Basic Remote Control Receiver Circuit
Figure 16 shows an
example of a basic remote
control receiver utilizing a
decoder chip from Holtek.
When a key is pressed at
the transmitter, a corres-
ponding pin at the receiver
goes high. A schematic for
the
transmitter/encoder
circuit may be found in
the LC transmitter guide.
These circuits can be
easily modified and clearly
demonstrate the ease of
using the Linx LC modules
for remote control appli-
cations.
1
2
3
4
5
6
7
8
NC
NC
NC
GND
VCC
PDN
NC
DATA
ANT
GND
NC
NC
NC
NC
NC
NC
16
15
14
13
12
11
10
9
HT658
4. Observe appropriate layout practice between the module and its antenna. A
simple trace may suffice for runs of less than 0.25" but longer distances should
be covered using 50 coax or a 50 microstrip transmission line. This is
because the trace leading to the module can effectively contribute to the length
of the antenna, thus lowering its resonant bandwidth. In order to minimize loss
and detuning, a microstrip transmission line is commonly utilized. The term
microstrip refers to a PCB trace running over a groundplane, the width of
which has been calculated to serve as a 50 transmission line between the
module and antenna. This effectively removes the trace as a source of
detuning.The correct trace width can be easily calculated using the information
below.
Figure 16: Basic Remote Control Receiver
BOARD LAYOUT CONSIDERATIONS
If you are at all familiar with RF devices you may be
concerned about specialized board layout
requirements. Fortunately, because of the care
taken by Linx in designing the LC Series,
integrating an LC-S receiver is very straightforward.
This ease of application is a result of the advanced
multi-layer isolated construction of the module. By
adhering to good layout principles and observing a
few basic design rules you can enjoy a
straightforward path to RF success.
1. No conductive items should be placed within 0.15
in. of the module’s top or sides.
GROUNDPLANE
ON LOWER LAYER
Figure 17: Microstrip Formulas (Er = Dielectric constant of pc board material)
Always incorporate
2. A groundplane should be placed under the
adequate groundplane
module as shown. In most cases, it will be placed
on the bottom layer. The amount of overall plane area is also critical for the
correct function of many antenna styles and is covered in the next section.
3. Keep receiver module away from interference sources. Any frequency of
sufficient amplitude to enter the receiver’s front end will reduce system range,
cause bit errors, and may even prevent reception entirely. There are many
possible sources of internally generated interference. High speed logic is one of
the worst in this respect, as fast logic edges have harmonics which extend into
the UHF band and the PCB tracks radiate these harmonics most efficiently.
Microprocessors with external busses are generally incompatible with sensitive
radio receivers. Single-chip microprocessors do not generally pose a problem.
Switching power supplies, oscillators, even relays can also be significant
sources of potential interference. Here again, the single best weapon against
such problems is attention to placement and layout. Filter the supply with a high-
frequency bypass capacitor as described above. Place adequate groundplane
under all potential sources of noise.
Dielectric
Constant
4.8
4
2.55
Width/Height
(W/d)
1.8
2
3
Effective
Dielectric
Constant
3.59
3.07
2.12
Characteristic
Impedance
50.0
51.0
48.0
RECEIVER ANTENNA CONSIDERATIONS
The choice of antennas is one of the most critical and often overlooked design
considerations. The range, performance, and legality of an RF link is critically
dependent upon the type of antenna employed. Proper design and matching of
an antenna is a complex task requiring sophisticated test equipment and a
strong background in principles of RF propagation. While adequate antenna
performance can often be obtained by trial and error methods, you may also
want to consider utilizing a professionally designed antenna such as those
offered by Linx. Our low-cost antenna line is designed to ensure maximum
performance and compliance with Part 15 attachment requirements.
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