Electronic – How are unbalanced coaxial cables used for broadcasting TV signals without any problems

coaxialstptelecommunications

As far as I know, in telephony STP or twisted pair cables are used. This creates balanced line impedances which is useful to mitigate for common mode related interference.

So using balanced cables in telephony and in audio is crucial to get rid of any EM or RF interference.

On the other hand, in TV broadcasting or many RF systems coaxial cables are used. And most of the coaxial cables I have seen are not balanced. I can see that the 50 Ohm concept is good to get rid of reflections in transmission line theory. But how come the unbalancedness of coaxial cables causes no problems with impedance balancing issues?

Best Answer

But how come unbalancedness of coaxial cables have no problem in impedance balancing issues?

The beautiful thing about coax is that the shield shunts mostly all external electric field interference to ground and the inner wire is largely unaffected. For an external magnetic field interference, a subtle thing happens; the current that flows in the shield due to the presence of the field creates a volt drop along the shield and, due to near 1:1 coupling between shield and inner, that identical volt drop is present on the inner core.

So, providing you use a differential receiver and the sending end has somewhat reasonably the same impedances to ground on both shield and inner, the differential receiver can reject the common mode interference.

If you do the math on the external fields produced by a regular signal sent down a coax and analysed the fields from send and return currents individually, you find that at all points outside the shield, the opposing magnetic fields exactly cancel to zero. There is no magnetic field outside a coax from a regular coax signal.

The impact of this is that the signal’s magnetic field is only produced in the gap between inner and outer shield. A repercussion of this is that the shield therefore has to have zero inductance. This is because the outer magnetic field is zero (aka zero induction) and the signal’s internal magnetic field has no effect on a tubular conductor (aka shield) hence, the shield behaves like an infinitely thick ground casing surrounding the inner.

That may be a little hard to swallow but if you go back to the theories of magnetic fields associated with a tubular flow of current, an external field is produced but no inner field. The reverse is entirely true; a magnetic field inside a tube induces no voltage along the tube AND, given there is no external field, the shield has zero inductance.

The upshot of all my rambling is that it works despite having a significantly unbalanced impedance regime between inner and outer shield. It’s not all that easy to immediately see I grant you so hopefully I’ve done it some justice.

Related Solutions

Nyquist’s Relation – Telegraph Speed of Transmission

Funnily enough if you search "Certain Factors Affecting Telegraph Speed." in the usual online sources, you will find the original text by Nyquist.

It includes a table:

Number of current values employed    Relative amount of intelligence ... transmitted
2                                    100
3                                    158
4                                    200
5                                    230
8                                    300
16                                   400

so the mistake is, as expected, not Nyquist's.

Amplifier – How to Pick an Amplifier for Very Fast Transient Signals

The world of amplifiers is vast and complex. I have had experience with a similar problem about 2 years ago, where I wanted to measure the impulse response of a fast photodetector (also in a research setting at university).

If you are using a 10GSPS acquisition system, then you must band-limit the analog signal to at most 5GHz to avoid aliasing. It's unlikely you were going to go higher, but it's something to keep in the back of your head.
At these frequencies, reflections of transmission lines usually become important in a measuring setup, especially if you plan on preserving the pulse shapes as much as possible. Impedance matching becomes important at all stages.
As you may have seen in the datasheets on Mini-Circuits, a fixed gain block will probably give you the best performance for wide bandwidths (I have used gain blocks from Mini-Circuits myself and they worked reasonably well for my setup). However, keep in mind that they assume both a \$50\Omega\$ input and output impedance to avoid reflections over \$50\Omega\$ SMA cables. This can pose some restrictions for you sample (eg. biasing).
If you need to strictly keep the bias voltage of the solar cell constant, then a transimpedance amplifier (TIA) as close to the sample as possible is in my eyes your only option.
If you're planning to use the fixed gain block as a transimpedance amplifier, the TIA gain will then at most be \$50\Omega\$ due to the termination resistance (and this TIA gain is usually considered low). The combination of high-gain, wide-band, low-noise and stability is notoriously difficult for transimpedance amplifiers. It is not just "use a fast opamp and put a resistor in the feedback loop".
It sounds to me that you're going to use a pulsed laser (I may be wrong), in which case you are in a position to average over many periods to drive down the noise. I'd say, use that to your advantage if possible. Note that averaging does not get rid of transmission line reflections, nonlinear behavior, aliasing, etc.!

At least, this is how I would summarize possible pitfalls of what I think you're trying to achieve. I've by now finished my PhD, but I remember the annoyance of trying to measure such a fast signal very clearly.