Hi, all!

I came across this circuit that generates a 250 Hz sine wave. I would like to change it to 500 Hz. According to the formula of the phase shift oscillator, the frequency is:

f=1/(2πRC√6).

So, I'm guessing the 47n capacitor and the 4K7 capacitor are used to generate the 250 Hz frequency (actually about 290Hz). I have 22nF caps and 5K1 resistors, that will get me to 580 Hz which is close to what I want (I just want to generate a tone, so it's not crucial that it is exactly 500 Hz).

However, I'm not sure what RC pairs generate the frequency. Is it C1 and R3? C2 and R4? Both pairs? What about R4 and C3?

To add more context, this is for a toy morse code generator (so not a real one that would go into a transmitter), and the input will be a push button. I like the characteristic sound of morse code of a sine wave more than the sound of a square wave.



https://makingcircuits.com/blog/single-transistor-sinewave-generator-circuit/

Thanks for your help!

 

Al three R's and C's add phase-shift to achieve the total 180° shift needed for oscillation, so each RC essentially provides 60° of shift.

 

C1 & R3 + C2 & R4. I think C3 is not critical.
---edited---
I was tough that all three caps are critical. 60+60+60 but right now I don't see what resistive load C3 has.

 

Al three R's and C's add phase-shift to achieve the total 180° shift needed for oscillation, so each RC essentially provides 60° of shift.

That's what I thought, but I'm not sure where R2 plays into the formula. So, I imagine I can do C1=C2=C3=22nF, and R3=R4=5K1. But what's R2?

 

C1 & R3 + C2 & R4. I think C3 is not critical.

So make C1=C2=22nF and R3=R4=5K1 for ~580 Hz, and leave C3 and R2 as they are?

 

But what's R2?

R2 is to get the transistor turned on.
If C3 has a resistor to work into (RC time constant) then maybe it is the resistance of the Base to Emitter junction of the transistor.

 

C3 just needs to be there to add a little more phase shift because as a practical matter, you can't quite get 180° from two resistor-capacitor stages.

 

same
22nF and 14.469kΩ three times
60° 120° 180° sections, a negative of 180°
the amplifier/ filter attenuator another 180° for a total of 360°
When the resistance goes ∇ Down the Frequency goes Δ up

Next calculate the resistor values to bias the transistor's base knowing the supply voltage.
The audio signal strength for toy sinewave output using one transistor.
Unwanted noise would not be a happy toy.

 

A bit off topic but here is a USB powered CW practice keyer module. They are listed for up to ~20USD when bought direct from china.
K4 CW Key Trainer CW Hand Key Oscillator Manual CW Key Oscillator Morse Code Telegraphy Morse Code Practice Sending HAM - AliExpress

 

This makes more sense, there are three identical RC pairs. The circuit I showed had two RC pairs and then a cap and a resistor in a different configuration and also a different value for the resistor.

 

The quality of both the carrier and in QRP and audio can make the difference in a readable transmission.
When built well the 3 phase oscillator is good choice for low distortion.
The external USB and PC software is available and inexpensive. The USB key is ADC to DAC and back out.
Video shows one of the common skip pathways sometimes capability can be from Washington State clear down to Texas.
The problem that he shows is the effect of the strong station. When the receiver gain goes up the noise also goes up.

If an ear trained for 600Hz QRP plus or minus 100Hz and a good headphone set, the signal might cut through the noise.