signal generator circuit

A 555 timer IC is the easiest solution, but will provide a rectangular wave of modest frequency stability (with the right timing components).

Tell us more:

1/ accuracy and stability 2/ waveform and allowable distortion if you need a sine wave output

Not sure what the frequency spec is for the 555, I thought it tailed off somewhere around 200kHz (according to the data sheet)

Many years ago I built the Manor supplies TV pattern generator which used

555s for both timebases - the 15.625kHz 555 couldn't be set reliably, the frequency was all over the place - in the end I had to build a XCO TTL timebase to replace the 555 circuits.

A great many "cheap gadgets" use 400kHz resonators well suited to an unbuffered CMOS inverter oscillator, the output is sort of sinusoidal if a bit low amplitude, but these days OP-AMPs that can handle those sort of frequencies aren't as expensive as they used to be.

I was thinking of the pre-CMOS version.

the multivibrator doesn't necessarily have to run at the signal generator's intended output frequency. check this out:

The discrete multivibrator on the left of the diagram obviously runs at a fraction of the 455 kHz output frequency. Maybe the OP could use a scheme like that?

"kell"

** The 455kHz oscillator is on the right side of the schem, consisting of the ceramic filter, T3 and the tank circuit.

The rest on the left is just an audio frequency modulator.

...... Phil

The resonant frequency of an LC tank circuit is f = 1/(2 Pi sqrt(LC))

It would help if you'd specified what you wanted/needed this for.

The easiest, for someone wondering about inductors, would likely be a crystal oscillator at some higher frequency, and then a divider to get it down to the desired frequency. So a 4MHz crystal oscillator and a decade counter would get you a nice 400KHz signal that only requires wiring. Get one of those 4-pin ttl oscillators to take care of the oscillator. You need to pick a frequency that will divide down to 400KHz, unless of course you can live with some frequency close to that and then you have a more open choice for the oscillator.

Someone mentioned ceramic resonators, and that would get you an exact

400KHz signal, though you might have to order the resonator. They are right, you do see them in things, but I can't say how often I see them so you might have to work through a lot of scrapped electronics before you find one at 400KHz (though, again if you can live with something a little different in frequency, you will have easier choices). You'll have to wire up a crystal oscillator to use it.

Most common radios have a 455KHz (or more recently, 450KHz) intermediate frequency, which means the IF transformers in them will likely be a good start to an LC oscillator, though you'll need to add an oscillator, and add some capacitance to tune the transformer down to 400KHz. And you have to pull out the right one, since there will likely be 10.7Mhz IF transformers in the same radios for the FM section.

If you don't need exactly 400KHz, you have more flexibility, since then you don't have to worry about tuning it. If it just has to be somewhere around there, the choices open up. If you need a sinewave, that will change the solution.

Michael

Ach! Ignore those 555ists. ;-)

For what you're asking, since you've asked nicely, the frequency is determined by

f = 1 / (2 * pi * (sqrt( R * C))

That's the resonant frequency. Their internal resistances, and the added resistor, will affect the Q, or "quality factor" of the circuit, which is related to how sharp of a resonance peak you have.

Well, that's your tuned circuit - that's how you select a frequency.

But, to oscillate, (which is what you're trying to do) you need some active element, to feed back the signal on itself, so that it will maintain the oscillation.

Google things like "RC oscillator circuit" - what you find will probably lead to further googling, and so on.

Good Luck! Rich

Why not make use of a Astable Multivibrator Circuit. Only two transistors, two capacitors, and four resistors are needed. This circuit produces a square wave. Regarding the frequency of 400 KHz, the product of the base resistor and base capacitor should equal 1/400 ms (Time Constant).

Sincerely,

Bugs_Bayani

Typo-- Delete R, insert L.

Yes. Thanks. :-)

Rich