• Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

Best way to generate 1Mhz-10MHz?

Status
Not open for further replies.

Speakerguy

Active Member
Hey,

I'm looking for a way to build a frequency generator that's variable in the low Mhz range.

Application is for the main frequency generator of a class E solid state Tesla coil driver. I have a rough idea of what the resonant frequency will be, but it would be nice to have some variability on the control side instead of always adjusting the # of turns on the secondary to change Fres.

CMOS 555 timers can do ~3MHz astable but that's really really pushing it. Any other ideas? It can be square wave, sine wave, whatever - and duty cycle doesn't really matter, so long as the fundamental frequency/period is correct.

And I guess it doesn't need to be variable over that total range (1-10MHz). If I could be able to adjust it from +/- 25% from a center frequency, or even +/10%, that might work too. I won't know til I wind it and fire it up and start drawing arcs what kind of frequency shift and error I might have vs. my calculations.

Any ideas? Thanks!
 

Sceadwian

Banned
Have you looked at Voltage controlled oscilators?
 

MikeMl

Well-Known Member
Most Helpful Member
10:1 frequency range is tough without bandswitching. Google "Hartley Oscillator" or "Colpitts Oscillator". Here is a good overview. ARRL Amateur Radio Handbook would have practical circuits.
 

Sceadwian

Banned
He did say he'd be happy with 10% though.
 

stevez

Active Member
Radio amateurs design/build VFOs (Variable Frequency Oscillators) to provide a source for transmitters and receivers. As already stated, wide frequency spans are tough without bandswitching. Today many vfos are digital and I have seen relatively simple kits for digital vfos with wide ranges. The output might be a bit dirty - suggesting that some filtration might be needed.

You can purchase a low cost signal generator. Alternatively you can find old tube type gear for very little money though getting it to work might be tough. What I have seen is guys take the old tube stuff - retain the coils and variable cap, hardware, etc and replace the other stuff with a solid state designs. I am guessing that these circuits have their problems - harmonics being one of them.
 

Speakerguy

Active Member
I think I will try to measure the Fres of my large coil first, which should be about 600kHz. I just hooked up and measured a 555 timer to see how fast I could get it to go so that I could measure the large coil, and with a CMOS 555 (TLC555 from TI) I can easily get 2MHz on a breadboard setup (about 60/40 duty cycle). I could probably go even higher but the smallest leaded cap I have onhand is 330pf - everything smaller is SMT. I may try and go higher, as the chip looks to have 20ns rise/fall times at 5V. So could maybe I can even get several more MHz out of it than is spec'd and might be able to use it to target the Fres of the smaller coil (shooting for ~4MHz).

Here are some scope shots. Don't ever let anyone tell you you can't get MHz signals out of a 555 - I just hooked this up with spare values on a solderless breadboard with all kinds of parasitics and managed just over 2MHz. Scope bandwidth is 300MHz, so nothing can hide.
 

Attachments

Last edited:

Mike - K8LH

Well-Known Member
Back to your original request --- there's a 1-60 MHz DDS (direct digital systhesis) kit available from the American QRP club that might fit the bill.

DDS-60 Kit


Regards, Mike
 

Sceadwian

Banned
wow.
For the price that's on my wish list.
One problem, though resolution, nothing is listed and the only example I could find had 10hz steps, is that as fine as it goes?
 

Hero999

Banned
An LC oscillator.

A 10µH inductor and a 500pF AM radio tuning capacitor should do the trick.
 

RadioRon

Well-Known Member
But seriously, if I wanted a low noise oscillator, I would build a VFO that tunes from 60 to 70 MHz and then put it into a mixer that is driven by a fixed 60MHz local oscillator. The output would then be low pass filtered below 10 Mhz. This is overkill for the application though. Like I said, just use the VCO in a 74HC4046. Easy as pie.
 

mneary

New Member
wow.
For the price that's on my wish list.
One problem, though resolution, nothing is listed and the only example I could find had 10hz steps, is that as fine as it goes?
The AD9851 chip can be set in incredibly small steps. The PIC software provided might be limited, but you do have direct control of the pins if you want full control of the chip.
 
Status
Not open for further replies.

Latest threads

EE World Online Articles

Loading
Top