Hi Mustafa,

your approach with coupling capacitors is correct provided the capacitors are large enough to constitute a low impedance (< some Ohm) for the lowest frequency in the signal.

I suspect that the impedance of your DC source as well as of the DC load is so low that your signal is nearly short circuited. Therefore, I would insert inductances in series with the source and with each load.

What is the baud rate?

It looks as the coupling capacitor is exhibiting high impedance and blocking your signal. So try some larger capacitor values.

Saharul Arof 1200 baud. Frequency of modulated signal is 1200 and 2200 Hz

Ashraf Yahya

Joerg Fricke Thanks for reply, so I must use higher capacitance, am I right? I used 1uF and for 1200 Hz Xc=132 ohm. Then I used 10uF and this is just 13,2 ohm.. Still didn't work. Is this resistance still too much?

Edit: Also I tried adding a serial inductance (100mH) and 100uF coupling capacitor and nothing is chanced.

Hello Mustafa Karahisar ,

if your DC circuit is loaded by n loads, you need n + 1 coils. Please see the attached sketch.

Another point which comes to mind: With 1200 baud @1200 and 2200 Hz, your 1200 Hz bits are just one fullwave long! As a consequence, when switching between 1200 and 2200 Hz there might occur voltage spikes behind the capacitor at the signal source, and these spikes might interfere with the filter in the receiver. I would try either to keep 1200 baud but to use 4800 and 8800 Hz (reward: smaller capacitors and coils) or to keep 1200 and 2200 Hz but to use 300 baud.

Joerg Fricke Thank you so much sir I'm grateful, your answers really helps. I did build with breadboard like schematic but I just see DC 0V on the red cable.

Actually first I'm giving AC signal without DC and capacitor charging with AC. Then I'm giving 24V supply to line after that capacitor voltage dropping slowly to 0V. And I don't know where is AC signal its just disappered. capacitors are 22uF and inductors are 100mH.

Edit: For just a moment it works nice but now signal is so noisy. I think I need to add a LPF. (It works after I supply modulator circuit with DC line but that was a short time)

Edit2: Meh.. I think i saw dream this isn't work.

If I understand correctly, common GND is connected to the minus terminal of the DC source. In this case, the capacitors should have their plus terminal on the DC line. But the capacitor on the left in your picture (AC in) has its minus terminal at the line. Does reversing (and better replacing) this capacitor change anything?

I tried that before nothing has changed. If I remove the capacitor (AC in) I can see input signal from black cable. When I connect again signal just lost. I am adding osciloscope pictures. Channel2 is AC input (blue signal). And Ch1 is output (yellow).

When I disconnect DC 24V from line, AC signal can pass the capacitor and I see the signal both input and output. I don't know how but DC is blocking AC.

Edit: I just see your editted answer. I did try to use 300 baud and I don't know why but I couldn't communicate with 300 baud. I will try soon as I can higher frequences.

How does your AC signal look like if you connect it directly to one of your 100 mH inductances (completely disconnected from the remaining circuit)?

Same as 3rd picture. AC signal not disappering

You had one terminal of the inductance at the orange cable and the other terminal at the blue one?

Sorry that I ask again but I don't trust your inductances. The colors for 100 mH should be brown black yellow + (black or silver or gold), and yours look like brown black brown gold to me which would mean 100 myH.

Ovv you are right my inductance is just 100uH. My biggest inductance i have now 4.7mH so i need to use higher frequnces. And I can see a signal now (ugly but OK) with 4.7mH i will just buy a higher inductance or i need to increase frequency right?

One more thing, why our previous pictures doesn't show anymore?

Edit: I add this answer too a picture but I can't see any picture...

Mustafa Karahisar , I would use higher frequencies (but still 1200 baud), then 4.7 mH will be o.k.

I cannot see any pictures, too. I guess that's a bug of the RG software, so hopefully it's just temporary.

Thank you for everything sir. Your answers really helps me so much.