50Ω in a GNSS antenna and where/when to use it - Page 1
Forum section: electronics / rf-microwave

author: rfleming
author_rank: Regular Contributor
timestamp: January 09, 2020, 11:17:32 pm
content: I currently have a gnss antenna based around a uBlox MaxM8W and only for the first time ever trying to design a antenna circuit around it. I have both options for an external an on board patch antenna, with the external likely to use an internal LNA and designed one board. My main concern is that I can get a gnss signal in general using both on-board and off-board antennas, but my on-board LNA design is terrible, I can't seem to push a signal through it at all, nor do i truly know where to ACTUALLY have my 50Ω resistance. I have chosen an inductor that has a 50Ω resistance at the rated frequency and the LNA app note recommends I use one. On the other hand, the on-board patch antenna already says that it has a 50Ω termination resistance. Same with all my on board connectors.Well enough spit balling, attached is my current PCB and schematic.The default build has R19 and C43 DNP so there is 2 separate circuit options. C43 allows the SAW filter to be used for the external antenna and R19 can be loaded if the LNA (U2) didn't work and I needed to bypass it.I am really looking for suggestions on how to improve this circuit and clearing up my knowledge on where/when to use the 50Ω terminations.Cheers,Ryan.

author: Yansi
author_rank: Super Contributor
timestamp: January 09, 2020, 11:42:59 pm
content: C5 is hopelessly far from the amplifier. Likely oscillating wildly.

author: pigrew
author_rank: Frequent Contributor
timestamp: January 10, 2020, 12:25:45 am
content: Unless I'm mistaken, you won't need a 50 ohm resistor... you should just rely on the proper matching components giving you the equivalent of 50 ohms (taking into account the input/output impedances of the LNA/SAW filter). The datasheets will nearly always suggest some particular matching components to transform the ports to be 50 ohm. If you're not using the PCB antenna, be aware that the length of track going to it will be a long RF stub in parallel to your LNA input. It'll have an effect on the input impedance of the board. On the other hand, the frequency is low-ish and the board smaller, so you likely can get away with not well controlled transmission line impedances.I feel like there's something that I'm missing, as I'm not able to make sense of the many of the components you have (L6,L30,L31,C43,C44,L29). The datasheet of the LNA shows that you need a 7.5 nH inductor in series with its input, in order for the input impedance to be 50 ohms. Also the DC-blocking series 1 nF is a good idea (though perhaps not required, depending on antenna).C42, C43, and L29 don't make sense to me. The SAW filter already has a 50-ohm input and output, so no matching should be needed there (nor any biasing). C42 and C43 should be more like 1 nF is order to pass the RF, but not apply an impedance transform. (use C0G/NP0 caps for this...).

author: rfleming
author_rank: Regular Contributor
timestamp: January 10, 2020, 01:25:37 am
content: Quote from: Yansi on January 09, 2020, 11:42:59 pmI have changed it to an 0402 from an 0603 and moved it significantly closer as suggested.Quote from: pigrew on January 10, 2020, 12:25:45 amThis is where my lack of knowledge comes in and I thank you for providing advice. My design intentions were as follows:- The external antenna with an internal LNA (assuming that doesn't have a SAW filter) would require a DC 3v3 pull up, though I didn't want the dc pull up to go through the saw filter, so I added a couple blocking capacitors to isolate this. It is possible it doesn't matter at all and I have added these components for no reason :|- The trace length unfortunately due to the space available is unavoidableBy removing some of the above components if not required in the end, I may be able to reduce this a little.- C14 is used as an additional DC filter like an LCL filter that is used for a stabler DC.- C43 is only used as an "option" just like R19 0R resistor if i cant get the LNA working or to incorporate the Saw filter with an external antenna.- L29 was intended to provide a stronger DC ground for the saw filter, but if the saw filter doesn't care about a DC offset on its input and output, I can remove, C42, C42 and L29.- I couldn't find any information if I should provide matching circuitry of any type of circuitry for the saw filter either.Originally I worked off the below google image for a general idea of how gnss circuits should look as a block diagram.https://encrypted-tbn0.gstatic.com/images?q=tbn:ANd9GcS1EvSfyrouXNbKGNxz4Fd85a_grsFkq1j6i69zZ5dJN9n9IlO_dw&sFollowing both of your suggestions I did the following:- Removed L6 and replaced L1 on the input to the LNA- Changed all my 0603 1nF to 0402, replaced C42 and C43 with 1nF instead of the 3.9pF- Moved C5 much closer to LNA VccIf I could get a suggestion on if the Saw filter requires a matching circuit or matters if it has a DC off set on onside and not the other that would be awesome. Looking at how they work functionally, I don't see any issue, but I am not as good at RF as I would like to be...Following the updates, is there anything else that I could improve on?Thanks once again,Ryan.

author: rfleming
author_rank: Regular Contributor
timestamp: January 10, 2020, 01:27:21 am
content: Sorry for double post, I couldn't attached items in an EDIT

author: pigrew
author_rank: Frequent Contributor
timestamp: January 11, 2020, 03:40:48 pm
content: For the antenna stub, add a 0-ohm jumper series resistor near the RF port so that the stub can be jumpered out (or just cut the track when unused).It looks like your SAW filter can handle up to 5V, but I'm not sure if it would connect the DC of the input and output, or not. It's a good idea to do what you're doing to add the ability to block DC from the SAW. The filter is already matched to 50 ohms (this is normal). You may want DC blocking capacitors, but won't need matching components. Everything in your schematic is already 50 ohm matched except for the input of the LNA. Use relatively "big" caps and inductors (perhaps 1 nF and 47nH) to make your bias-tees, but otherwise you should only use a single matching component (the 7.5uH on the LNA input). If you have the 6.8uH already, feel free to use it which will result in slightly worse performance.The Vcc decoupling cap of the LNA was suggested to be >10nF (feel free to use 10nF to 100nF X7R).Specifying inductors as "1 kohm" seems to be implying that you're using ferrite beads? Don't do that in this case, you want low-loss RF inductors for any node touching the RF lines. You can use ferrites in the DC biasing nets, just not touching the RF.It seems that your M8W module has an internal bias-tee to apply DC bias to the antenna port. It can be up to 6V, but using your circuit, it'll default to 3.3V. What voltage does your external antenna need?Review the M8W "Hardware Integration Manual" for more examples on how to configure your module.Assuming that you already have a working prototype in hand, I think that you can rework your prototype for improved performance (e.g. by cutting tracks and substituting components).

author: rfleming
author_rank: Regular Contributor
timestamp: January 12, 2020, 11:39:00 pm
content: Thanks again pigrew.At this stage I am YET to get my LNA to function at all, this begs the question whether I:- Chose the correct part- have the correct pinout- have a good foot print- if its ACTUALLY soldered down, though I have 4 board and they all 'look' good.The first scematic/pcb attached in the thread has already been made, so I have started making manual changes to one of those board to try and get the LNA working. Scratched a few traces to make the 100nF sit right next to the LNA input, replaced the original R19 with the 6.8uH inductor that I had on hand and removed C43/L29/C14/L30 to get a simple signal through. i am also using an off-board basic patch antenna and left the device close to a window for an hour to see if it would connect, unfortunately not. On the other hand, if I bypass the lna with a wire directly to the SAW, I at least got a connection after a few minutes. I also cut the bypass stub to see if itcould have "slightly" better performance as well.Patch:https://www.digikey.com.au/products/en?keywords=627-1084-ndRegarding the design suggestions, I have upped the decoupling capacitor to the LNA 100nF X7R since I will be using this part elsewhere in the circuit. In terms of the Ferrites I am using (as you correctly identified), once again beyond my knowledge, isn't the purpose of these parts low DC resistance but high AC resistance? The part used in the schematic (below) is 1kΩ at 1Mhz and 580mΩ at DC. As you mentioned to have a low loss part, doesn't this meet the specifications well?Ferrite:https://www.digikey.com.au/products/en?keywords=587-1841-1-NDReviewing a generic googled graph of the inductor vs ferrite application (below) I can see at the higher frequencies my generic choice might not be a good choice for 1.6Ghz though still the ferrite does show better targetted results than the inductor for a 'Bias Tee' or the 'DC ground Offset' inductor?https://micro.rohm.com/en/techweb/upload/2018/10/EMC-16_fig1.gifEDIT: I just found some footprint errors, the pads may have been too spaced apart.

author: pigrew
author_rank: Frequent Contributor
timestamp: January 14, 2020, 06:50:27 pm
content: Perhaps I was too harsh on the ferrite, though your choice seems to have a bit low impedance (~250 ohm at 1.5 GHz). My guess is that it'll reduce your signal power by one or two dB. I'd need to do a s-parameter simulation to know for sure. The |S11| is about 0.8 (higher would be better). Often, a "ferrite bead" is designed to absorb RF energy in a lossy way (low Q), whereas a "ferrite inductor" (high Q) will reflect the power (not convert it to heat). By low-loss, I mean high Q at RF frequencies. I don't know the details of inductor design well enough to say much more.As long as the pads are electrically connected, I wouldn't worry about small footprint issues, though it's hard to know for sure without an X-ray (since it'l a pullback QFN). I suggest probing your DC tracks to verify that the enable and VDD are being applied to the LNA properly.