Folded Dipole Antenna for CC2520

Hi,

The impedance is different from CC2520 than CC2500, CC2510, CC2511 & CC2550 so the antenna can not be copied exactly. The antenna element could be used but the impedance matching part of the tracks leading to the antenna element from the chip would have to be changed slightly to adjust for the impedance change. You would need to simulate the new impedance matching part of the tracks in ADS or similiar software.

Regards,   Richard.

Greetings!

What typical differential impedance value for CC2520? We calculate folded dipole model in HFSS.

Thank you,

Roma

Hi Richard, do you have any reference design of CC2520 RF matching network using lump components instead of transmission line. Thanks.

Hi, You can try these values instead for the balun circuit:

C shunt = 0.5pF

L series = 2.7nH

L shunt = 1.8nH

C series = 0.75pF

Regards,

   Richard

UGlee,

Its pretty easy to match this up to the CC2520.  The traces H1 in the app note actually is what matches to the antenna.  The actual antenna starts at the pads H2 which adjust the final phase. 

H1 adjust the imedance to match the input impedance of the antenna.  The antenna has input impedance of 277 ohms the imgaginary part or j is close to adjust to zero by H2 distance.    If you use a line calculator you can get the degrees of change of H1 and the impedance change from 80 to  differential value of 277 this means you have single-ended 138.5 = sqrt of (80 * value)  or a value of 239 for single-ended input or differentiall 2*239 for the end of the H1 impedance. 

The phase of the CC2500 is 80+j74 moves by its length of H1 will subtract phase because of the capacitance of between the H1 lines.  This means H2 must add additional phase of jx value to adjust the j value back to +j74 correct loading and differenitally 277. 

I hope you follow this.  If you use a good line calculator which gives degrees of line length it is easy to calculator.  Appleworks has a good one, also Agilent,  AWR only allows one way calculations.   Do not change the antenna unless you are changing the stack up.  The stack up will change the capacitance value therefore the antenna impedance will change.  So please be careful when copying.  If you change the stackup you will need to retune the antenna.   It is possible to have a perfect match without using components. 

If you want to make it more compact you can substitute for H2 two series capacitors to adjust the phase.    A differential inductor acrossed the antenna input allows for adjustment of the real values impedance match between the CC2520 and the antenna. 

Rgds,

Hi,

I'm trying to use TI folded dipole antenna with CC2500 transceiver for a school project, but I have a hard time understanding the way in witch the matching was done.

In the previous posts it was said that  the antenna has an impedance of about 277ohm and the chip has 80+j74ohm. The design note also specifies the traces dimensions H1, H2, H3 etc. From posts on other forums i concluded that for H1, w=0.25(trace width) and s=0.5(spacing), and for H2 and H3, w=0.5 and s=1.5, this with a dielectric substrate of 0.8mm(height). 

I calculated the impedance values using TXline from AWR, assuming that the the lines are microstrip coupled lines, and the values are, for:

H1: Zodd=86ohm, Zeven=144ohm, Zdiff=2*Zodd=172

H2:Zodd=80ohm, Zeven=94ohm, Zdiff=160

The chip and antenna both have differential ports, so I'm thinking that the antenna should "see" a 277 impedance value and the transceiver should "see" a 80+j74 impedance value, but it doesen't result from calculus. Also I've considered the transmission line as a quarter wave transformer but the length is too short compered to wavelength  (aprox. lambad/16) and from what i know the trace widths should be inveresed (the traces with small w and greater impedance should be connected to the antenna). 

Could someone explain to me how the impedance matching is done because I want to use a dielectric substrate with a 1.6mm height and I have to recalculate the traces dimensions and it is not clear to me how. I'm knew to RF design and I'm sure something escapes me :)

Thank you,  Andrei

Andrei,

Here are the results of our measured data for the antenna.  We match the antenna directly to the impedance of the radio.  Of course the trace layout is critical in matching this to the radio.   The antenna has a high impedance but as you apply microstip line techniques you add the necessary lines to match or present a load to the radio output for best power transfer.   I know this is a short answer but the antenna is correctly designed and matched to the intended radio. 

Refer to David Pozer book on Microwave Design. 

6116.folded dipole .pdf