two elementary Question In telephone system and modulation

[baby_1]

Hello
i want to work telephone elementary theory.we have input circuit

i check the phone line voltage via oscilloscope and see a 148 volt on it if i connect this circuit to phone line what capacitor's voltage should i choose?(how can find the max voltage at capacitor?)

i=150/(61000+10000)=2mA

so the cap voltage is 123.6 volt?it right or we can we assume the input voltage is (150-48)=102 volt? (48 the dc volt of telephone line)?


as you know in FSK systems that play a roll in telephone systems the PSTN send CID data after the first ring (between first and second ring) i see the output of telephone via oscilloscope but i can't see when the CID transfer because i can only see (ringing tone) a sin with 26hz frequency.
what is wrong?

 

[FvM]

I can't follow your capacitor calculation exactly, I don't know what the said 148 V number means (Vp, Vrms, AC only, AC+DC) and don't understand how you are mixing AC and DC values. If you are just targetting to capacitor voltage rating, I suggest some margin, e.g. 250 VDC.

Regarding on-hook signalling, I guess you didn't look sharp. The signal level is pretty low, -13.5 dBm (about 150 mVrms).

 

[zorro]

Hi Baby_1,

The correct value if impedance is not 61000+10000 ohms, but (10000+j*61000) ohms, i.e. its absolute value is sqrt(R^2+X^2)=61.8Kohms aprox. with phase -81 degrees.
In order to specify the capacitor you should consider the maximum voltage, that is DC voltage (48 V) plus the peak AC on the capacitor, and take a good margin, as FvM suggest. In other telephone lines the voltage can be higher, according to specifications of telephony system (varies from a country to another).
In order to separate the CID you should filter with a simple high pass, rejecting low frequencies (dc and ringing tone).
Regards

Z