Independently filtering the two tones will certainly work but bear in mind for radio purposes they are typically 170Hz apart so you need some very narrow band and sharp edged filters to split them. If the tones are within the audio range I would suggest using them to modulate a HF oscillator then using two quartz crystals to filter the resulting output frequencies. You will have difficulty achieving the very high 'Q' needed with discrete component audio filters without them ringing.
1200 Bauds FSK modems can of course use any frequency pair if you are linking point to point with wires but consider the bandwidth needed if you intend to use phone lines or radio links.
Brian.
A 555 can be made discrete, so if this is a solution I might really try it. I want to send data through the GSM network, not landlines.MANY years ago I built an FSK modulator/demodulator to store 'programs' from my COSMAC onto a tape recorder and back again using a 555.
As such it was very similar to your requirement. Basically the demodulator was a PLL that followed the input frequency and you used the error voltage to see if it was 'high' or 'low'.
Try a search for FSK demodulation using a 555 and you will see various circuits.
I'm not sure if you count using a 555 (or 565 or similar) as using 'discrete' components but it could well be a start.
Susan
GSM might pose other difficulties, some frequencies are deliberately filtered out and the time multiplexing will shift the bits to some degree, making the data more difficult to recover. I suppose you know that you can use GSM as a data modem without using FSK already. Plug a phone into a USB port, using a 'full' USB cable (ie not one that only has the power wires in it) and the computer should enumerate it as a serial port. You can then use 'AT' commands to control the phone, including making calls and sending data.
Brian.
A pure envelope detector probably wouldn't work, the data rate is too close to the carrier frequency to be able to follow it without significant ripple. If you want to go down that route, use two detectors and a bistable circuit.
circuit that responds quite fast to audio amplitude variations of a signal, detecting them into DC to drive the comparator. I do not want to use LM567's for this purpose really.
Consider that if you use say 1200/2400Hz and 1200 Bauds, there will only be ONE cycle of audio at 1200Hz and HALF a cycle at 2400Hz, if you try to rectify and store the result in a 'smoothing' capacitor it won't have time to charge up and discharge. This is why other modulation/demodulation systems are used as the speed increases in a limited bandwidth.Brian please explain the operation of such a circuit?
So there is absolutely no way of doing it at these speeds using this filter/detector method?Consider that if you use say 1200/2400Hz and 1200 Bauds, there will only be ONE cycle of audio at 1200Hz and HALF a cycle at 2400Hz, if you try to rectify and store the result in a 'smoothing' capacitor it won't have time to charge up and discharge. This is why other modulation/demodulation systems are used as the speed increases in a limited bandwidth.
Brian.
This ordinary bandpass filter may have sufficiently narrow bandwidth that you only need to add a diode detector and smoothing capacitor.
Then feed it to a Schmitt trigger. Peak amplitudes at center frequency should cause it to change state same as a 567 IC.
The 567 requires a certain number of cycles to pass before it locks onto its center frequency, and switches its output. By running comparisons you'll discover whether the bandpass filter responds in more or fewer cycles.
Sine sweep covers 30% to each side of the center frequency.
View attachment 167547
Try it and see but you will find you can make almost any theoretical 'Q' and frequency in a simulation but making it work in real life is a different matter. Very low value input resistors followed by very high gain is a recipe for instability and loading problems.
If I was forced to design an FSK demodulator in discrete components I think I would go down the PLL and lock detector route or a PLL and coincidence detector.
Brian.
Just a thought, but PLLs are popular for fm work. But they suffer from
lasrge latency, eg. number of cycles to achieve lock. But if you use
reciprocal counting techniques to measure PW, thereby frequency,
you easily achieve 1 cycle latency. Of course this requires a freq/T
counter, and a properly extracted and conditioned input to the counter.
BPF takes care of most of that.
There is always Foster Seeley detector, or the ratio detector approach.
Another very interesting approach -
Regards, Dana.
I can imagine there is much difference from theory to pragtice, but I did so in order to aboid having to cascade more such filters. I do not know how well will it work in a single stage or if it turns out to be an oscillator. But I have to test it to know for sure.
I have chosen to vary the input resistor, to not reduce gain much, but the shunt resistor can be altered instead, with the same effect in the frequency and Q, if one wants to preserve stability. But I have to do real life tests.
I think that the PLL idea requires so many components that wouldn't worth it...
--- Updated ---
Thanks so much for this video! Now I know how to build a transformerless FM detector.
I am trying to think of how this could apply to the fsk demodulator. Well, we do not have the IF now, we do it directly in audio so we should expect a larger deviation, but I am considering the first limitation out of the 3 that he mentions.
How about using this average voltage produced by this detector on a comparator?
But where would the input of this pulse demodulator will be connected, to both filter outputs?
Thats then fed to the
two transistor pulse to audio circuit. In other words deviation to DC value, in his example music.
Regards, Dana.
So could I just simply fed the output of this two transistor fm demodulator to your comparator input https://www.edaboard.com/threads/rf-detector-with-my-comparator.396013/#post-1701310 to convert it to digital data stream?
Remember, my input is two tones audio, not radio, so the pulse counter will work in the audio frequency spectrum directly, generating the relevant pulse widths as the audio frequency changes.
I can imagine there is much difference from theory to pragtice, but I did so in order to aboid having to cascade more such filters. I do not know how well will it work in a single stage or if it turns out to be an oscillator. But I have to test it to know for sure.
I have chosen to vary the input resistor, to not reduce gain much, but the shunt resistor can be altered instead, with the same effect in the frequency and Q, if one wants to preserve stability. But I have to do real life tests.
I think that the PLL idea requires so many components that wouldn't worth it...
--- Updated ---
Thanks so much for this video! Now I know how to build a transformerless FM detector.
I am trying to think of how this could apply to the fsk demodulator. Well, we do not have the IF now, we do it directly in audio so we should expect a larger deviation, but I am considering the first limitation out of the 3 that he mentions.
How about using this average voltage produced by this detector on a comparator?
But where would the input of this pulse demodulator will be connected, to both filter outputs?
Yes, you have two deviations (tones), and out of this circuit will be two dc values you can feed
to a comparator to discriminate between tones (which have been mapped to distinct
DC values). There is of course a design consideration of what the DC values are, they are
derived from duty cycle, and fact you will be using CMOS, probably at 5 V, out of comparator.
Note comparator has to look at duty cycle integrated to get DC to compare against to determine
which tone, so latency is involved. Or you use a reciprocal counter implementation to measure the
duty cycle to determine which tone. Personally i would go that route, use a micro, to do that. That
latency is 1 cycle of the tone being sent.
Also keep in mind a carrier freq is needed for the mixer, so you have error considerations deviation
of Tx carrier from recovery carrier used in mixer.
Regards, Dana.
--- Updated ---
The input to the discriminator takes the modulated FM
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