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    FSK 1200baud discrete "modulators". What do you think?

    Hi
    I want to investigate the possibility of making a discrete FSK 1200 baud modulator operating from 5v and connected to the TTL UART of an MCU. The circuits have to be made as simple as possible (fewer parts).
    Here are two of my rough ideas. I have tested the oscillator parts and they work and they can be varied nicely too.
    1200baud is a much higher speed than 300 and I do not know how well these will behave until I test them.

    The first circuit uses 2 oscillators and a switch to switch audio outputs.
    The second circuit uses a single oscillator and a way to vary it's frequency.

    I have thought that at 1200baud, varying the oscillator frequency might cause the oscillator not to be able to switch that fast from one frequency to the other (mark/space), so that is why I have designed the first circuit, which does not mess with the oscillator resonators at all, although I like the simplicity of the second one.

    Any comments/suggestions would be valuable so as how to tackle the problem.
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    Re: FSK 1200baud discrete "modulators". What do you think?

    At least at 1200 Baud, the oscillator phase need to synchronized with the data. That's not possible with the suggested circuit.


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    Re: FSK 1200baud discrete "modulators". What do you think?

    Quote Originally Posted by FvM View Post
    At least at 1200 Baud, the oscillator phase need to synchronized with the data. That's not possible with the suggested circuit.
    I should have posted the whole "modem" block diagram before I got you into trouble. Here it is. The RX will use two narrow filters and a simple detector which will convert audio tones to voltage levels (data). I do not have any idea why the phase needs to be synchronized, but by just thinking logically, I thought it could be possible to work in such a simple arrangement.

    - - - Updated - - -

    I have found this document now. What I have thought for the receiver, is described in figure 15. It is a non-coherent detector. 1200baud-capable?
    Professional engineering is the top, but amateur engineering is more fun.
    It is when you cross the barrier between these two, that things become really fascinating!



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    Re: FSK 1200baud discrete "modulators". What do you think?

    The problem is the frequencies are low compared to the data rate. A change to logic high or low is likely to result in partial cycles of the tone and hence all kinds of spurious other frequencies. Remember you are only selecting one cycle or almost two cycles of the tone per bit so you need to ensure the data only changes at a suitable point in the tone waveform.

    Brian.
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    Re: FSK 1200baud discrete "modulators". What do you think?

    I don't think the phase must be synchronized with the data, but there should be no discontinuities, so switching between two different oscillators is a bad idea.


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    Re: FSK 1200baud discrete "modulators". What do you think?

    Hm...
    So 3 basic problems have reported to me. One is the phase (which based on the paper, could be ignored with suitable detection scheme), the other is the effectiveness of the detector when talking about high speeds (1200 baud) and the third is the switching between marks and spaces, which must contain no "clicks".
    There may be another one, the codecs of the GSM which may make the system unusable through this network or maybe through every digital line.
    Last edited by neazoi; 10th October 2019 at 08:51.
    Professional engineering is the top, but amateur engineering is more fun.
    It is when you cross the barrier between these two, that things become really fascinating!



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