Basic parameters of these devices are loss (attenuation) and matching (return loss). In this part we will describe the loss parameters.
  • As - coupling loss - signal loss between tap-off input or cross socket, and its branch output - tap loss,
  • Ap - cross-talk attenuation- signal loss between outputs - isolation,
  • Ao - loss between a branch output and the main output,
  • A - through loss - signal loss between the main input and output.
In some catalogs we can come across a parameter called Directivity (D=Ao-A), telling how return signals are attenuated compared with signal coming from the passive input of the device.
The tap-off (or simply tap) is a device with one input, one main output, and multiple branch outputs - the types with 2-4 outputs have identical tap attenuation (in one device) and are available in several versions with various attenuation levels.
Multitaps (4+ outputs) can have outputs with different tap loss. On the illustration, differences between these elements are presented on Satel devices.
Impedance matching is also a very important parameter, because it guarantees providing maximum power to the receiver, and it prevents reflections. The parameter defining matching quality is mismatch loss - "a" [dB].
For splitters:
a>=14 dB for f = 5-40 MHz,
a.>=14 dB-1,5 dB/octave in 40-2150 MHz band, but not less than 10 dB,
for tap-offs
a>=10 dB.
These values are required for elements used in building networks, and are achieved when all outputs of a splitter are terminated with 75 ohm resistance, and - in the case of a tap - the main output is used, and the branch outputs are open or shorted. Though, it is good to close the unused outputs of taps as well, because the mismatch attenuation becomes much higher and the overall network parameters are better.
Analysis of splitter and tap-off parameters shows why subscriber sockets should not be connected to splitters. To such a socket a receiver can be connected or not, and - as receivers do not assure impedance matching in the whole band - such socket does not guarantee proper matching. In tap-offs, impedance matching of the main input and output practically does not depend on the load impedance in the branches, and the reflections beyond tap outputs do not influence the rest of the network.
Taps have one or more branch outputs - the types with 1, 2, 3 and 4 outputs are available with different level of tap loss. E.g. Satel POS-02xx (where xx means attenuation in a particular version) can have tap loss of 8, 12, 16, 20 or 24 dB.
This allows to equalize the signal level in each spot. On the input of the cascade of tap-offs the signal level is highest, so the first tap-off must have the largest attenuation. With every further step, the attenuation lowers, similar to the signal level - to ensure maximum uniformity of output signals at subscribers' outlets.
Example of cascade of taps
Taps - cascading. Most of networks are installed in apartment houses, and due to row structure, cross connections must be used to connect the tap-offs. Mind that the through loss may become very high, so there can appear problems with equalizing signal level for each subscriber.
That is why not more than 5-6 taps should be connected. If a larger number is needed, the only solution is to divide the network into two sub-networks using a tap-off. It allows to use amplifiers with lower output level, and it is easier to design the whole network. More>>
An example of cross-branch network divided into sub-networks
Taps - isolation. Isolation between tap inputs (Ap) and the Ao attenuation (separation) are extremely important parameters in all cases when the network provides many channels. The reason is the interferences generated by the RF tuners of the connected televisions, VCRs etc.
The tuners generate signals at heterodyne frequency, which makes some channels (e.g. those 5 channels higher) extremely vulnerable to interference. Previously these channels were advised to be left unused (sometimes called "prohibited", but due to variety of programs and the necessity of using as many channels as possible, nowadays they have to be used as well.
To damp the signals, the isolation between any two receivers should be at least 50 dB. In the situation where receivers are connected to a single tap-off, the isolation depends on the tap-off attenuation and lengths of the cable between tap outputs and subscriber sockets.
Isolation between receivers
Taps - isolation between receivers connected to different taps. To ensure proper S/N ratio, the isolation between any two receivers should be also at least 50 dB. In this case the isolation depends on the Ao, A, As attenuation, and the lengths of the cable between tap outputs and subscriber outlets.
The diagram shows that the isolation is at least 52 dB (4+30+4+4+6+4=52 dB) - for the worst case. So practically the problem of isolation should only be taken into consideration for the cases of connecting the receivers to the same tap-off.
Typical separation (Ao) is 30-40 dB, pass-through attenuation (A): 1-4 dB, tap attenuation (As): 6-24 dB. Typical cable loss between tap-off and receiver is 4 dB.
isolation between receivers
Multitaps. The tap-offs with 4 or more tap outputs often have different loss of the outputs. It allows to compensate loss differences in outlets installed at different distances from the device. They are usually used in apartment houses, hotels etc., where cables go through long corridors, and/or the outlets are placed on different floors.
Use of 8-out taps for connecting outlets placed on two floors