NerdKits - electronics education for a digital generation

You are not logged in. [log in]

NEW: Learning electronics? Ask your questions on the new Electronics Questions & Answers site hosted by CircuitLab.

Basic Electronics » Oscilation circuits

March 29, 2011
by Hexorg
Hexorg's Avatar

Hello everyone, I've come up together with a few circuits that will produce an oscillating signal with variable frequency. Unfortunately, I couldn't remember some formulas, and I don't have the time right now to look for them online. You are more then welcome to correct me, and add anything. :)

Here we go.

Manual starting Wien-bridge sine oscillator:

C1 = C2 = C
R1 = R2 = R
V1 = V2 = Vout (peak)
F = 1 / (2*pi*R*C)

To begin oscilations, set (Rf/Ri) > 3, then to keep the stable oscillations, set (Rf/Ri) = 3

Self starting Wien-bridge sine oscillator using zener-diodes:

A little modification allows for a self-starting oscillator. 
C1 = C2 = C
R1 = R2 = R
V1 = V2 = Vout (peak)
F = 1 / (2*pi*R*C)

Zeners' Vref should be smaller then V1
This is easy to build, but will NOT result in a clean sine-wave

Self starting Wien-bridge sine oscillator using JFET:

C1 = C2 = C
R1 = R2 = R
V1 = V2 = Vout (peak)
C3 is electrolytic (!)
F = 1 / (2*pi*R*C)
Rf= 2 * (R3 + r'ds)  !!! r'ds is a drain-source resistance of JFET (look in JFET's datasheet for this value)

Triangular-wave oscillator:

The first op-amp works as a comparator, second - as an integrator.
f = (R2 / R3) * ( 1 / (4*R1*c))

Square-wave oscillator:

Unfortunately no formulas.
March 30, 2011
by Ralphxyz
Ralphxyz's Avatar

Nice Stan, thank you.

The op-amp is a 741 or equivalent correct?

V2 would be a negative power supply correct?

Now some circuits driven by the mcu would be interesting.

Pin change high/low and PWM signals.

Thanks to the formulas I should be able to work out the parameters so that I can learn how to read/understand my oscilloscope.

I have my Forrest Mims Electronics Tablets with similar circuits but I like having them here.

Ralph

March 30, 2011
by Hexorg
Hexorg's Avatar

Yes, op-amp is 741 (those guys are really useful).

Yes, V2 is negative, sorry I didn't mark them, but generally on a DC power supply symbol the side that ends with a longer line is positive, and the side that ends with a shorter - negative.

If you want something cool, driven by the MCU, try this:

Integrator:

C1 = C
R1 = R
Vc = ( Ic / C ) * time
Iin = ( Vin / R )
Ic = Iin

( Change in Vout ) / ( change in time) = - ( Vin / ( Ri*C ))

Differentiator:

R1 = R
C1 = C
Vc = Vin
Ic = ( Vc / t )*C
Vout = Ic * R
Vout = -( Vc / t )*RC

Digital to Analog converter:

This can be a really useful circuit. Not too expensive either - for an N-bit converter you need 1 op-amp and N+1 resistors. It just works as weighted summing amplifier. Resistor 2R is twice as resistive as R, 4R is 4 times, and so on. You HAVE to keep this pattern.

LSB Vout = -Vin*(Rf / R), since TTL levels are 0 and +5v, we can rewrite that
Vout = -5*( Rf / R ) when we input 1 on LSB

The whole expression is (D0 - voltage at LSB; D8 - voltage at MSB):

Vout = -( D0*(Rf/R) + D1*(Rf/2R) + D2*(Rf/4R) + D3*(Rf/8R) + 
           D4*(Rf/16R) + D5*(Rf/32R) + D6*(Rf/64R) + D1*(Rf/128R))

Note the "-" at the beginning. If you want positive output, just pass it through an inverting amplifier. The output of this will be composed of a series of "steps" of voltages, instead of smooth analog signal, this is known as alaising. To smooth out the signal, (operation known as anti-alaising), just connect the output of the DAC to an integrator mentioned above.

March 30, 2011
by Hexorg
Hexorg's Avatar

Oh! it's off-topic, but I'm getting the same scope XD

March 30, 2011
by bretm
bretm's Avatar

The main problem with that type of DAC is that you need very accurate resistors. In order for R and 128R to be in the same ratio, you need better than 1% tolerance, or else test a lot of resistors to find ones that match the best.

If you actually want to build this circuit for use with the MCU, I'd stick to 6 or fewer bits (stop at 32R).

March 30, 2011
by bretm
bretm's Avatar

(or use a resistor ladder)

March 30, 2011
by Hexorg
Hexorg's Avatar

bretm, Yea you are right, if you really need a precise DAC. If someone just wanted to test their oscilloscope with it, or just to play around, that'd work with any 10% tolerance resistors :)

March 30, 2011
by bretm
bretm's Avatar

But if you don't need a precise DAC, you can just use 5 bits instead of 8 in which case 10% is appropriate anyway.

Post a Reply

Please log in to post a reply.

Did you know that the printf format string "%.3f" will show three digits after the decimal point? Learn more...