acoscillator
Can the rms AC voltage change with frequency?
I have connected a DSO to the audio oscillator(having 10V p-p). Peak to peak voltage remains constant on changing frequency(10 Hz to 100KHz) but on measuring the rms voltage with a multimeter, the voltage varies drastically(falls off from 3.4V at 500Hz to 0.5V at 50KHz).
Is something wrong with the oscillator/multimeter? Can AC rms voltage vary even when peak to peak value remains constant?
For a low distortion audio sinewave, the Wien Bridge oscillator is widely used:
The RC filters provide a 0 degree phase shift at the desired frequency providing positive feedback to keep the oscillation going. You can think of them like a high pass filter followed by low pass filter to give a bandpass response.
The negative feedback needs an exact gain of 3. Since component values vary in practice, we can't just use two resistors, we need an AGC (automatic gain control) In the circuit shown this is achieved using the lamp as the bottom of the resistive divider. The lamp is like a resistor with a PTC (positive thermal coefficient) or a PTC thermistor. So when the voltage rises on the output, the lamp heats up and the resistance rises. This causes the voltage drop across it to rise and more negative feedback to be applied to the opamp inverting input, therefore reducing the gain and keeping the circuit stable.
The frequency is controlled with a dual gang pot (VR1A and VR1B)
The circuit shown should give from ~145Hz to ~1590Hz. Obviously you can adjust the component values to give different ranges. The formula is: f = 1/(2 * pi * R * C) with R1 = R2 and C1 = C2.
So for the pot at max (100k + 10k) we get:
1 / (6.28 * 10e-9 * 110e3) = 144Hz
and with the 100k pot at minimum (10k) we get:
1 / (6.28 * 10e-9 * 10e3) = 1592Hz
ESP has a some good info on audio oscillators and plenty of example circuits.
The other option if you are familiar with microcontrollers is digital synthesis. You can get far more control but you need a quality DAC to give comparable THD to the circuit above.
I made a little audio test oscillator which gave excellent results from a dsPIC33FJ64GP802 which has 2 good quality 16-bit audio DACs onboard. Simply feed the outputs into an opamp (and write the code of course)
There are also function generator ICs out there that could do the job, have a look on Mouser, Farnell, etc. You could also add a (preferably quite sharp) low pass filter to Russell's 555 circuit to give you your sine wave if you don't mind a bit of extra distortion.
This is some pretty old advice, and I don't know if it will be relevant for your micro, but about 4 years ago I did a project with a PIC18F which encountered strange spurious resets. After reading the report and re-jogging my memory here is what seems to have solved it:
Do you have the Low Voltage Programming Enable configuration bit enabled? Is your PGM pin on PORTB? If so, you may want to consider disabling both Low Voltage Programming Enable and Port B A/D Enable to digital inputs on reset. According to my old report, what was happening was we left PORTB floating while they were analog inputs and triggered the PGM pin. Looking back I don't know if this diagnosis was correct, but we did end up finishing that project successfully so it may be worth a shot.
Best Answer
Most multimeters aren't designed for the frequencies you are using. They are generally intended to be used at around line frequency (50hz or 60Hz.) Some will also work at higher frequencies, some won't. Around 10Hz you aren't too far from what the meter expects, so it works OK. At higher frequencies, it won't work so good.
The multimeter will have filters on its input that limit the frequency range of the input signal. Check the specifications of your meter to see what range it is guaranteed to work in. You may find that it is only specified for, say, under 200Hz or maybe just at 60Hz or just at 50Hz.
Here is a good multimeter, and it only goes upto 30kHz.
In response to comment:
Your Fluke 115 is only specified for up to 1kHz when measuring AC RMS voltage. Above that, anything goes. So, your meter is misleading you.