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SN74LV4046A: FSK Modulation / Demodulation Troubles

Part Number: SN74LV4046A

I'm trying to do binary FSK modulation / demodulation using two SN74LV4046A, one to modulate and one to demodulate.

I built a test circuit based on the example in SLAA618 (Implementation of FSK Modulation and Demodulation using CD74HC4046A).

I've also pored over SCHA003B (CMOS Phase-Locked-Loop Applications Using the CD54/74HC/HCT4046A and CD54/74HC/HCT7046A).

VCC = 5V

On both modulator and demodulator, I am using:
R1 = 4.42kOhm
R2 = open
C1 = 27pF

On demodulator, the low pass filter is connected as shown in figure 4 of above application note, with:
R3 = 36kOhm
C2 = 100pF

Input to the modulator is:
1.36V to represent binary 0 -> produces 10MHz at modulator VCO output
3.1V to represent binary 1 -> produces 20MHz at modulator VCO output


I am having trouble at the demodulator. I was hoping to recover approximately the same voltages as input to the modulator (or at least have good voltage separation), then use a Schmitt trigger logic gate to bring that output back to normal TTL levels.

I alternately tried using Phase Comparator 1, and Phase Comparator 2.


When using Phase Comparator 1:
* When input to modulator is 1.36V producing 10MHz, output voltage at DEMout is about 2.75V.
* When input to modulator is 3.1V producing 20MHz, output voltage at DEMout about 2.28V.

Therefore, there is not enough voltage separation, so the Schmitt trigger logic gate does not distinguish between the two states (its output is always high).


When using Phase Comparator 2:
* When input to modulator is 1.36V producing 10MHz, output voltage at DEMout seems to oscillate at high frequency (too high for my scope) between 880mV and 2V.
* When input to modulator is 3.1V producing 20MHz, output voltage at DEMout oscillates similarly between 2.2V and 4V.

This is producing totally meaningless output.


Suggestions?

Scope images:

All of the below are using Phase Comparator 1:

When input to modulator is 1.36V representing "binary 0":

Input to modulator VCO, 1.36V:

Modulated signal, 10MHz (9.8 MHz in this photo):

Output of Phase Comparator 1:

Output at DEMout:

When input to modulator is 3.1V representing "binary 1":

Input to modulator VCO, 3.1V:

Modulated waveform, 20 MHz:

Output of Phase Comparator 1:

Output at DEMout:

  • In reply to twelve12pm:

    No - I'm afraid that I have not had time to get into the lab yet. I'm currently planning to spend all day tomorrow in there catching up on all the tests I need to do, including this one.

    I just completed a final exam for my analog IC deesign course (hooray!), which I had taken a day off work to study for. Working full time and working towards a master's degree is rough!


    Looking for a low voltage translator? Check out the AXC family that supports 0.7V to 3.3V translation!

    The Logic Minute training page has videos on many interesting topics that all are kept shorter than 5 minutes.

  • In reply to Emrys Maier:

    For an update: I was able to get my bench setup about 1/2 completed today. It was quite an exciting day (unfortunately) and a bunch of issues that required my immediate attention kept popping up.

    I'm going to be back in the lab tomorrow to get some data.


    Looking for a low voltage translator? Check out the AXC family that supports 0.7V to 3.3V translation!

    The Logic Minute training page has videos on many interesting topics that all are kept shorter than 5 minutes.

  • In reply to Emrys Maier:

    To give you a brief update:

    I was able to test the device on the bench today. I found that the buffer used only has a valid input range from 0.7V to ~3.6V -- beyond 3.6V input I see a ton of noise/oscillation on the output. It looks like you are using it in the right range from the scope shots, so I doubt that's the problem.

    My initial tests were all done with the VCO disabled (no RC components attached), so my next step will be to get it running at a similar frequency to yours and see if I can reproduce the noise you're seeing.


    Looking for a low voltage translator? Check out the AXC family that supports 0.7V to 3.3V translation!

    The Logic Minute training page has videos on many interesting topics that all are kept shorter than 5 minutes.

  • In reply to Emrys Maier:

    Thank you for the update. I am glad that we are operating within the valid input range on the buffer.

    Looking forward to hearing if your setup, with VCO enabled at similar frequencies to ours, exhibits the same issues we're seeing. Thanks again.
  • In reply to twelve12pm:

    Hey,
    I was able to get the device operating in a very similar condition to yours (10 MHz VCO frequency, slightly different RC values), but I wasn't able to get the same type of instability for the DEM_OUT buffer.

    I've tried adjusting the output loading with different capacitances and resistances (no load to ~100pF, open to 100kohm).

    I'm not sure what else I can try. Sorry I'm not more help!


    Looking for a low voltage translator? Check out the AXC family that supports 0.7V to 3.3V translation!

    The Logic Minute training page has videos on many interesting topics that all are kept shorter than 5 minutes.