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Electrical Transmitter-Receiver Prototype Document
Design
A circuit design was considered which did not use a micro-controller. This was an effort to save power
and layout space if the transmitter/receiver pair designed utilizes a simple encoder-transmitter and
decoder receiver topology. Our most common feedback comment centered around a desire for a
smaller device, so a smaller board and in turn, a smaller transmitter would go a long way towards
satisfying the customer.
After consulting many manufacturers design guides and taking cues from product data sheet design tips,
the following circuit design was formulated. Note the low number of components and lack of a microcontroller.
The external oscillator values were chosen in accordance with the associated TX and RX IC datasheets to
ensure proper operation at 433 MHz.
Component Selection
A major decision in component selection centered around whether or not to use TX and RX chips with
internal oscillators, or to use those which require external crystal oscillators. After considering the price
difference, it was seen that using internal oscillators would add approximately $2 of overall cost to the
devices as opposed to using external oscillators.
The next item to consider was which components to choose. There was a consideration to use
components all from the same manufacturer in an effort to make integration easier and more reliable,
but ultimately this was not done due to cost considerations. This may have contributed significantly to
the reasons why the prototype was nonfunctional at the end of the allotted time. The bill of materials
and unit prices are shown below.
Component
Transmitter IC
Receiver IC
Encoder
Decoder
Crystal 13.56 MHz
Crystal 6.74 MHz
One-Off Cost
1,000 Unit Run
Cost
Part No.
576-1973-5-ND
576-2421-ND
LICAL-ENC-LS001ND
LICAL-DEC-LS001ND
887-1015-ND
535-10210-ND
14.74
Unit
Extended
Vendor Price
Price
DigiKey
2.38
1.75
DigiKey
4.55
2.28
DigiKey
3.47
2.89
DigiKey
DigiKey
DigiKey
3.47
0.46
0.41
2.89
0.26
0.26
10.33
Breakout boards for the surface mount package TX and RX ICs were ordered to speed up the bread
boarding integration effort, but are not included in the cost figures above.
Assembly and Debug
The circuits were constructed according to the diagram on page 1. Upon initial testing, no output was
seen at the LED. Probing with a multimeter indicated that when the buttons were depressed, the
voltage at the output pin of the transmitter chip changed from 0.01V to 0.35V. This voltage was seen at
the input to the encoder, however nothing of significance was seen at the output of the decoder. The
encoder output was probed using an oscilloscope, but no signal was seen. This lead me to believe that
the Linx Technologies encoder that we were using was not suitable to take data directly from our Micrel
transmitter chip. It could be a slight mismatch of baud rate, or other factors. This would be easily
remedied by constructing the circuits laid out below which use components only from Linx. Another
problem encountered was with the receiver. Certain capacitors needed to be chosen according to the
data sheet that were not observed when making the initial design. CTH which is the Data Slicing
Threshold Capacitor (Analog I/O. The capacitor connected to this pin extracts the dc average value from
the demodulated waveform which becomes the reference for the internal data slicing comparator. This
should have been a cap with a value of 47nF. Another capacitor that was overlooked was the CAGC
capacitor. This is the Automatic Gain Control (Analog I/O). You should connect an external capacitor to
set the attack/decay rate of the on-chip automatic gain control. This should have been a value of 2.2 uF.
See the table below:
Conclusion
The information laid out above paints a picture of the progress made on assembling the functional
prototype. For a thorough look at lessons learned and electrical suggestions for future work, please refer
to the 'Lessons Learned and Advice for the Future' node on the P11032 EDGE Homepage.