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LM331: Generation of harmonic signals of controlled frequency and amplitude

Leonardo Marrone
Prodigy 20 points
Part Number: LM331
Other Parts Discussed in Thread: VCA810

Dear all,

my need is to generate an harmonic signal (sine wave) with a frequency tuned between 0 and 1 kHz and the amplitude controlled between 0 and 1 Vrms. As analogic inputs to control frequency and amplitude, I can provide current and/or voltage (dc) signals. 

I am guessing to use the LM331 to control the frequency and feed the generated signal to a VCA810 (or equivalent) to control the amplitude. 

What is the amplitude of the signal generated by the LM331? Does it depend somehow on the analogic input signal as the frequency (formula to calculate?)  or is it constant?

Could you please confirm that this solution make sense?

Thank you 

Leonardo

  • 0
    TI__Guru* 93105 points

    Hello Leonardo,

    What is the amplitude of the signal generated by the LM331? Does it depend somehow on the analogic input signal as the frequency (formula to calculate?)  or is it constant?

    The LM331 in voltage-to-frequency (V-to-F) mode produces a digital output. The output levels are that generated by the NPN output transistor circuit depicted in Figure 13 the Simplified Block Diagram of Stand-Alone Voltage-to-Frequency Converter and External Components. Therefore, if VLOGIC is +5 V the digital waveform's amplitude will be near 0 V for a low, and +5 V for a high.

    Could you please confirm that this solution make sense?

    Since the LM331 output will be a digital stream it will contain not only the fundamental frequency, but multiple harmonics as well. If you need a sine wave extensive output filtering would need to be used to remove the harmonics. Covering a frequency range of 0 Hz to 1 kHz and providing an effective filter at any particular frequency of interest could prove a difficult challenge. It would seem that a direct digital synthesis (DDS) solution would provide a more direct approach to a sine wave output over the frequency range of interest.

    Regards, Thomas

    Precision Amplifiers Applications Engineering