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input impedance CC2592

Other Parts Discussed in Thread: CC2592, CC2591

Dear Sir,

Can TI specifiy to my the input impedance (pin 10) of the CC2592.I would like to investigate the matching towards the antenna in more detail.

Thanks

M. Struik

  • No. The network between the ANT pin and the SMA connector is designed based on load pull measurements to give the most optimal load impedance for the PA (high output power, low harmonics, low current consumption) and also the required filtering to pass ETSI/ FCC requirements. What the impedance in on the PA output is not relevant since s-parameters are linear, small signal parameters and the swing on the PA is large. The PA is in saturation and hence not linear. The PA requirements dominates and the LNA is fairly forgiving on the source impedance.

  • So to summarize TER's post: Our recommendation is to follow the reference design as closely as possible. This includes schematic, BOM, layout, and stack-up.

    Cheers,
    Fredrik

  • Actually Fredrik, we can make a reasonable estimate of the output impedance of the CC2592 based on the information given.  What do we know?

    1) Comments for the CC2591 specify it as either 13 or 25 ohms with very little imaginary component

    2) The reference circuit appears to be both a low pass filter AND matching circuit

    3) S-parameters are small signal, and the power amp is a non-linear large signal device

    Through simulations of the reference circuit (including the 4.7nH bias) I found that a 15 ohm source produced very nice results.  The plot below shows this.  S11 is given in green and S21 in red (plus 3dB).  It looks to me like somebody did a very nice job of tuning this circuit for a good impedance match at 2.45G and as much attenuation at the 3rd harmonic as possible.

    We also know an antenna is not a 50 ohm load all all frequencies, so this plot is only valid (if assuming a quarter wave dipole antenna) at odd harmonics.  The means there was no point trying to do anything for the 2nd harmonic.  The 3rd is 18dB down, which is quite good.

    I can arrive at two conclusions: the CC2592 looks like 15 ohms; and the reference circuit is a very good design.

    However, if you need to simplify because of board space or some other reason, there are simpler matching networks to try.  Since we're translating from a low impedance (15) to high (50), the simplest network is a series component and then a shunt component (in parallel with antenna).  The high pass version of a matching network would be a series 2.2pF capacitor and a 2.7nH shunt inductor.  This gives the response below.

    The return loss (S11) is excellent and I think would provide a wonderful match.  The added bonus is that is also provides an in herent DC block.  However, there is no attenuation of upper harmonics. 

    The other option is a series inductor of 1.0nH and shunt capacitance of 1.8pF (my own tuning).  This gives a matching as good as the reference with far fewer components.  The 3rd harmonic reduction is only 9dB.  That makes the reference circuit 9dB better in that respect. 

    Jim