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TV transmitter for video recorders This TV transmitter enables you to transmit signals from the UHF output of your VCR to a second TV set in the house without messy splitters and cables. It uses two ICs & has a range of about 20 metres. By JOHN CLARKE How many times have you wanted to watch something from your VCR on another TV set located in another room in your house? It is possible to do so, of course, by running a cable from your VCR to the second TV set. Or you can transmit the signal to the set using this simpie transmitter. The first option of running a cable 14 SILICON CHIP . can involve a lot of installation. First, you have to split the signal from the VCR to obtain two outputs, one for the main TV set and the other for the second set. After that, you have to run the cable and, in some cases, this can be very difficult since it may involve drilling through brickwork or crawling around in confined spaces in the ceiling or under the house. Another problem with cable is that it can look unsightly if it is not installed properly. A cable outlet also ties the second set to a particular room, at least as far as receiving VCR signals is concerned. Using the TV Transmitter solves these problems. You do not need to run a cable and the TV set can be moved freely about the house without the need to relocate any wiring. Fig.1 shows the general arrangement of the two schemes. Fig. l(a) shows the conventional splitter and cable method while Fig.l(b) shows the arrangement using the TV Transmitter. The TV Transmitter operates by amplifying the UHF signal from the RF output of a VCR and then coupling it to a small antenna. This antenna then radiates the signal to a second identical antenna at the remote TV set. The antennas are 3-element Yagis which have a nominal gain of about 7dB. Because the transmission is at ultra high frequency (UHF), the antennas are quite small and relatively unobtrusive. Their main advantages when compared to a simple half-wave dipole are that they are quite directional and provide greater range. Note that, strictly speaking, the TV Transmitter is really a high gain amplifier, since it does not contain a modulator stage. Nor does it need one in this application, since it is fed from the modulated output of the VCR. Also, because of antenna size limitations, the device is suitable only for use with VCRs that have a UHF output (see panel). Circuit details Refer now to Fig.2 for the circuit details. As can be seen, the TV Transmitter is a relatively simple device based on two OM350 hybrid wideband amplifier ICs. This IC is a Philips device and has a nominal gain of 18dB over the frequency range from 40-860MHz. Our circuit uses two OM350 ICs in series to provide a gain of 36dB. The RF output from the VCR is coupled to IC1 via a 220pF capacitor. Following amplification in IC1, the signal appears at pin 5 and drives inductive load 11, a Philips wideband choke. The amplified signal is then coupled to a 220pF capacitor and split two ways: (1) to the input of IC2; and (2) to the main TV set via an s2on resistor and another 220pF capacitor. The 820Q resistor provides about 21dB of signal attenuation when connected to a 75Q TV input. This means that the overall signal loss from the VCR to the TV is about 3dB, due to the 18dB of gain in IC1. This 3dB signal loss is not important since the output from the VCR is more than sufficient to drive a TV input. The amplified output from IC2 is coupled via yet another 220pF capacitor to the output socket and from there to the antenna. Inductor 12 is another Philips choke which performs the same function as 11. Power for the circuit is provided by a 12V DC plugpack supply. A 47Q TV ANTENNA SPLITTER MAIN TV SET VCR REMOTE TV SET Fig.l(a): the conventional splitter & cable method for connecting two TV sets to a VCR. This scheme can become unwieldy if the remote TV set is a long way away from the VCR & the main set. TV ANTENNA TV TRANSMITTER TV IN OUT MAIN TV SET VCR REMOTE TV SET Fig.l(b): the alternative scheme using the TV Transmitter. It operates by amplifying the output from the VCR & then coupling the signal to a small Yagi antenna. The broadcast signal is then picked up by an identical antenna at the remote set. limiting resistor and 12V 1W zener diode regulate the supply to +12V, while two .00lµF capacitors decouple the supply at inductors 11 and 12. Transmit switch Sl switches the supply rail to IC2. When this switch is closed, the circuit is in transmit mode but when Sl is open, IC2 is left unpowered and so no signals reach the antenna. IC1, however, is paw- ered continuously so that signals are always available for the main TV set. Construction Because the circuit operates at UHF, special RF construction techniques are required. All the parts, except the switch and power socket, are mounted on a double-sided PC board coded SC02212911 and measuring 30 x - TRANSMIT ON/OFF S1 47!1 12V FROM •----W,._.-+-------+-----' PLUG-PACK - ~ .,. .001! .001! • II II L1 II II L2 II II 1BdB GAIN 1BdB GAIN r 'T 220 ~ S Q (!UTPUT TOANTENNA -21dB 220pFT L1, L2 : PHILIPS 4312·020-3670 CHOKE ( 6) 75ll OUTPUT 'T TO TV INPUT TV TRANSMITTER Fig.2: the circuit uses two OM350 hybrid wideband amplifier ICs in cascade to provide a nominal gain of 36dB. As well as driving IC2, ICl also drives a 21dB attenuator circuit to provide signal feed for the main TV set. DECEMBEH 1991 15 S1 Warning! ~]-\ • I J·~zot JtJt~2rK~ + • • TOP VIEW , - - - - - - - - - - O U T P U T _ _ _ _ _ _ _~ OUTPUT T O ~ ANTENN/\~ 220p~~ .0 0 1 ~ ~1:-· b•~ mti\to 220pF. .001~ • BASE VIEW Fig.3: install the parts on the two sides of the PC board as shown here & don't forget to install the "pin throughs" where indicated. Be careful with the two ICs, as they can easily be damaged if overheated. The two RF chokes, L1 & L2, are supplied ready wound. 97mm. This is housed in a plastic case measuring 130 x 67 x 43mm and fitted with an adhesive label. Begin construction by carefully affixing the label to the case and drilling out the switch mounting holes. The cutout for the switch can be made by first drilling a hole at the position indicated and then carefully filing it to the required shape. This done, use a sharp utility knife to cut away the labelling from the four corner mounting holes of the lid. The next step is to drill the three mounting holes in the side of the case for the 75Q PAL sockets. After mounting, these sockets are soldered directly to the PC board, with their centre pins passing through the board from the base side to the top side (see Fig.3). The socket positions are marked using the PC board as a template. To do this, place the board flat against the side of the case with the base side of the board facing outwards (orient the board as shown in Fig.3 and centre it top and bottom). Check that the socket holes line up with the labelling on the front panel, then carefully mark the holes with a pencil and Only four components are mounted on the top side of the PC board - the two OM350 ICs, ZD1 & the 47Q resistor. 16 SILICON CHIP This project is suitable only for use with VCRs that have a UHF output (ie, VCRs that require you to set your TV set to a UHF channel). It will not work satisfactorily with VCRs that have a VHF output. Check that your VCR is suitable before rushing out to buy parts for this project. Your VCR manual should state whether the modulator outputs at UHF or VHF and this will also normally be indicated on the back of the VCR. Depending on parts availability, we hope to publish a TV transmitter suitable for VCRs with VHF outputs in a later issue. lightly centre-punch them. The socket holes can now be drilled and reamed to size and holes drilled in one end of the case for mounting the DC power socket. This done , mount the 75Q PAL sockets in position with their solder lugs oriented as shown in one of the photographs. The DC power socket and the on/ off switch can also be mounted at this stage. Fig.3 shows the assembly details for the PC board. Note that some of the parts are mounted on the top side of the board while the remainder are mounted on the base side. There are also several "pin through" points , where a short piece of tinned copper wire is passed through the hole and soldered to both sides of the board. Here's how the parts are installed on the base side of the PC board. Keep all component leads as short as possible. PARTS LIST 1 double-sided PC board, code SC02212911, 30 x 97mm 1 plastic case, 130 x 67 x 43mm 1 Dynamark front panel label, 125 x 64mm 1 12V DC 300mA plug pack 1 2.1 mm DC panel socket 1 2.1 mm DC line plug 3 panel-mount 750 PAL sockets 1 DPDT slider switch 2 Philips 4312 020 3670 chokes 2 Philips OM350 wideband amplifier ICs (IC1, IC2) 1 12V 1W zener diode (ZD1) 1 8200 0.25W 5% resistor 1 47Q 0.25W 5% resistor 2 .001 µF ceramic capacitors 4 220pF ceramic capacitors The PC board is installed in the case by fitting it to the three PAL sockets & then soldering the centre pins. The socket solder lugs are soldered to the ground plane on the base side of the board (tin the board first to make this job easier). Antenna components Version 1 (with steel rods) 2 double-sided PC boards, code SC02212912, 220 x 13mm 6 250mm x 1.6mm-diameter steel rods 1 2-metre length 75Q coax cable 2 75Q line PAL plugs (for antenna leads) 1 60mm length of 0.8mm enamelled copper wire 2 3.3pF ceramic capacitors 1 120mm-length of plastic sleeving for wire ends 1 500mm-length 12 x 12mm wood 1 240mm-length of 75 x 20mm wood 2 20mm wood screws 2 15mm long x 3mm diameter machine screws and nuts 2 cable clamps or cable ties 8 rubber feet Version 2 (PCB version) The external wires from the board are connected to the TRANSMIT switch & to the power socket. Check the plug polarity carefully before applying power. Start the board assembly by install. ing the 470 resistor, ZD1 and the two ICs. ZDl's anode lead must be soldered to both sides of the board, while its cathode lead should be looped to provide thermal stress relief (see photo). The two ICs should be pushed down as far as they will go and their pins soldered on the underside of the board only. Take care not to overheat the two ICs, since these are hybrid devices and are easily damaged. In fact, it's best to tin the copper pads on the board before installing the ICs, to make the soldering as fast as possible . The remaining parts are mounted on the base side of the PC board. Note that some of the component leads do not pass through holes in the PC board but simply solder direct to the pads. Keep all component leads as short as possible. The power supply leads can now be soldered to the PC board and the free ends soldered to the switch and socket. Make sure that the socket polarity is correct, with the centre pin 2 PC boards, code SC02212913, 250 x 211mm 1 2-metre length 75Q coax cable 2 75Q line PAL plugs 2 3.3pF ceramic capacitors positive as shown on the front panel label. At this stage, it is a good idea to check that the centre contact of the DC plug is also positive with respect to the outer contact. If it isn't, swap the leads around now. Inspect the PC board carefully for possible wiring errors before installing it in the case. When you are cerDECEMBER 1991 17 Fig.4: this simple 600MHz Yagi antenna consists of three steel rods soldered to a PC board, plus a feeder wire & a 3.3pF capacitor. PUSH 10mm LONG PLASTIC SLEEVING ON ALL WIRE ENDS 100 100 ALL ELEMENTS 1.6mm DIA STEEL WIRE 600MHz YAGI ANTENNA The completed Yagi antenna fits into a slot at the top of a wooden stand & can be adjusted up or down so that it points directly at the opposing antenna. tain that everything is correct, fit the board to the PAL sockets and solder the centre pins. Finally, solder the socket solder lugs to the ground plane on the base side of the PC board. Antennas Two different types of antenna can be used with the TV Transmitter, al18 SILICON CHIP though both are 3-element Yagis. The first version uses a long narrow double-sided PC board coded SC02212912, to which are soldered rods made out of piano wire [available from hobby shops) for the reflector, dipole and director. This arrangement is mounted on a wooden stand and is pivoted at one end so that the trans- mitting and receiving antennas can be pointed towards each other. The second version of the Yagi aritenna uses a large PC board coded SC02212913 and this has etched copper tracks for the elements. Although rather ugly in its appearance, this version can be easily concealed on top of a wall unit or bookcase and aimed at the opposing antenna. The antenna version that you choose is based mainly on personal preference and whether you want a free-standing unit or one that can be concealed. You could even use a different version at either end of the link. There is virtually no difference in performance between the two units. Both antennas are constructed to the same dfmensions and are designed around a centre frequency of 600MHz (near channel 37). The wavelength (lambda) is therefore 500mm. Both the director and reflector are located 0.2 lambda from the dipole, with the reflector measuring 0.485 lambda, the dipole 0.466 lambda and the director 0.458 lambda in length. If your VCR outputs at some frequency other than channel 3 7, then the antenna dimensions will have to be re-calculated to obtain the optimum range. This will not usually be a problem since many VCRs with UHF modulators can be adjusted to transmit at any frequency between channels 36 and 40. Because the 75Q feeder cable used with the antenna is unbalanced, we Fig.5: here are the dimensions for the wooden stand. The upright is slotted at the top to accept the PC board & is secured to the base using a wood screw. Although not shown here, four rubber feet should also be fitted to the base to prevent scratches to furniture or to the top ofyourVCR. 3 DIA. UPRIGHT 12x12x250 ANTENNA STANO MATERIAL : woqg Another view of the finished 600MHz Yagi antenna. If your VCR outputs at a frequency other than channel 37, the antenna dimensions will have to be adjusted to suit. SECURE UPRIGHT TO BASE WITH WOOD SCREW FROM UNDERSIDE OF BASE DIMENSIONS IN MILLIMETRES The TO TV output must be fitted with a 75Q dummy load (ie, a 75Q resistor) if the main TV is not connected. decided to use an unbalanced feed arrangement. The main advantage of this arrangement is that the dipole can be a single solid length instead of the two separate lengths required for a balanced feed. The shield of the 75Q cable connects directly to the centre of the dipole, while the centre core connects to the 75Q position on the dipole via a series 3.3pF capacitor and feeder wire. This capacitor cancels out the inductive component of the antenna connection . Building antenna version 1 The first step in building version 1 is to cut each element to the dimensions shown in Fig.4. These lengths should be made to within 1mm for best results. This done , centre each element exactly in the PC board and solder it on both sides of the board. Now solder in the capacitor and feeder wire. This wire is soldered to the PC board as shown in Fig.4 and then soldered to the dipole 25mm away. The 75Q cable can now be attached and plastic sleeving pushed over the ends of the steel rods. Don't leave the plastic sleeving off. It's there to reduce the possibility of eye injury_and to prevent the rods from snagging on clothing or curtains. Fig.5 shows the dimensions of the wooden stand used to support the antenna. It uses a rectangular base to support a 250mm-long square upright and this is slotted at the top to accept one end of the PC board. The board is then secured to the stand using a screw and nut. Tighten the nut firmly so that the antenna can still be swivelled up or down for aiming but will remain stationary after adjustment. Finally, fit four rubber feet to the DECEMBER 1991 19 a II QUALITY NO BRAND DISKETTES 5.25" DSDD 5.25" DSHD 3.5" DSDD 3.5" -DSHD $4.95 $9.30 $8 .75 $17.50 *************** JAPANESE BRAND , ':', ,,' r / 1~ FLOPPY DRIVES f ' SUPER %_ 3.5" 1.44M $99.00 / . 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Building version 2 There's not much to building this version, since the antenna elements are already etched into the board (SC02212913). All you have to do is install the 3.3pF capacitor and attach the 75Q cable. Fig.6 shows the construction details for this antenna. Testing The TV transmitter is now ready for testing but first you must tune your second TV set to the signal from the VCR. This is best done by bring- ing the two together and connecting the RF output from the VCR to the TV antenna input with a lead. The TV set is then tuned to the VCR signal in the usual manner (ie, using a test pattern or by playing a tape). To test the TV Transmitter, connect it as shown in Fig.1 and install the two small Yagi antennas. Now apply power and check that both TV sets display the VCR program when the TRANSMIT switch is set to ON. Only the main TV set should display the picture when the switch is set to OFF. Important: if the main TV set is not connected, the TO TV output on the TV Transmitter must be terminated with a 75Q resistor (otherwise the picture on the remote TV will be a mess). This can be done by fitting a 75Q I . 5%0FF . . ' ALL ORDERS · / - _IF YOU MENTION~ 'h \ UHF CABLE * ** ** ** ** ** ** ** , .~;:1 :R :!:~~~t~ nr~ .,,;e..,,,-•~- - This version of the antenna can be hidden on top of a wardrobe or bookshelf but is not as easy to aim as the first version. TOTV INPUT (FROM VCR RF OUT) 0 OUTPUT 0 0 (TO ANTENNA) TRANSMIT OFF ON + 0 12V DC INPUT ~ =IIHD==== .,.; = = -.... ~ TV TRANSMITTER ,... ,... C\I en § C\I .=:= 0 (.) "'... C\I C\I Cl) ~ > Fig. 7: actual size front panel artwork. s'° ~ -= ~ ~ 0 "Cl ~ "' ...= "Cl '° 0 .,Q u ~ '' - "Cl ~ ::? "'d, :a = ;1 0 "Cl ~ -=... <E "'... ~ 0 i... '° ~ -~ '{' ] Fig.8: here are the full-size artworks for the double-sided PC board. Check your finished board carefully against these patterns before installing any of the parts. ... N 0== N Q) N N 0 ~ ,... '° Ol ,... .,Q oi 0b ~ resistor inside a 75Q PAL plug, which can then be plugged into the TO TV socket as the need arises. Ifit doesn't work, check the supply voltage to the OM350 ICs by measuring the voltage across ZDl. You should get a reading of 12V. If the reading is low, check the orientation of ZD1. Range The maximum range will be obtained when the two antennas are pointing directly at each other. You should have little difficulty in estab- lishing a range of about 20 metres in free space but this will be reduced if there are obstructions in the transmission path (particularly metal). Our tests showed that the prototype had a range of about 15 metres through 30cm-thick concrete-brick wall, while plaster walls had little (if any) affect on range. In practice, the range achievable should be adequate for most homes. Note that antenna placement can have a big affect on picture quality. Just moving the antenna forwards or (.) If) 0 • 0 backwards by 10-15cm can make all the difference between an unwatchable picture and noise-free reception, so it's worth experimenting. SC DECEMBER1991 21