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FSK 1200baud discrete "modulators". What do you think?
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sv3ora:
Hi, this is my first post.
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.
mikerj:
Use a timer peripheral in the micro to generate your FSK frequencies, and either use a bit based UART function to switch between the two FSK frequencies (easy at just 1200Baud) or tie the UART output back into an external interrupt and switch timer frequencies in the interrupt.

Using two separate oscillators means you get an unpredictable phase change at the transition i.e. you might get an extra short or extra long cycle, which may or may not be acceptable.  A single oscillator switching between frequencies will give a smooth transition.
sv3ora:

--- Quote from: mikerj on October 09, 2019, 11:35:22 am ---Use a timer peripheral in the micro to generate your FSK frequencies, and either use a bit based UART function to switch between the two FSK frequencies (easy at just 1200Baud) or tie the UART output back into an external interrupt and switch timer frequencies in the interrupt.

Using two separate oscillators means you get an unpredictable phase change at the transition i.e. you might get an extra short or extra long cycle, which may or may not be acceptable.  A single oscillator switching between frequencies will give a smooth transition.

--- End quote ---

I think I can't go that far with the MCU solution you propose, it seems too complex for me and the "firmware" is already written by someone else in assembly! :(

Interesting the thing you mentioned about the single oscillator. I attach a paper I found (80's) about the different demodulation schemes. I am trying to build something that on the detector side, does not depend on the phase changing of the signal, like shown on figure 15.
amyk:
What frequencies are you using?
sv3ora:

--- Quote from: amyk on October 09, 2019, 11:58:48 am ---What frequencies are you using?

--- End quote ---

1200 baud asynchronous half duplex (bell202). 1200Hz and 2200Hz. But since I haven't still built it, I can change standard, or even do not follow any standard, as long as the modulator/demodulator can represent 1200baud in audio tones ok. I really do not mind.
It will be used as a modem through the audio transferred through mobile phones.
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