u-blox EVA-8M Series Hardware Integration Manual

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u-blox EVA-8M Series Hardware Integration Manual Manual pdf 14 page image

EVA-8M and EVA-M8 series - Hardware Integration ManualUBX-16010593 - R06 Design-in Page 14 of 47Early Production Information2.3.3.2 3-pin antenna supervisorThe 3-pin antenna supervisor is comprised of the ANT_DET (active antenna detection), ANT_OK(short detection) and ANT_OFF (antenna on/off control) pins. This function must be activated bysending the following sequence to the EVA-8M / EVA-M8 series receivers in production:• B5 62 06 41 0C 00 00 00 03 1F CD 1A 38 57 FF FF F6 FF DE 11☞ Applying this sequence results in a permanent change and cannot be reversed. An unstable supplyvoltage at the VCC_IO pin while applying this sequence can also damage the receiver.Function I/O Description RemarksANT_DET I(pull-up)Antenna detected“high” = Antenna detected“low” = Antenna not detectedByte sequence given in section 2.3.3.2should be applied.ANT_OK I(pull-up)Antenna not shorted“high” = antenna has no short“low” = antenna has a shortByte sequence given in section 2.3.3.2should be applied.ANT_OFF O Control signal to turn on and off the antennasupply“high” = turn off antenna supply“low” = short to GNDByte sequence given in section 2.3.3.2should be applied.Table 4: 3-pin Antenna supervisor pinsThe external circuitry, as shown in Appendix A.8, (see Figure 20) provides detection of an activeantenna connection status. If the active antenna is present, the DC supply current exceeds a presetthreshold defined by R4, R5, and R6. It will shut down the antenna via transistor T1 if a short circuithas been detected via U7 or if it’s not required (e.g. in Power Save Mode).The status of the active antenna can be checked by the UBX-MON-HW message. More informationsee theu-blox 8 / u-blox M8 Receiver Description Including Protocol Specification [3].The open drain buffers U4, U7 and U8 (e.g. Fairchild NC7WZ07) are needed to shift the voltage levels.R3 is required as a passive pull-up to control T1 because U4 has an open drain output. R4 serves as acurrent limiter in the event of a short circuit.2.3.4 Electromagnetic interference and I/O linesAny I/O signal line (length > ~3 mm) can act as an antenna and may pick up arbitrary RF signalstransferring them as noise into the GNSS receiver. This specifically applies to unshielded lines, lineswhere the corresponding GND layer is remote or missing entirely, and lines close to the edges of theprinted circuit board. If for example, a cellular signal radiates into an unshielded high-impedance line,it is possible to generate noise in the order of volts and not only distort receiver operation but alsodamage it permanently.On the other hand, noise generated at the I/O pins will emit from unshielded I/O lines. Receiverperformance may be degraded when this noise is coupled into the GNSS antenna (see Figure 9).In case of improper shielding, it is recommended to use resistors or ferrite beads (see Appendix B.12)on the I/O lines in series. These components should be chosen with care because they will affect alsothe signal rise times. Alternatively, feed-thru capacitors with good GND connection close to the GNSSreceiver can be used (see Appendix B.13).EMI protection measures are particularly useful when RF emitting devices are placed next to theGNSS receiver and/or to minimize the risk of EMI degradation due to self-jamming. An adequatelayout with a robust grounding concept is essential in order to protect against EMI. More informationcan be found in subsection 2.13.6.3.