This is only a preview of the December 1991 issue of Silicon Chip. You can view 49 of the 96 pages in the full issue, including the advertisments. For full access, purchase the issue for $10.00 or subscribe for access to the latest issues. Articles in this series:
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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!
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•
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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
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.SILICO N CHIP
FRONT OF ANTENNA
Fig.6: the second version of the antenna has its elements etched into a large PC
board. You simply install a 3.3pF capacitor & attach the feeder cable.
underside of the base, attach the 75Q
lead to the side of the upright with a
U-shaped cable clamp or cable tie,
and fit the end of the cable with a 75Q
PAL plug.
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
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-
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)
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Fig. 7: actual size front panel artwork.
s'°
~
-=
~
~
0
"Cl
~
"'
...=
"Cl
'°
0
.,Q
u
~
''
-
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0
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-=...
<E
"'...
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0
i...
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]
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
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