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I am wondering why telephone cables are having the diameter or exact 0.44mm and 0.63mm? Is that any special reason?
I have seen some of the cable have up to 2400 twisted pairs, could I know is the number of pairs will affect performance of cable?
Thank you.
The diameters are almost certainly for historical reasons. The sizes you've given correspond to 1/40 inch and 1/60 inch. Probably these looked like good round numbers to someone who was making the decision a long time ago, and we've stuck with them ever since. Any new cable would have to have the same characteristic impedance as the old, fit into the same connectors, have the same or better bend radius etc.
Each twisted pair in a cable carries a signal. More pairs, more signals. More pairs can slightly reduce the performance of a cable due to crosstalk, but it's generally worth it to carry the extra signals.
The characteristic impedance of a telephone cable is very important to get right when it comes to minimizing side-tone (that's the handset microphone producing an amplified sound in your ear). This is achieved by a side-tone cancellation circuit: -
When the line impedance matches Rw in series with Cw there is no microphone signal transferred to the earpiece connected to winding A. The microphone feeds the centre tap between S and P windings.
So, for this to work effectively, the impedance of the telephone line has to "look like" Rw in series with Cw across the range of useful speech frequencies. Here's what a cable impedance might typically look like: -
As you should be able to see, the nominal impedance at 1 kHz is about 600 ohms in magnitude but is in fact similar to the complex impedance mentioned above.
The characteristic impedance of any cable at low frequencies is \$\sqrt{\dfrac{R}{j\omega C}}\$ where R is the series resistance of the loop per metre and C is the parallel capacitance per metre. Any change in conductor spacing or diameter means the capacitance per metre will change and the approximation of Rw in series with Cw won't be as ideal.
Capacitance for twisted pair for example: -
\$C_{twistedpair}\left ( \frac{pF}{inch} \right )=\left ( \frac{.7065}{\ln \left ( \frac{2s}{d} \right )} \right )\cdot \epsilon_r\$
In order to maintain a specific impedeance all properties of a telephone cable are specific. So the wire diameter,the wire distance to each other, the insulation thickness and type. The way of twisting and so on. This results from the past in the thickness indicated
