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Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
I've made frequency modulated plasma speaker where I input the audio to the Rt pin of the SMPS IC (SG3525). I understand this is not the best approach, and I was wondering what chip would you recommend for this task (voltage controlled frequency modulation)?
Registered Member #1232
Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881
I have used the technique you described and it works fine. The important thing is to inject the audio via a resistor and capacitor in series to the Rt pin, with the usual resistor still in place to set the quiescent frequency.
...but you could search for dedicated VCO chips if you prefer. The popular 4046 PLL device incorporates a Voltage-Controlled-Oscillator which has good linearity.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
GeordieBoy wrote ...
I have used the technique you described and it works fine. The important thing is to inject the audio via a resistor and capacitor in series to the Rt pin, with the usual resistor still in place to set the quiescent frequency.
One thing I have noticed is that there is some difference in sound perofrmance if you use larger Ct and smaller Rt, or smaller Ct and larger Rt, to get the same running frequency. From my observation, larger Ct and smaller Rt results in more volume. However I'm not 100% sure about that.
How important is the resistor in series with the audio input?
Registered Member #1232
Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881
The ramp generator in PWM controller ICs usually uses something called a "current mirror". This arrangement of transistors charges the timing capacitor Ct with a current identical to that current which flows out of the Rt pin and through the timing resistor Rt. It is done this way so the ramp is highly linear because thats what you ideally want for PWM.
When you feed current from an audio source into the Rt pin of the chip you are adding or subtracting from the current already been driven down through Rt. The current mirror copies this new varying current through Ct and hence the charging slope (and the frequency of oscillation) changes.
The relative sizes of Rt, Ct and the resistor through which you introduce the audio modulation dictate the quiescent operating frequency and the depth of the frequency modulation. Since you are modulating the charging rate instead of the period, the modulation should be quite linear.
It's still a rough way to achieve FM, but it is sometimes used in SMPSUs to modulate the switching frequency in response to the mains voltage waveform. Kind of "spread spectrum" for SMPSUs in order to lower the peak EMI emissions! (A dirty trick in my opinion )
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Control the frequency with voltage
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