Moving S11 response with matching / tuning components - Page 1
Forum section: electronics / rf-microwave

author: okw
author_rank: Frequent Contributor
timestamp: June 14, 2023, 01:46:37 pm
content: I have a circuit with a GPS module, SAW filter and an external active antenna. I have added 0402 footprints for PI on left and PI/T-filter on the right side of the SAW (don't mind which R/C/L they are or their values, they are only placeholders). I have tested a lot, but can't seem to "move" the dip of the S11 curve to the desired 1575.42MHz for GPS. It stays at around -2.5 to -3dB. Some matching values gives me a whopping -35 to -40dB, but far away for my desired center frequency.I did three tests (neither had GPS module attached):(1) Tested first left sub-block (L1/L2/C1/SAW). Removed RE1/RE2. Placed my PS200 (vector antenna analyzer) in the point L1/C1. Changed a lot of values for L1/L2/C1, without any luck. Each picture is named after the component values and the measurements.(2) Tested only SAW. Removed C1/L2/RE1/RE2, put a 50 ohms resistor to ground (L3), measured in point C1/L2. Terrible VSWR (~8, but datasheet says 1.4).(3) The whole circuit, including 3V to the antenna. Nothing to write home about, forgot to take picture.Any idea how to improve this?

author: okw
author_rank: Frequent Contributor
timestamp: June 14, 2023, 01:48:22 pm
content: Test 1.

author: okw
author_rank: Frequent Contributor
timestamp: June 14, 2023, 01:49:32 pm
content: Test 1.

author: okw
author_rank: Frequent Contributor
timestamp: June 14, 2023, 01:50:06 pm
content: Test 1

author: okw
author_rank: Frequent Contributor
timestamp: June 14, 2023, 01:56:48 pm
content: Test 2 (only SAW)

author: szoftveres
author_rank: Regular Contributor
timestamp: June 14, 2023, 02:26:08 pm
content: SAW filters are high-Q (way higher than quartz), they don't tune. Where is your original SAW filter coming from, and why is it not centered at GPS, as it is supposed to be? Maybe replace it?

author: okw
author_rank: Frequent Contributor
timestamp: June 14, 2023, 02:35:39 pm
content: Quote from: szoftveres on June 14, 2023, 02:26:08 pmI didn't understand your reply. I attached a schematic where the SAW filter is shown.

author: RFDx
author_rank: Regular Contributor
timestamp: June 14, 2023, 06:18:02 pm
content: The SAW filter is, according to Murata, a 50 Ohm device. The VSWR is (or at least should be) typ. 1.4 @ 1575MHz. Why do you need matching networks?

author: szoftveres
author_rank: Regular Contributor
timestamp: June 14, 2023, 06:26:39 pm
content: This is what I'm getting with your input- and output network

author: okw
author_rank: Frequent Contributor
timestamp: June 14, 2023, 06:54:21 pm
content: Quote from: RFDx on June 14, 2023, 06:18:02 pmI misunderstood the datasheet, thinking unbalanced would mean I needed to balance it (match).So you mean I should drop the matching networks on both sides? Perhaps leave C3 for DC block since it has an active antenna? Any suggestion on value here?Also, suggestions on L5/C4-values for the antenna power?

author: RFDx
author_rank: Regular Contributor
timestamp: June 14, 2023, 08:51:28 pm
content: Quote from: okw on June 14, 2023, 06:54:21 pmYes. The SAW filter has two 50 Ohm IO and three grounding pads although your schematic shows only one.Quote from: okw on June 14, 2023, 06:54:21 pmKeep C3 as a DC block. If you can find an inductor (L5) with a self resonant frequency (SRF) at around 1575MHz, increase C3 to at least 100...150pF. In case L5 has a SRF >= 3GHz use C3 together with L5 to build a 50 Ohm 2nd order Butterworth HP-filter. Examples would be 47nH/22pF or 22nH/10pF for L5/C3 (RL>20dB @ 1575MHz).

author: okw
author_rank: Frequent Contributor
timestamp: June 14, 2023, 09:42:50 pm
content: Quote from: RFDx on June 14, 2023, 08:51:28 pmThanks. Would it make most sense leaving the DC block on the side it is now (right)? Or between the SAW and receiver (left)?Did you mean C4 instead of C3 for the Butterworth? If not, what about the value of C4?Are there any pros/cons of going up or down in inductance, except lower DC resistance for lower inductance.

author: RFDx
author_rank: Regular Contributor
timestamp: June 16, 2023, 01:41:24 am
content: Quote from: okw on June 14, 2023, 09:42:50 pmThe DC block (C3) is on the antenna (right) side. The receiver probably has it's own input capacitor so you woudn't need a DC block at the receiver (left) side.Quote from: okw on June 14, 2023, 09:42:50 pmC3 and L5 form the 50 Ohm highpass filter. C4 (e.g. 1nF or similar) is grounding one side of L5 for AC/RF signals. L5's SRF needs to be above ~3GHz.I see you chose L5=27nH wich has a SRF of ~1.6GHz. In this case increase C3 to at least 150pF.Quote from: okw on June 14, 2023, 09:42:50 pm1nF (0402 size) would be OK. You could increase the value of the bias resistor R1 to something like 10 Ohm.Quote from: okw on June 14, 2023, 09:42:50 pmThe quality factor of an inductor decreases with higher DC resistance. Insertion and return loss get worse. SRF decreases with higher inductance values.

author: okw
author_rank: Frequent Contributor
timestamp: June 16, 2023, 06:05:09 pm
content: Quote from: RFDx on June 16, 2023, 01:41:24 amWould I be better off using 27nF/100-150pF (SRF ~1.6GHz, Q=~20@1.5GHz) or 47nH/22pF (SRF ~2.9GHz, Q=~65@1.5GHz)? I suspect the latter?

author: RFDx
author_rank: Regular Contributor
timestamp: June 19, 2023, 02:28:50 pm
content: Quote from: okw on June 16, 2023, 06:05:09 pmThe HP-filter version would imo be easier to implement. With the 27nH/150pF combination you would have to be sure (e.g. with a measurement) that the SRF of the inductor is indeed ~1.6GHz.A third option would be to use a 1/4 lambda transmission line instead of the inductor. Unfortunately a transmission line needs substantially more space on the PCB.