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# T7024 2.4GHz front-end doubt

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#### acemax

##### New Member
Hi,

I am designing a zigbee application using the T7024 (PA+LNA) but I don't understood the switch driver application on page 10 (Figure 11) of the Design Guide:

https://www.electro-tech-online.com/custompdfs/2009/08/doc4549.pdf

The idea is to use the MC13224 or CC2530 transceiver and the T7024 to increase zigbee range.

Could someone please give an idea of how to implement the 1/4 wave circuit of the design guide?

Thanks,

Andre Fernandes

This circuit is a standard diode Transmit/Receive switch used in many radios. In your case the quarter wavelength line is most easily implemented in microstrip. Microstrip line is a pcb trace on the surface of your board, backed by a ground plane under it. The width of the trace, and the distance to the ground plane are both critical dimensions, so we need to know what the dimension is, in your pcb, from the underside of the top copper layer to the closest surface of the ground plane layer. From this, we calculate the width of the trace. Once you know that, then the remaining critical dimension is the length of this trace, which we estimate using the velocity factor of your particular pcb. For this, we need to know what dielectric is used in your pcb.

Knowing the width and length, it is just a matter of laying this out on your board. When laying out microstrip, you have to keep any ground plane that is on the same layer as the microstrip trace away from that trace by at least the width of the trace. You may turn corners with the trace, but it is better to keep it straight if this is convenient.

If you are not laying out a pcb, then the obvious alternate is to use a piece of small coaxial cable, and cut a coax jumper that is one quarter wavelength long. Calculate length using wavelength=speed of light divided by frequency, then multiply by 0.25 to get one quarter, then multiply by the velocity factor of the cable you use.

The way this T/R switch works is when no current is put through the PIN diodes it is in Rx mode with just the capacitance of the diodes in effect that should be resonated out.

When current is put through PIN diodes they become low resistance. The path for Tx is connected to antenna and Rx port is 'shorted' to ground through bypass cap on power supply side of Rx PIN diode. The short on Rx port is rotated to open circuit at the antenna through the quarter wave line.

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