edge a transistor that when it turns on will pull down the voltage.  It is important that both transistors do not conduct at the same time.  If they were to do that, then they would present a short circuit across the power supply which would be hard on the transistors and very wasteful.  Devices like the ICs I suggested impose a little delay, IE dead time between when they turn off one transistor, and when they turn the other transistor on to prevent simultaneous conduction and the resulting destructive shoot-through current.

If you have a two winding coil operating in a push-pull configuration, then when one transistor turns on, the other transistor's collector (bipolar) or drain (MOSFET) will fly up to twice Vcc.  When one of the transistors / MOSFETs turns off, then the voltage at the transistor turning off rises up and the voltage at the off transistor falls down until the magnetizing current in the coil finds a discharge path.  If you turn on both transistors at the same time then the magnetizing flux in each of the two coils cancel and the current through each coil builds-up very quickly to destructive levels.

The bottom line:  Never turn both transistors on at the same time.  Make sure that you insert enough dead time between when you turn the on transistor off and the off transistor on so that they do not both conduct at the same time.

1MHz would be fast for most MEG constructions that I have seen.

This unit for $65.00 will make pulses up to 40MHz for you:  http://www.ebay.com/itm/40MHZ-DDS-FUNCTION-SIGNAL-GENERATOR-DIY-KIT-SPI-I2C-MASTER-SLAVE-PULSE-GENERATOR-/261383840015?pt=LH_DefaultDomain_0&hash=item3cdbb0e10f

[Farmhand]

I built a variable frequency and variable pulse width oscillator with an SG3525, it has two outputs out of phase, so it can run an inverter and such (low side switches) I use mosfets and drive the gates directly from the SG3525 outputs which works OK up to over 110 kHz I can adjust the circuit from less than 25 Hz per switch to 115 Khz per switch using a dip switch with some capacitors and resistors for set timings and some pots for adjustments. It imposes a built in dead time that is adjustable by a resistor. Mine is at about 40 uS I think as far as I can tell from memory, I used a 100 Ohm resistor for the dead time control. If I want just one signal I can double the frequency and halve the load on each mosfet by connecting The mosfet drains together, in that case it is vital to have the "dead" time for the reasons Mark describes, (shoot through current).

AN SG3524 has different outputs to the SG3525 chip and will not provide such a square wave when driven directly from the outputs where the SG3525 will up to a degree, the SG3524 on the other hand will require, (turn off sharpeners of some kind), or mosfet drivers. Both chips have their merits though I prefer the SG3525 for driving mosfets.

One thing I noticed when going into the mHz range with square wave oscillators a while ago is ringing on the circuit rails and in the signal, which heats up the mosfet drivers unnecessarily, which seem to protect the mosfets they don't get hot. Significant current is required to drive the mosfets at that frequency, the drivers need to be up to the job.

There is a very cheap part called a CD4047 Which provides a close to 50/50 duty as standard but has some built in dead time I believe. The 4000 series parts are dirt cheap and do good signals. they can also drive mosfets directly from the outputs of the CD4047.

Get the Book called "The CMOS Cookbook" by Don Lancaster, if you can, borrow or beg for it if need be, it has endless detailed info on Logic circuits.

Cheers

[MarkE]

The 3525 and 3524 are early 1980s era components designed for 20kHz to about 100kHz operating frequencies.   They work great if that is the frequency range where one wants to operate.  Things like the 5102 and 5105 operate comfortably at much higher frequencies.  The important thing is to ensure that there is enough dead time so that no shoot-through occurs.  With modern drivers and MOSFETs 100ns of dead time is plenty.  30ns is often enough.

Ringing in fast rising / falling current circuits happens for a number of different reasons.  Stray circuit inductance is always a factor which comes back to layout.  Beware that stray circuit capacitance is also a factor.  There are lots of techniques for mitigating ringing.  Each has its own trade-offs.  RC snubbers from the drain of each MOSFET to ground are fairly common as are parallel resistor / Schottky diode combinations between the gate driver and the MOSFET gate.