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Support Circuit Design



                                                                          Support Circuit Design  345

                             (The 50-mA value is PIN3’s forward current draw, while PIN1 and PIN2
                           will each be on by 25 mA, for a low value of on resistance. DC bias is in
                           volts.)
                        4. The design must have a positive voltage to turn on the series PINs and turn
                           off the shunt PIN for an on switch condition; a negative voltage will switch
                           off the series diodes and turn on the shunt diode, thus creating a high-iso-
                           lation RF switch for the off condition. However, for low-level RF input sig-
                           nals, only a positive on bias is required, while off simply needs 0 V (if there
                           is no negative bias, then high-level input signals can self-bias the PINs to
                           on, which will lower the switch’s off isolation drastically).

                        High isolation series/shunt RF electronic switch (Fig. 8.13). When this switch
                        receives a V       of  10 V, the series RF  PIN1 becomes reversed biased
                                    CONTROL                      IN
                        (causing a high resistance), while the shunt PIN2 is forward biased (creating
                        a low resistance), shunting any input to ground that may get through the
                        series diode. When 	10 V is placed at V       , the series PIN1 is forward
                                                               CONTROL
                        biased (giving a low resistance), while the shunt PIN2 is reverse biased (block-
                        ing the input signal from shunting to ground), allowing the RF IN to pass
                        through to RF OUT.
                        Wideband diode RF switch (Fig. 8.14). This switch is capable of operating up to
                        1 GHz with low distortion and high isolation (up to 40 dB in the off condition).
































                        Figure 8.12 A high-isolation RF PIN diode switch.



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