Circuit Notebook 90 - Veroboard Video Distribution Amplifier

Impedance matching

In professional video equipment all video outputs have a 75R source resistance and all inputs have a 75R input resistance. This is to ensure that when equipment is connected together using 75R coaxial cable, the coaxial cable is terminated into 75R at both ends and signal reflections are prevented.

Distribution Amplifier

The little distribution amplifier to be described has the required 75R input resistance and has two outputs each having a 75R source resistance. It could be used to feed simultaneously a TX and video monitor, or two video monitors.

The circuit is shown in Fig. 1. The amplifier consists of a video op-amp which is connected to provide a voltage gain of X2. A 1V peak-peak signal at the input of the op-amp results in a 2V p-p signal at its output. This signal increase is necessary because the output signal is sent through a 75R source resistor through a co-ax cable into a 75R load in the next piece of equipment. This results in the signal being halved - back to 1V p-p.

The distribution amplifier uses the Analog Devices AD817AN op-amp [1] (Rapid Electronics 82-0482). I would normally use an Elantic EL2020, but the manufacturers say that it 'should not be used for new designs'.

Construction

The unit is built on a piece of 0.1" matrix Veroboard, 7 strips wide by 15 holes long. The component side of the layout is shown in Fig. 2. The overall size is 0.8" by 1.6" (17.8mm x 40.7mm). As it is so small, it can be mounted by using a single M3 screw and an M3 mounting pillar. In the layout, an 'X' indicates a break in the copper track, a black blob indicates a soldered joint and a line between tracks indicates a wire link. I use pins to make external connections to the tracks but you could solder the wires directly. The completed amplifier is shown in Fig.3.

Performance

In basic tests, the amplifier using the AD817AN has a bandwidth exceeding 6MHz +/- 0.3 dB when driving into two 75R loads. The performance using an EL2020 is marginally better.

Supplies

The amplifier requires supplies of +5V and -5V, both at 7mA. The easiest way of generating these (from the usual +12V supply) is to use a DC-DC converter, 12V in, +/- 5V out, such as the Newport NMA 1205S (Rapid 84-0315). This DC-DC converter does not have an internal regulator so it must be supplied from 12V +/- 10%. A warning - if you use a plug-top power supply, then make sure it is a 12V regulated type. Unregulated types may produce an output in excess of 20V when lightly loaded. If this is the case then the DC-DC converter could be fed from a +12V linear regulator IC, such as the 7812. To avoid damage due to the accidental reversal of the supply it is prudent to include an 'Idiot Diode' in series with the 12V supply. The various options are shown in Fig. 4.

Distribution Amplifier Application

A suggested application for the Veroboard Distribution Amplifier is shown in Fig. 5. This is a very basic video switch box (metal preferred) which could be used in a simple home station or when out portable, to avoid having to disconnect and reconnect cables.

The three inputs are from, for example, a camera, a caption generator and a receiver. A single-pole 3 way switch routes the video signal to a monitor. The monitor can then display the camera, caption, or received signal. The other output goes to the TX. A two way switch allows either the camera or caption to be selected.

A suitable supply option may be included in the switch box to allow shack or portable use. All coax connectors have their outer connections joined together through the metal of the box, to which is also connected the 0V of the power supply.

Interconnecting Equipment

As mentioned previously, coax cable with a characteristic impedance of 75R and fitted with BNC plugs is usually used for interconnecting video equipment. For short cable lengths up to, say, 2 metres or so, the cable impedance is less critical and thinner cables fitted with phono plugs may be used. These cables are typical of those supplied with domestic video equipment, camcorders etc.

As domestic video equipment is often used in the ATV shack, together with some professional or semi-professional equipment, it is necessary to keep a selection of leads or adaptors to hand. For example, 'phono female to BNC male' (Rapid 16-0135) [3] and 'BNC female to phono male' (RS Components 406-470). Phono connectors of reasonable quality are much cheaper than BNC types and very much easier to fit. In general, good quality domestic phono to phono leads are quite adequate for use in the ATV shack.

After many years of building video equipment using only BNC sockets, I am considering fitting phono sockets in parallel with BNC sockets on future equipment, to avoid having to use adaptors and converter patch leads.

Figures

Fig.1. Circuit of Video Distribution Amplifier

Fig.2. Veroboard Layout, component side

Fig.3. Completed Distribution Amplifier

Fig.4. Power supply options

Fig.5. Application, Video Switch Box

References

[1] AD817 Data sheet, Google type in 'AD817' [2] NMA 1205S Data, Google type in 'nma1205s' [3] Rapid Electronics Ltd, www.rapidonline.com/components/