Hello ST Team and Community,
We are in the process of designing a new product line for my company using ST's ISM transceiver S2-LP along with BALNF-SPI2-01D3 broad band BALUN. This BALUN has 50 Ohm impedance matched with S2-LP transceiver for 860-930 MHz frequency range. We plan to use BALUN (instead of discrete components for matching) because it reduces the PCB board space.
We also plan to use a broad band antenna by TE, part no. 2195835. This is a flexible PC unbalanced antenna with 1.13mm cable from 50-200mm lengths. I have a number of unknowns/questions.
1) The data sheet (DS) page 3 of BALUN shows insertion loss -1.6dB to -1.7dB, and Return loss on antenna -14dB to -18dB. The data sheet does not show, but I guess it has SWR of 1.3:1 to 1.5:1.
2) The TE antenna DS page 2 shows VSWR 1.3 to 1.9 for cable length 50-150mm; and Return loss -15dB to -10dB. These loss numbers are slightly higher for 200mm cable length.
3) For one of our products, the cable length is less than 50mm, and for another much more than 200mm. And the antenna is defined as unbalanced in the DS, which is another unknown.
4) Once we build the products prototypes, the plastic case and the ground plane will have impact on the matching of the antenna to the device components. That means, we will need to add a pie/L network between the BALUN and the antenna to match the two for 50 Ohm nominal impedance. This will reduce the reflection losses etc. and help increase the range of operation for our devices, plus reduce the battery power consumption.
Q1) The BALUN data sheet does not show any matching components between the BALUN and the antenna. (There is only C16 decoupling capacitor.) And I could not find any reference design at ST.com website. Also, the datasheet does not show the discrete components values/types. Is there any reference design for S2-LP transceiver with BALUN? Without that, we won't be able to use these ICs.
Q2) Both the BALUN and the antenna are designed for the broad band frequency range 860-930 MHz. Antenna data sheet shows the resonant frequency with lowest Return Loss and SWR varies with the cable length. To get the minimum return loss, we are thinking to use an RF switch (Skyworks AKYA21024) and two pie matching networks (discrete components) - one for resonant frequency 868 MHz (Europe) and other for 915 MHz (USA). I am guessing... I can install the RF switch and the two Pie component sets between the BALUN and the antenna (the same broad band antenna). We will manually try different L and C values to match with each frequency. (I have seen such configuration in a TI's broad band transceiver CC1350 reference design, but with all discrete components - no BALUN.) I need community feedback on this issue, and a possible solution please.
Thanks.
