NRX1 Datasheet, PDF (4/8 Pages) Radiometrix Ltd – Low cost VHF Narrow Band FM receiver

Applications information
RX Received Signal Strength Indicator (RSSI)
The NRX1 has a wide range RSSI which measures the strength of an incoming signal over a range of
60dB or more. This allows assessment of link quality and available margin and is useful when
performing range tests.
The output on pin 11 of the module has a standing DC bias of up to 0.5V with no signal, rising to 2.4V
at maximum indication. ∆Vmin-max is typically 1V and is largely independent of standing bias variations.
Output impedance is 56kΩ. Pin 11 can drive a 100µA meter directly, for simple monitoring.
Typical RSSI characteristic is as shown below:
2.4
2.2
2
1.8
1.6
1.4
1.2
1
0.8
0.6
0.4
0.2
0
RF Level (dBm)
Figure 4: RSSI level with respect to received RF level at NRX1 antenna pin
Packet data
In general, data to be sent via a radio link is formed into a serial "packet" of the form :-
Preamble - Control - Address - Data - CRC
Where: Preamble:
This is mandatory for the adaptive data slicer in the receiver in the NRX1 to
stabilise. The NRX1 will be stable after 10ms. Additional preamble time may be
desired for decoder bit synchronisation, firmware carrier detection or receiver
wake up.
Control:
The minimum requirement is a single bit or unique bit pattern to
indicate the start of message (frame sync.). Additionally, decoder
information is often placed here such as: packet count, byte count, flow
control bits (e.g. ACK, repeat count), repeater control, scrambler
information etc.
Address:
This information is used for identification purposes and would at least
contain a 16/24 bit source address, additionally - destination address,
site / system code , unit number and repeater address's may be placed
here.
Data:
User data , generally limited to 256 bytes or less (very long packets
should be avoided to minimise repeat overheads on CRC failure and
channel hogging).
CRC:
16/24 Bit CRC or Checksum of control-address-data fields used by the
decoder to verify the integrity of the packet.
The exact makeup of the packet depends upon the system requirements and may involve some complex
air-traffic density statistics to optimise through-put in large networked systems.
Radiometrix Ltd.
NRX1 receiver data sheet
Page 4