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Technical support > Support selection > Manufacturer Tech Tips What is a CCTV Balun? 
Why Balance? From the drawing, one can see that a balanced signal has two matched conductors. They are matched because their paths to ground are identical, both at DC and at high frequencies. Any interference that couples into one conductor will cause it to move in voltage. But with Unshielded Twisted Pair (UTP) wire, the interference couples into the other conductor too. Since both conductors move, it’s important that the devices at the end of the wire have absolutely identical paths to ground, and that the circuit receive only the voltage DIFFERENCE between the conductors, while ignoring their ‘common’ voltage to ground. The ability to reject this ‘common’ signal, while receiving the ‘differential’ signal is the basis of any balun (or active differential amplifier.) Again, a balun is required at each end of the wire. All of these circuits perform this function, but always in a less than perfect way. The closer we come to perfect balance, the better we can reject interference. A good balance circuit is tricky, and is one difference between NVT and the competition. There is a measurement for how good the balance is. It’s called Common-Mode Rejection Ratio (CMRR). On the balanced side of the balun, one puts a zero amplitude differential signal on the two wires (they’re shorted together), plus a large amplitude common signal (big voltage relative to ground). Now we measure the small voltage on the unbalanced (coax) side. Under ideally balanced conditions, the coax voltage should be zero. To the extent that there’s imbalance, a small error voltage will appear. We define CMRR as the ratio of this error signal (Vdm) to the amplitude of the common (interfering) signal (Vcm):
Now this number is usually very small, so engineers convert it to decibels to make it more manageable. [For reference, 6 dB is a doubling, 20 dB is a factor of 10, 60 dB is a factor of 1000.] CMRR = 20 * Log (Vdm / Vcm) In the case of NVT, we have 60 dB of CMRR over the range of 15KHz to 5MHz. This means that a 10 volt common-mode noise will generate a 10 millivolt error in our video signal. By contrast, our competitors’ baluns have typically around 30 dB of CMRR (a factor of 30) which isn’t good enough when there are ringing telephones, data such as RS-422, 24VAC, RF, or other video signals in the wire bundle. Note that for passive NVT devices, the CMRR falls away below 15KHz, meaning that it doesn’t reject 60 Hz power-line groundloops. However, by employing our active receiver, its balance extends down to DC, and ground-loops are rejected.
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The term "balun" is short for "Balance-Unbalance". It’s typically (but not always) a transformer where one side is connected to a signal and a ground (such as coax), and the other side has both conductors floating relative to ground, with our signal between them. For correct operation one always needs a balun at each end of the wire.
Vdm / Vcm
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| | | Copyright © 2008 Norbain SD Ltd: updated on Jul 1 2008 | |||||