Technical Manual
Table Of Contents
- Wireless Sensor Interface A720 (addIT™)
- 1. About the A720
- 2. Hardware
- 4. Software
- 4.1. AMOS
- 4.2. Mode Check
- 4.3. A/D Task
- 4.4. The Terminal Task
- 4.5. The Radio Interface Task
- 4.5.1. Digital Squelch
- 4.5.2. Modulation Technique Used
- 4.5.3. Generic Format of a Radio Frame
- 4.5.4. Data Frames
- 4.5.5. Frame Types
- 4.5.5.1. Request
- 4.5.5.2. Broadcast Answer
- 4.5.5.3. Set I/O Request
- 4.5.5.4. Read I/O Answer
- 4.5.5.5. Broadcast Request
- 4.5.5.6. Ping
- 4.5.5.7. Pong
- 4.5.5.8. Memory Dump Request
- 4.5.5.9. Memory Dump Answer
- 4.5.5.10. Data
- 4.5.5.11. Set ID
- 4.5.5.12. Set Slot Time and Sample Rate
- 4.5.5.13. Set Frequency
- 4.5.5.14. Set Battery Charge Levels
- 4.5.5.15. Set Pulse Counters Parameters
- 4.5.5.16. General Acknowledge
9
The Modem Interface
2.1.3. Transmitter Section
The transmitter section uses the VCO composed by Q2/Q1. This VCO is modulated
on the anode of its varicap diode (D3) by means of a small amount of the data signal.
Before modulation, the data signal is filtered by a four-pole low-pass filter built
around U1. The filter effectively removes the harmonics on the data signal in order
to keep the adjacent channel power under the required level.
The output of the VCO/buffer amplifier is applied to the power amplifier, operating
in class A thus minimizing the harmonics (Q3). The output of the amplifier is direct-
ed to the antenna through the antenna switch and the low pas filter. The antenna is
switched on transmit mode when a positive current is applied on the anode of D1 via
R12. The DC component is obtained from the collector of the power amplifier (Q3).
C11 is used to pass along the RF component.
The transmitter is activated by the microcontroller in two steps:
• The PLL is switched on (by means of Q5);
• After a delay of about 5 mS when the PLL has settled, the power amplifier is
switched on (by means of Q7).
The above mechanism assures a clean transmission start, avoiding that the transmit-
ter “spills-out” its carrier on several channels before stabilizing. The switch off fol-
lows the same principles, but in a reverse order.
2.2. The Modem Interface
The modem operates with two tones: 1 kHz (representing the 1 bits) and 2 kHz (rep-
resenting the 0 bits). A bit cell is represented by a complete time period (
1/f
), thus the
raw throughput varies between 1 and 2 kbps (average 1.5 kbps). The modem func-
tionality is essentially implemented in software. However, a signal conditioning is
performed on both receive and transmit paths.
On receive, the buffered analog data signal from U8:D is applied to a 3 kHz low pass
filter (U8:A). The filter output is further fed both to a Schmidt trigger (U8:C) and a 50
Hz low pass filter (U8:B), the output of the latter being used as a reference for the slic-
er (the Schmidt trigger). The TTL data,
RXDO
, is obtained at the output of the slicer
(i.e. on TP2). The microcontroller overtakes the decoding operation (see also “Mod-
ulation Technique Used” on page 43).
On transmit, only the low pass filter built around U1 is used: its role is to “smooth”
the square signal
TXDI
generated by the microcontroller.