Hi,
We have again done careful calculation on the impedance on two crucial tracks or groups of tracks on CC2500-realted RF modules. We calculated based on copper thickness, FR4 thickness, ϵr, track with and separation, track distance to surrounding GND copper in the same layer in the case of coplanar waveguide. If ϵr is not provided in the readme file, we calculate for both 4.2 and 4.5. If the distance between track and surrounding GND copper in the same layer is non-uniform, we calculate for both the smallest and the largest distance.
CC2500 datasheet only have two numerical information for impedance:
- The differential load impedance for RF_P/RF_N port is 80+j74, page 11
- "The balun transforms the impedance to match 50 ohm antenna”, page 18
The two crucial tracks or groups of tracks we calculated are
a) Between RF_P/RF_N port and balun input
b) Between balun output to antenna
For a), we also calculated differential impedance in addition to microstrip impedance because the RF_P and RF_N are differential and tracks run close in parallel; for b), we also calculated coplanar waveguide impedance for the USB non-Nano dongle, CC2500EM-31mil, CC2500EM-62mil and RF2500T because the feed line between balun output and antenna are surrounded by GND copper on the same layer.
Z(RF_P/RF_N-balun)
For RF_P/RF_N to balun line, all ways of calculation give significant large value, among which the smallest one is 71.78 for USB non-Nano dongle treated as microstrip. Other values are all larger than 100ohm.
The magnitude for datasheet’s “differential load impedance for RF_P/RF_N 80+j74” is root(80^2+74^2)=108.97, which is closer to many values for Z(RF_P/RF_N-balun) above. Is this NOT a coincidence?
TER http://e2e.ti.com/support/low_power_rf/f/155/p/199899/712114.aspx#711454 If this distance is the same in your board as in the reference design you don't have to change the balun or the tracewidths. Note that it is only for traces that are longer than 0.1 times the wavelength you need to consider 50ohm traces.
The very basic wave propagation principle shows that if the medium or path is not uniform there is DEFINITELY reflection. Is TER’s 0.1λ comment related to diffraction? Are we licensed to NOT consider the impedance between RF_P/RF_N and the balun as long as the trace is short? We raise this question because we will use JTI chip balun and does not directly copy the reference design.
Z(balun-antenna)
For Z(balun-antenna), coplanar waveguide calculations are closer to 50ohm requirement in all cases. The 44.01 value occurs when we use the smallest trace-GND-copper separation, and if we take the largest 37mil separation the 49.23 result is very close to 50. If we use microstrip calculation, for CC2500EM-062 and RF2500T (which it not a proper design for RF performance), the deviation form 50ohm is very large.
So could someone confirm with us whether coplanar waveguide calculation should be used for Z(balun-antenna)?
Matt
