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CD4046B: CD4046B for FSK Demodulation

Bartosz Soltowski39
Prodigy 125 points
Part Number: CD4046B

Hi,

I designed an FSK system with a 555 timer as a modulator and CD4046B for demodulation.

The system sends signals at around 45kHz for binary '1' and around 25kHz for binary '0'.

When I send information over the power line everything works fine as long as my frequency during 'sleep mode' is still being generated, corresponding to 25kHz (binary '0').

I am now trying to diesocnnect my 555 timer whenever information is not being send and activate it only when I need to send a package of data. My 555 timer produces appropriate signals over the power line. Unfortunately, my PLL does not seem to work anymore under this scenario.

Does anyone know how to fix this problem?

Bartek

  • 0
    TI__Guru* 97046 points
    Hi Bartosz,
    Can you provide a schematic for your system?
  • 0 in reply to Emrys Maier
    Prodigy 50 points

    Hi Emrys,

    You can find schematics in the attachment.

    I supply my 555 timer with 3.3V and PLL with 5V. First picture is a model which works fine but it constantly sends binary '0' (around 25kHz) in a sleep mode. As a result, for the second experiment I added a transistor which disconnects 555 timer from the power line when no information over the power line is being sent. It seems to work fine but just for a certain period of time. I send around 20 packages of data per second. The system works usually for 15-20 seconds, sometimes longer (a couple of minutes). After that the PLL stops reading data and starts reading again when I restart it. Do you have any suggestions why does it happen?

    One more thing I don't understand is a starting sequence of PLL. I need to completely restart the module in order to make it work properly. Otherwise it would never synchronize with the rest of the circuit.

    I also don't understand why voltage on pin 14 is 2 Volts, even when 555 timer is completely off.

    I am looking forward to hearing from you.

    BartoszPLC.docx

  • 0 in reply to Bartosz Soltowski
    Prodigy 50 points
    oh, and the demodulated output which I am reading is a voltage on C26 capacitor. It is then connected to the comparator and a buffer to give me binary '0' and '1'
  • 0 in reply to Bartosz Soltowski
    TI__Guru* 97046 points

    Hey Bartosz,

    That schematic is a bit hard to read, but it looks like there's no connection between the demod output and R48/C26.  Also, there are several jumpers that go nowhere... do you have this setup on a breadboard?

    Hopefully I will have some free time this weekend to decipher the schematic, or if you can provide an improved one, I might be able to help sooner.


    Can you also provide scope shots of the operation of the modulated signal when it's working properly and when it's failing?

  • 0 in reply to Emrys Maier
    TI__Guru* 97046 points

    Hey Bartosz,
    To answer your question about why you see 2 volts on pin 14, that's because pin 14 is a CMOS input, which acts like a small capacitance to ground. You have AC coupled the line, which essentially makes the input floating. Since the input has no DC path to charge or discharge, the capacitance there will maintain whatever voltage was there before the pin was left floating. This is generally very bad for CMOS devices, and cause a lot of problems in circuits. You might want to check out this application report on the topic: www.ti.com/.../scba004d.pdf


    After looking over your schematic, I can make a couple of recommendations.

    First, I would recommend removing C19. This AC couples the signal going into the CD4046B, and that device is supposed to only have logic level inputs - no negative values. Negative voltages on this pin can cause damage to the device and possibly latch-up if enough current is present.

    Since you want to have only a binary output of the CD4046B, I would recommend holding the VCO at a constant frequency (between your logic HIGH and logic LOW frequencies). This will keep the CD4046B running all the time, and your output will be one of two DC values (after filtering, of course).

    The remainder of my recommendations are about your schematic. You don't show what connects to JP1, JP2, JP32, JP40. There are several places where wires cross without junction dots, so I assume they have no connection, however I see at least one place where that would me a wire is completely floating (R48 connection to R46), so it leaves me quite confused.

    In addition to making sure the schematic clearly shows all connections (on and off the schematic), I would recommend not running wires through components, as this can get confusing as well.

    I hope some of this helps you.

    Good luck with your project!

  • 0 in reply to Emrys Maier
    Prodigy 125 points
    Thanks for your help Emrys.
    I looked at your comments, modified the circuit a bit and the Demodulator works fine over the power line.
    Thank you very much for your support.