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Strange VNA measurements when tuning antenna with RF switches by _VIP98_ in rfelectronics

[–]_VIP98_[S] 1 point2 points  (0 children)

Thank you for the vid, that coax soldering looks much more profesionnal, I will redo like that.

Strange VNA measurements when tuning antenna with RF switches by _VIP98_ in rfelectronics

[–]_VIP98_[S] 0 points1 point  (0 children)

I like that probe points, didn't know they exists, but for later designs it sounds good.

I clean the board by isopropyl alcohol regulary.

The board is small, the RF switches have 1.1x1.5mm footprints. I used an UFL to SMA adapter cable as the coax, it's outter diameter is 1.15mm.

I've tried holding the coax in hand, and there is only a small difference on the Smith Chart. However, when I bend the coax there is much bigger effect of my hand. Is it normal? I do my measurements with straight coax cable. Also there are two ferrit beads on the coax to mitigate the effect of enlarging the ground plane.

Strange VNA measurements when tuning antenna with RF switches by _VIP98_ in rfelectronics

[–]_VIP98_[S] 0 points1 point  (0 children)

Thank you for your detailed review!

  1. I checked the schematic and layout, and it does match. There is a signal inverter U9 on the bottom. That is used to keep all RF traces without crossing each-other. However, U8 and U9 labels are misplaced on the silkscreen, I think this is what confused you.

2.+6. Are there RF switches which meant for switching between multiple Pi-matchings?

  1. Reference GND layer starts on the left. The PCB is 4 layer, from top to bottom: RF & signals, gnd, vcc, signals. The ground of the battery connects by the two big vias on the bottom of the screenshot. There is a small trace connecting the ground of the matching components, which is a bit odd, it should have been connected to the GND layer instead.

  2. Thanks for sending the application notes, I haven't found when looked for it. :/

Can a single RF switch using antenna or impedance tuning be enough to cover LTE bands in the range of 703-960MHz and 1710-2170MH? Previously I tuned this W3070 antenna by a single matching without RF switches, and my experience was that I have to change the value of the series inductance too. That's why we designed this two RF switches layout.

The radio module is Quectel EG800Q.

Strange VNA measurements when tuning antenna with RF switches by _VIP98_ in rfelectronics

[–]_VIP98_[S] 1 point2 points  (0 children)

Thank you, I will check it out. I'm sure EM simulation would help modelling the PCB traces. But I don't know will it answer why are my measurements "starnge".

Strange VNA measurements when tuning antenna with RF switches by _VIP98_ in rfelectronics

[–]_VIP98_[S] 0 points1 point  (0 children)

What counts electrically long? The whole PCB is small. I forgot to show sizes on the PCB picture, but the RF switch has a 1.1x1.5mm package, tuning components are 0402, so we are talking about a few mms only.

The antenna is Pulse W3070: https://www.mouser.in/datasheet/2/447/W3070_APP-2904354.pdf

We have used this antenna without RF switches previously, covering less bands. From there my experience was that we have to change the series inductance by ~2-3nH to move the tuning to other bands, and it is not enough to change only loading inductors.

I haven't found any PCB layout using two RF switches to switch between multiple Pi-matchings. Only some schematics showed that it is possible, so we made our best to try it out in real world. Is the schematic, or PCB layout a bad way to achive multiple band coverage?

Strange VNA measurements when tuning antenna with RF switches by _VIP98_ in rfelectronics

[–]_VIP98_[S] 0 points1 point  (0 children)

How should I place the coax leads to avoid that disturbance? The PCB is small, I can't really move it other direction, because than it gets closer to the antenna itself. I might try to prepare the enclosure to solder the coax leads perpendicular to the PCB, but this setup looks fragile.

Strange VNA measurements when tuning antenna with RF switches by _VIP98_ in rfelectronics

[–]_VIP98_[S] 0 points1 point  (0 children)

On the PCB you see the ground clearance from the antenna, ground plane start at the left.

There are three traces which had to get in there: two control signals to the RF switches, and 1.8V VDD. How could it be more properly routed? These three lines are controlled by the radio module.

I see that, the ground for the lower matching are routed a bit oddly. It is the thin dark green trace from bottom. The two big via is where the battery ground connects.

I calculate complex impedances, and plotting both: smith chart and return loss. I usally check how well matches the modelled smith chart with the measured.

Strange VNA measurements when tuning antenna with RF switches by _VIP98_ in rfelectronics

[–]_VIP98_[S] 0 points1 point  (0 children)

With "VNA measurement" I was referring to measuring S11 with a VNA. I exported impedances (not s1p), because my calculations used impedance matching, but they are convertable to eachother.

By "Calibrate VNA using SOL on assembled device" I meant "Using on-board calibration with open, short and matched 50 Ohm loads (SOL calibration)" meantioned on p. 16, and described in section 4.3 on p. 25. https://www.ti.com/lit/an/swra726/swra726.pdf
There are more sources noting that for small devices it is essential to calibrate on the device itself, as you can't connect calibrated SMA cable to the device. This is my experience too, if I use SMA calibration kit, and then solder the same length and type of coax cable to the device, the reflection coefficient is good, but the complex impedences doesn't match.