Can the transmitters and receivers be used to make a transceiver?

A transceiver can be made by placing a transmitter, receiver, and antenna switch on each side of the link. The antenna switch is an integrated circuit that connects the antenna to either the transmitter or receiver based on the logic state of one or two control lines. The switch serves two primary purposes. First, it maintains a 50-ohm match between the antenna and the modules. If you were to connect both modules directly to the antenna, each part would see a 25-ohm impedance. This mismatch would result in losses that would degrade the range and performance of the system.

Second, it prevents the full output power of the transmitter from being placed directly on to the sensitive front end of the receiver. While this probably will not damage the receiver, it will saturate the front end and will take several tens of milliseconds for the receiver's sensitivity to return to the point where it can see a signal.

Antenna switches (also called T/R switches or RF switches) are inexpensive, small, typically six-pin ICs. They are available from several manufacturers, most notably NEC, Macom and Peregrine Semiconductor.

Linx does not recommend a specific part over another because most are very similar in performance. One application may benefit from one package or pin-out while another application may benefit from a different package or pin-out. Look for the highest isolation between the Tx and Rx ports (typically around 30dB) and the lowest insertion loss between the Tx or Rx port and the antenna port (typically around 0.5dB) at the frequency and impedance of interest. Some have one control line and a Vcc line, others have two control lines, but all will have the same basic operation. It is up to the designer to select the package and manufacturer that works best for their application.

Many antenna switch data sheets will recommend coupling capacitors on the port lines to prevent the DC bias in the switch from getting into other stages. The RF modules have an internal series capacitor as a part of the output match that can also act as the coupling capacitors, so no additional components are needed on these lines.

A coupling capacitor may be needed on the antenna line depending on the type of antenna. Our monopole antennas are essentially a wire, which is an open circuit at DC so no capacitor is required. Loop trace antennas, such as our Splatch Series, are short circuits at DC and would require a capacitor.

When making the transceiver, the control lines for the switch can be connected to the power down lines of the RF modules. This will throw the switch and power the modules up and down at the same time. The transmitter will need to be powered down while the unit is in receive mode to ensure the sensitivity of the receiver. The antenna switch will provide about 30dB of attenuation, so if the transmitter is set to 0dBm, the intended signal will need to be greater than -30dBm for the receiver to pick it up. Since most of the receivers have a sensitivity near or in excess of -100dBm, this will seriously degrade the range and performance of the system.

This is also important in the case of the Linx LT Series and LR Series modules. The LT and LR transmitters will have the oscillator active unless either the PDN or Vcc line is low. The DATA line will activate the amplifier, so it will be on when the DATA line is high and off when the DATA line is low. When the amplifier is off, the oscillator will be on the output at around -70dBm. With around 30dB of isolation through the switch, the receiver will see a signal of about -100dBm on its input. With a typical sensitivity of -112dBm, this will reduce the range of the system, so it is important to make an allowance to power the transmitter down when in receive mode. Connecting the control lines to the power down lines of the modules is an easy way to take care of this in one step. See AN-00128: Data and Bidirectional Transmissions under Part 15.231 for more information.