Functional overview
The transmit section of the BiM3B consists of a frequency modulated crystal locked PLL feeding a
buffer amplifier and RF filter. A Tx select line controls operation. The transmitter achieves full RF
output typically within 1ms of this line being pulled low. Modulation is applied at the TXD input and
may be either a serial digital stream toggling between 0V and 3V (digital drive) or a high level analogue
waveform with the same peak limits (linear drive). Modulation shaping is performed internally by a 2nd
order lowpass filter, which minimises spectral spreading. The RF output is filtered to ensure compliance
with the appropriate regulations and fed via a Tx/Rx changeover switch to the antenna pin.
The receive section is a single conversion FM superhet with an IF of 10.7MHz. A SAW bandpass filter in
the receiver front-end provides image rejection and suppression of other unwanted out-of-band signals.
Like the transmitter, the receiver is controlled by its own active low RX select line. A post-detection
lowpass filter establishes the signal bandwidth and ensures clean operation of the subsequent adaptive
data slicer. The slicer is optimised for balanced data such as bi-phase code. The standard module
features a received signal strength (RSSI) output with 60dB of range. A version of BiM3B featuring a
fast acting Carrier Detect (CD) output on the same RSSI pin is also available. The CD output will
indicate the presence of any RF signals on the carrier frequency.
matching
& lpf
PA
VCO
÷
64
Loop
Filter
φ
ref
osc
35kHz
LPF
TXD
Antenna
Tx/Rx switch
÷
32
φ
Loop
Filter
ref
osc
Vcc
2.9 - 16V
Regulator
& Tx/Rx
logic
Tx select
Rx select
VCO
RSSI/CD
869MHz
BPF
LPF
data
slicer
preamp
IF Amp
Demod
RXD
Mixer
10.7MHz
AF
Pin description
Fig. 2: Block diagram
RF GND
pins 1 & 3
RF ground pins, internally connected to the module screen and to pins 5, 9, 10 & 18 (0V). These pins
should be connected directly to the RF return path (e.g. coax braid, main PCB ground plane etc).
Antenna
pin 2
50Ω RF connection to the antenna, DC-isolated. See pages 7 & 8 for details of suitable antennas and
feeds.
0V (GND)
pins 5, 9, 10 & 18
Supply ground connection and screen.
RSSI
pin 11
Received Signal Strength Indicator with 60dB range, operational when Rx is enabled. Output voltage
nominally 0.15 - 0.40Vdc (no signal), 1.40Vdc (maximum). See page.6 for typical characteristics.
CD
pin 11
Carrier Detect - When the receiver is enabled, a low indicates a signal above the detection threshold is
being received. The output is high impedance (50kΩ) and should only be used to drive a CMOS logic
input.
Radiometrix Ltd, BiM3B Data Sheet
page 2
RADIOMETRIX [ RADIOMETRIX LTD ]
