T7024 2.4GHz front-end doubt

[acemax]

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:

http://www.atmel.com/dyn/resources/prod_documents/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

 

[VEBill]

It's one-quarter wavelength of transmission line, including consideration of velocity factor. When one end is essentially shorted to ground, then other end will present a good open circuit. I'm assuming those diodes are being used as RF switches.

Stand by for others' contributions.

 

[biff44]

well, it is a basic way to select how your antenna is connected to the chip. When DC current is flowing in the two pin diodes, then the PA is connected to the antenna, and the LNA input is short circuited to ground via the capacitor to ground right next to the second diode. If any of the PA output power goes down that line to the LNA, it sees the RF short circuit, and no big transmit voltage can develop to hurt the LNA input.

For any RF energy going down the line (with both PIN diodes forward biased), it has a 90 degree phase shift, hits the pin diode RF-shorted to ground and bounces back with 180 degrees, and travels back to the common junction with another 90 degrees. So, the reflection coefficient looking down the line toward the LNA is p=1 with an angle of 360 degrees, or equivalently p=1 with an angle of 0 degrees. That is also the definition of an "open circuit". So any RF energy looking down that line sees an open circuit, and instead goes toward the antenna.

Now the reality is that you can not make the line be 90 degrees in electrical length from 2400 to 2500 MHz--the best you can do is to make the line 90 degrees at midband (2450 MHz). Also, there are other complications. The PIN diode has some series inductance, which causes phase shift. The capacitor to ground, and the ground via it uses, also have some inductance. So you really do not want 90 degrees at 2450 MHz, but something slightly different from 90 degrees to compensate for it.

The proper way to do the design is to use a RF simulator program, with models of the diode, capacitor, via hole, and microstrip transmission lines, and tweak the transmission line length to center the insertion loss from PA to antenna at 2450 MHz.

A rough guess, if you keep the pin diode, capacitor, and via hole very close together, is that they might look like perhaps a 1 nH inductance to ground when the PIN diode is forward biased. That is J15 ohms of reactance. Using a smith chart, you can see that you need a line length of 0.453 wavelength for the line in question.

You can calculate how long this is if you know the dielectric constant of the board you are using, the width needed to make it 50 ohm, and the effective dielectric constant. You can probably find a free program online somewhere to calculate those things, maybe apcad, etc. In FR4 board, 14 mils thick, the line would be around 25 mils wide and 547 mils long.

http://www.MaguffinMicrowave.com

Maguffin Microwave wireless design consulting

 

[Comcokid]

What Atmel is doing is using PIN diodes to switch the TX and RX to the antenna. A quick search found the following circuit (somewhat similar) that has more explanation to the 1/4 wavelength line to cancel out reflections from the TX port(or RX port if receiving).

http://www.nitehawk.com/sm5bsz/pindiode.htm

Given that the Chipcon (now TI) RF ports are a little iffy on spectrum and matching, and Atmel is not necessarily known as an RF fabrication company, I suspect you will have your work cut out for you.

I successfully designed a LNA/PA for a 802.15.4 module using a Hittite PA chip, a Hittite LNA chip, and a pair of NEC SPDT RF switches, and implemented it on a double sided design incorporing an on-board F antenna using a Freescale 802.15.4 chip and no special voltage levels to support the LNA/PA/SW. Got a full mile + range with only about 70 mA additional current. However, the company direction fell apart before this particular module went to FCC testing, so I don't know how good the spectrum was.