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arms22 / SoftModem

Audio Jack Modem Library for Arduino
BSD 3-Clause "New" or "Revised" License
184 stars 36 forks source link

Filters over FSKIN and FSKOUT #9

Open hoonkai opened 7 years ago

hoonkai commented 7 years ago

Hi

I have a question about the shield's filters over FSKIN and FSKOUT. It looks like FSKOUT has a high-pass filter, but why is it needed when the Arduino can generate 2.4khz with PWM?

With FSKIN, how come it has a low-pass filter? Are the two resistors for dividing voltage?

Also, what's the solder joint for?

Thanks

clacktronics commented 7 years ago

I can try to help, I may not be 100% right but I read the schematic like this.

On FSKIN I think C1, R1 and R2 are for biasing the signal as it should be an AC waveform with no DC on it, it puts the waveform approximatley 1/2 of VCC (2.5v). C3 is probably for oscillation prevention.

on FSKOUT R3 and R4 are a voltage divider to lower the signal and C2 removes DC putting it back to AC.

SJ1 solder point is to choose if you want it to output or input there is no duplexing

hoonkai commented 7 years ago

Thanks. With SJ1, my reading is that it's a 3-way jumper, but shorting all 3 pads would effectively short the MJ-2135's pins 1 and 4. Shorting SJ1's middle and bottom pads effectively would be redundant. Why are there 3 pads rather than 2?

Bangybug commented 6 years ago

SJ1 is likely for swapping channels, because audio jack pinouts may be different.