u-blox TOBY-L4 Series System Integration Manual

Contents

TOBY-L4 series - System Integration ManualUBX-16024839 - R04 Design-inPage 78 of 143The DC impedance at the RF port for some antennas may be a DC open (e.g. linear monopole) or a DC short toreference GND (e.g. PIFA antenna). For those antennas without the diagnostic circuit of Figure 36, the measuredDC resistance is always at the limits of the measurement range (respectively open or short), and there is nomeans to distinguish between a defect on the antenna path with similar characteristics (respectively: removal oflinear antenna or RF cable shorted to GND for a PIFA antenna).Furthermore, any other DC signal injected to the RF connection from an ANT connector to a radiating elementwill alter the measurement and produce invalid results for antenna detection.It is recommended to use an antenna with a built-in diagnostic resistor in the range from 5 k to 30 kto assure good antenna detection functionality and avoid a reduction of module RF performance. Thechoke inductor should exhibit a parallel Self Resonance Frequency (SRF) in the range of 1 GHz to improvethe RF isolation of the load resistor.For example:Consider an antenna with a built-in DC load resistor of 15 k. Using the +UANTR AT command, the modulereports the resistance value evaluated from the antenna connector provided on the application board to GND: Reported values close to the used diagnostic resistor nominal value (i.e. values from 13 k to 17 k if a15 k diagnostic resistor is used) indicate that the antenna is properly connected. Values close to the measurement range maximum limit (approximately 50 k) or an open-circuit“over range” report (see the u-blox AT Commands Manual [2]) means that that the antenna is notconnected or the RF cable is broken. Reported values below the measurement range minimum limit (1 k) indicate a short to GND at theantenna or along the RF cable. Measurement inside the valid measurement range and outside the expected range may indicate an incorrectconnection, damaged antenna or wrong value of antenna load resistor for diagnostics. The reported value could differ from the real resistance value of the diagnostic resistor mounted inside theantenna assembly due to the antenna cable length, the antenna cable capacity or the measurement methodused.If the primary / secondary antenna detection function is not required by the customer application, theANT_DET pin can be left unconnected and the ANT1 / ANT2 pins can be directly connected to therelated antenna connector by means of a 50  transmission line as illustrated in Figure 35.2.4.2.2 Guidelines for ANT_DET layout designThe recommended layout for the primary antenna detection circuit to be provided on the application board toachieve the primary antenna detection functionality, implementing the recommended schematic illustrated inFigure 36 and Table 27, is explained here: The ANT1 / ANT2 pins must be connected to the antenna connector by means of a 50  transmission line,implementing the design guidelines described in section 2.4.1 and the recommendations of the SMAconnector manufacturer. DC blocking capacitor at ANT1 / ANT2 pins (C2, C3) must be placed in series to the 50  RF line. The ANT_DET pin must be connected to the 50  transmission line by means of a sense line. Choke inductors in series at the ANT_DET pin (L1, L2) must be placed so that one pad is on the 50 transmission line and the other pad represents the start of the sense line to the ANT_DET pin. The additional components (R1, C1 and D1) on the ANT_DET line must be placed as ESD protection.