Multiplying two signals

Discussion in 'Wireless & RF Design' started by peter_morley, Jul 19, 2011.

  1. peter_morley

    Thread Starter Member

    Mar 12, 2011
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    I am essentially trying to modulate a signal. I want the carrier signal to be multiplied by the audible signal. I've read a little bit about amplitude modulation but I'm having trouble getting a good result. I did a test in multisim with my own idea of how to modulate the signal but its not quite right. I am not getting a signal on top of the carrier wave but it looks like my carrier signal is being multiplied by the audible signal because the amplitude is changing. I've attached my really simple schematic of my attempt. Help please!
     
  2. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    As far as I know you have to add the second frequency, carrier in your case at the emitter. See the attached picture.

    Please, no comments about the values of capacitors/resistors/or that the circuit doesn't work like this, because the transistor is not correctly biased, no power supply, etc. It's obvious :D. I just threw the values in there to demonstrate the idea. one frequency at the base, the other at the emitter.

    C1 removes the modulating signal freq.
     
  3. peter_morley

    Thread Starter Member

    Mar 12, 2011
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    Thanks for the advice but I don't quite understand what you mean by correct biasing. I changed my schematic and took out the cap and the resistor connected to the collector. I then put a DC source and a resistor in series to the collector. I am getting a weird wave form that doesn't look right. Do I need a decoupling cap to block dc reversal?
     
  4. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    I meant that 1. there is no power supply in the simulation I made, 2. if I couple AC into the input without biasing the transistor it wouldn't work. In the simulation it's achieved because the input signal generator has a positive DC offset.

    The capacitor at the output removes the modulating frequency.

    Did you actually build a circuit, or is it all on simulation? Post the changes you've made.
     
  5. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    You can also do it with the modulation & Carrier drive points reversed. I've used a "tuned" collector load as a variation.
     
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  6. peter_morley

    Thread Starter Member

    Mar 12, 2011
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    I tried to see if my circuit would work with an AM radio receiver because I was having trouble making a low frequency 100kHz receiver. So I'm following the FCC rules with a low power transmitter that has less range than a car transmitter. I first used a sig gen as my carrier frequency (1Mhz) then I wanted to see if I could get the same results with a 555 timer as my carrier. Knowing it doesn't create AC I assumed it wouldn't work but for some reason it does. And I know that it will splatter all across the AM band because of the nature of the square wave ie its harmonics. I attached my schematic with an oscope shot of it. Why is this working?
     
  7. t_n_k

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    Mar 6, 2009
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    I don't see anything on the scope that looks like an AM signal. Although it will probably work in principle. The problem will be that the modulated carrier component may not be a nice sinusoid - hence leading to the presence of other harmonics in the AM signal. You would need to take a zoom in on the AM waveform to confirm the purity (or otherwise) of the underlying signal.

    You can get around that harmonic problem somewhat by making the collector load a tuned circuit - thereby restricting the unwanted harmonic amplitudes. I'd use something less excessive than the 5mH choke in the collector. Perhaps 500uH with 5nF as the resonating capacitance for a 100kHz carrier (collector to ground). For a 1MHz carrier you might use 100uH with 250pF.

    Obtaining a clean AM signal by using a tuned load for an amplifier operating in non-linear mode is typical say of the Class C AM final stage power amplifier.
     
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