• State TI Thinks Resolved
  • Locked Locked
  • Replies 16 replies
  • Answers 1 answer
  • Subscribers 50 subscribers
  • Views 1888 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

MPY634: Need help in resolving the asymmetrical output problem in using MPY634 as an AM modulator.

Prakash Rodrigues
Prodigy 80 points
Part Number: MPY634

I would like to use the MPY634 as a AM modulator.  The circuit is wired exactly as mentioned in the datasheet figure 9 page 8.

Modulation input Em is + - 5V @ 150 Hz (square wave); Carrier input  Ec= + - 8V @ 2 Khz (square wave) ; Power supply is + - 15V DC.

SF pin is left open.

Problem:
When Em is 0; and Ec is + -8V @ 2KHz; The output is same as the input (Ec) as expected and clean and symmetrical.

Now when Em is slowly increased to + -1V / sq wave ;  the output is AM modulated wave; but there is asymmetry in the waveform with the negative peak at ~-13 V pk and positive peak at ~+9 V pk.

Also the depth of modulation is unequal across the x axis.

Addition to this, there is a waveform distortion at the output. 

Query:

1) Any help on what could be the issue?

2) What is the role of the SF pin? The datasheet is not very clear on this.

3) How to smoothly vary the Em signal and obtain a nice AM moduated waveform without distortion?

Thanks in advance for the help.

  • 0
    Guru 140200 points
    Hi Prakash,

    how are you driving the inputs?

    Kai
  • 0
    TI__Guru* 93105 points
    Hello Prakash,

    You mention that your modulation signal Em is a square wave. As you are certainly aware the spectral content of the square wave is more complex than that of a simple sine wave. You are now dealing with the waveform symmetries of the all the odd harmonics comprising Em. I suspect that any non-symmetry in the square wave content, including the rise and fall characteristics, will result in output asymmetry.

    Try changing the modulating signal to a symmetrical about zero sine wave and see if the output attains symmetry and distortion is reduced. If it does, then this may provide some hints on how to improve the amplitude modulation performance. If it doesn't improve, we'll have to look elsewhere for a reason.

    The MPY634 Scale Factor (SF) is equivalent to a gain factor in the multiplier equation. It is set to 10 V, but can be set to any value between 3 and 10 V. Since it is in the numerator of the equation decreasing SF, is equivalent to increasing the gain.

    Regards, Thomas
    Precision Amplifiers Applications Engineering
  • 0 in reply to kai klaas69
    Prodigy 80 points

    Hi Kai,

    The Ec is generated from a OPAMP square wave oscillator circuit with peak of + - 13.5 V ; while the Em is from a SRS function generator.

    They are both connected via a coupling capacitor of 0.1 microF.

    Output of the MPY634 is directly observed on the osilloscope.

    -Prakash

  • 0 in reply to Thomas Kuehl
    Prodigy 80 points
    Hi Thomas,
    Just a clarification; the symmetry I am mentioning is about the amplitude of the modulated output which is asymmetrical about the x axis.

    When the modulating signal is changed to Sine wave; --- does not improve the distortion nor affect the symmetric nature of the output.

    The clarification is - the output of the MPY634 when no Em is applied is still asymmetrical and not symmetrical as mentioned in my previous query.

    Thanks for the information on SF. Will try to change the gain and see if it affects the output.

    Regards,
    -Prakash
  • 0 in reply to Prakash Rodrigues
    Guru 140200 points
    Hi Prakash,

    and you have forgotten the resistors form the inputs of MPY634 to ground like Petr in this thread?

    e2e.ti.com/.../338979

    Kai
  • 0 in reply to Prakash Rodrigues
    TI__Guru* 93105 points
    Hello Prakash,

    Please proivde the schematic of your MPY634 AM modulator circuit. Maybe there is some detail that has been overlooked.

    Regards, Thomas
    Precision Amplifiers Applications Engineering
  • 0 in reply to kai klaas69
    Prodigy 80 points
    Hi Kai,
    Thanks for the info. I have attached the schematic below. Please see. I will put 10 K at the inputs and see the performance.
    Regards,
    Prakash.
  • 0 in reply to Prakash Rodrigues
    Guru 140200 points
    Hi Prakash,

    yes, the resistors are missing! This should have been the reason for the strange behaviour...

    Kai
  • 0 in reply to kai klaas69
    Prodigy 80 points
    Hi Kai,
    Yes, the resistors helped to dramatically reduce the asymmetry. Thanks.

    However the distortion is still present in the output of the MPY634. This is more pronounced at the negative side of the modulated signal output. And it does not change much even if a sine wave is fed instead of a square wave.

    Does one need a similar resistor at the Z input side too? Z1 is shorted to the Output while Z2 is connected to Y1 and already there is a resistor at Y1. ??
  • 0 in reply to Prakash Rodrigues
    Guru 140200 points

    Hi Prakash,

    you need only two resistors, one from X1 to ground and another from Y1 to ground. Have you played a bit with the resistor values? Does this have an impact on the distortion?

    I have seen that you have an astable multivibrator with an OPAmp in your circuit. Take care, these circuits can sometimes produce lots of noise, which can be problematic when breadboarding. So, check the performance with another signal generator.

    Kai

  • 0 in reply to kai klaas69
    Prodigy 80 points
    Hi Kai,
    I did play around with the resistor values; 100 K seems fine while 10K does still retains a bit of asymmetry and distortion.
    Yes. I too feel this could be an issue. I will soon transfer this into a proper circuit board but I am working on 'not so high' frequencies though.

    Regarding the SF , resistor potentiometer is not a linear gain transfer. Any thoughts about the reason for this???

    - Prakash
  • 0 in reply to Prakash Rodrigues
    Guru 140200 points
    Hi Prakash,

    datasheet says that there are "internal device tolerances" which come into play when using an external resistor. This could also mean a linearity issue. Datasheet also mentiones an alternative scheme with a voltage divider. Have you checked it?

    Kai
  • 0 in reply to Prakash Rodrigues
    TI__Guru* 93105 points
    Hi Prakash,

    The MPY634 Z1 input, being connected to Out pin, does not need a resistor to ground. The Out pin provides a low impedance current return for Z1 input current. Yes, Z2 is taken care of by any resistor from Y1 to ground because they are connected together.

    Is it okay with you to close this E2E inquiry?

    Regards, Thomas
    Precision Amplifiers Applications Engineering
  • 0 in reply to Thomas Kuehl
    Prodigy 80 points

    Hi Thomas,

    Seems okay to close this e2e enquiry. The distortion in the negative side of the modulated envelope is still persisting though.

    In case any query; I will post again.

    Thanks Kai.

    Thanks Thomas.

    Regards,

    -Prakash

  • 0 in reply to Prakash Rodrigues
    TI__Guru* 93105 points
    Hi Prakesh,

    Are you sure you are aren't exceeding 100 % modulation? That results in clipping the Emin portion of the AM envelope. If that is the case, distortion will be generated.

    For a sine wave modulating a carrier the maximum sideband amplitude for 100 % modulation is -6 dB, relative to the carrier amplitude. You are using a square-wave for the modulation so there a numerous sidebands being generated. The maximum percent modulation would then depend on how the sidebands add amplitudes.

    Try reducing the square-wave amplitude and see if the distortion becomes reduced. If you find the distortion disappears, then I suspect the problem is over-modulation.

    Regards, Thomas
    Precision Amplifiers Applications Engineering