Question

The tone ringer unit in telephone generates frequency in the ________ range.

• A. 20 Hz to 20 kHz
• B. 330 Hz to 3.3 kHz
• C. 16 Hz to 60 Hz
• D. 20 Hz to 3.3 kHz

Hint:

To protect the sensitive speech circuits in a telephone, a high-pass capacitor is placed in series with the tone ringer unit. This blocks DC supply currents while allowing the $16\text{--}60\text{ Hz}$ AC ringing signal to pass through.

Answer 1

The Correct Option is C

Step 1: Analyzing Telephone Ringing Signals:
When an incoming call is routed to a landline telephone, the central telephone exchange sends a high-voltage AC ring signal to the subscriber's line to trigger the ringer.

Step 2: Standard Ringing Frequencies:
Under international and Bureau of Indian Standards (BIS) telecom guidelines, this incoming ringing signal has an AC voltage of approximately $75\text{--}90\text{ V}_{rms}$ and a low frequency in the range of 16 Hz to 60 Hz (typically $20\text{ Hz}$).

Step 3: Evaluating the Role of the Tone Ringer Unit:
In older telephones, this $20\text{ Hz}$ AC signal directly rang a mechanical bell. In modern electronic telephones, a Tone Ringer IC (such as the ML8205 or CS8205) detects this low-frequency ($16\text{--}60\text{ Hz}$) incoming AC signal and converts it into a higher-frequency dual-tone melody ($500\text{ Hz}$ to $1200\text{ Hz}$) to drive a piezo ringer.

Step 4: Conclusion:
Since the tone ringer unit is designed to detect and operate on the incoming ringing frequency, the primary range of the ring signal is 16 Hz to 60 Hz, corresponding to option (C).