Radiometrix Ltd, RXM-UHF Data Sheet

page 7

ANTENNA TYPES

Any of the integral antenna shown in the data sheet for the transmitter module (TXM-418-5) is suitable

for use on the receiver. Additionally a coax fed external dipole or ¼ wave ground plane antenna may be

considered if system range is paramount. A 2.2kΩ resistor wired across the coax at the antenna end

will allow tamper detection (cutting) of the coax using at the tamper sense facility in the receive module;

pin 8 will go low if the coax is cut.

DATA SLICER

A CMOS compatible data output is available on pin 6, this output is normally used to drive a digital

decoder IC or a microprocessor which is performing the data decoding. The signal detect output on pin

11 may be used to gate the data output before it is supplied to a decoder, however this should only be

done on systems with ‘weak’ digital coding i.e where there is a danger of the decoder giving false

outputs on the ‘noise data’ which is present on pin 6 when no valid signals are being received. Systems

with good CRC, checksum or repeat code verification will not require the ‘noise data’ to be gated off and

as a result will be able to decode weaker signals (ie greater range).

The data detect output on pin 11 is normally used for duty cycle power saving for portable equipment

where battery life is a problem. By pulsing the receiver on/off the average supply current may often be

reduced by a factor of 20 or more depending upon the system requirements, the data detect output is

valid 5ms after application of the supply and is used to inhibit the power saving while data decoding is

done.

The data slicer in the receiver module is designed to accept data with a wide range of pulse widths and

mark: space ratio’s. The voltage waveform on pin 4 (AF) is fed to 2 peak detectors, one +ve, one -ve and

a comparator threshold is set half way between the max and min voltage, a small amount of hysterisis is

applied. The data on pin 6 is the output of this comparator.

The date slicer has a transient response time, this is the settling time/hold time of the peak detectors. It

is programmable by an external resistor between pin 5 & 7 (min & max). With no resistor fitted

(normal use) the data slicer settles in approx 300ms from reception of a coded signal (ie the first 300ms

of signal may be corrupt at the data output) and will pass pulse widths up to 50 ms of continuous 1 or 0.

A resistor between pin’s 5 & 7 shortens these time ie

Resistor Value

(pin 5 to 7)

Code Preamble

(minimum length)

Longest 1 or 0 allowed

Open Circuit

300ms

50ms

1 MΩ

150ms

25ms

220 kΩ

30ms

5ms

47 kΩ

7.5ms

1.3ms