How do I match between those two? - Page 1
Forum section: electronics / rf-microwave

author: technix
author_rank: Super Contributor
timestamp: May 06, 2020, 06:55:13 pm
content: I want to match STBlueNRG-132 to a balun for use with a chip antenna. While ST did specify a certain balun of their own, I am not comfortable with using it as that is a small BGA style part which I can not solder manually. So I am trying to find alternatives that can match the (25.4 + j20.8)Ω 2440MHz differential RF port to a single ended 50Ω chip antenna.When I threw the numbers intoAnalog Devices' RF Impedance Matching Calculatorpage, it gave me an option: two inductors at 137pH and a capacitor at 1.3pF, when matching a 50Ω differential port to that complex impedance. 50Ω differential happens to be a standard value for chip baluns, so if this match works I got my balun immediately. Now how should I interpret this? Do those numbers suggest that I can get sway without matching, match it with 0603 lump passives at those values, or just wing it with a slightly longer stretch of differential traces on the PCB? (I can not solder 0402 or smaller parts reliably. The PCB stackup is 2-layer 1.2mm or 1.6mm FR4.)This project is intended to create a hassle-free bare-bones BLE to UART bridge module that works with iOS devices.

author: E Kafeman
author_rank: Regular Contributor
timestamp: May 10, 2020, 04:23:36 pm
content: It is rather common to use lattice type LC balun for these kinds of chips and frequencies. Se example:https://analog.intgckts.com/impedance-matching/lumped-lc-balun/It is cheap and simple and if you is clever can impedance matching be included as part of the balun.A such network need short distances between components and 0603 is a bit on the big side.Good light, tweezers and a solder pen in good shape or heat-gun and 0201 is no problem but it need some practise.Notice also that in GHz world is size and type of inductor important factors. A big inductor requires more space and bigger pads which increases stray-losses and with component size does ESR and SRF change.Just for the matching part is it correct that a capacitor can be enough. Check if output pins are DC biased so that they not becomes shortcut or have other DC requirements.It looks like below Smith Chart before and after. Not 100% 50+j0 Ohm but close enough.Any requirement for RX impedance which not is same as TX impedance?How is this impedance measured? Near IC pins or 5 mm away at a defined PCB? Measured result will differ. If measured near IC pins and balun and matching circuit is placed 5 mm away can that result in a measured impedance of 25+j50 Ohm instead as a rough example, due to added time delay.I prefer to measure myself even if RX and TX impedances are given. Every PCB adds its own parameters due to ground-plane design, dielectrica and trace width.If you not have any measurement possibilities are chances for any impedance matching to improve performance near zero.However can a balun and DC decoupling can be necessary to not destroy the chip when load it with external antennas.

author: technix
author_rank: Super Contributor
timestamp: May 15, 2020, 01:55:28 pm
content: Quote from: E Kafeman on May 10, 2020, 04:23:36 pmI am hoping I can get away with one 0603 cap and the chip balun right next to the RF pins from that chip, within 1mm of each other. The ST reference design with their balun put their specifically matched balun less than one millimeter away from the edge of the chip.

author: cdev
author_rank: Super Contributor
timestamp: May 15, 2020, 09:00:50 pm
content: for 2400 Mhz very thin FR4 board (and the resultant thinner traces) are better than thick.So far I have not been able to successfully solder the really small parts by hand. Hot plate reflow is the only way Ive been able to.

author: technix
author_rank: Super Contributor
timestamp: May 16, 2020, 07:20:21 am
content: Quote from: cdev on May 15, 2020, 09:00:50 pmCan I use a regular thinkness 4 layer board instead, with the 2nd layer being entirely the ground for this purpose?Quote from: cdev on May 15, 2020, 09:00:50 pmThis is why I prefer larger components like 0603 and above

author: OwO
author_rank: Super Contributor
timestamp: May 16, 2020, 09:15:15 am
content: 0603 1pF at 2.4GHz is pushing it. The capacitance can be as high as 1.3pF depending on how you mount the capacitor. 0402 0.8pF might do.

author: technix
author_rank: Super Contributor
timestamp: May 16, 2020, 03:11:24 pm
content: Can I just run the traces as 5mil wide with 5mil spacing differential traces on a 2-layer or 4-layer PCB for a longer stretch than usual to get the desired inductance and capacitance?

author: OwO
author_rank: Super Contributor
timestamp: May 16, 2020, 04:28:50 pm
content: 1pF @ 2.4GHz is still a bit much to create with distributed components. 1.0pF 0402 should be ok, it doesn't need to be very accurate. No one uses 0603 components at 2.4GHz other than for decoupling and DC blocking.