Do you think my circuit design can work ? Should I use SAW Filters on LNA input and output (I think they are build in into LNA already) ?

I need to use the dipole wire antenna which has in ideal condition the sensitivity 2 dB only. I tested it with the SparkFun GPS Breakout - ZOE-M8Q and can't get valid GPS position because of the low GPS signal. It works only when I use active GPS antenna with build in LNA 27 dB (TDS here).

 

Why do you have the antenna constraint?

 

I am working on the GPS/LoRa cat tracker 12 x 26 mm (0.472 x 1 inches) sewn into cat collar and can't use standard GPS patch antenna. The dipole wire allows me to put antenna around the whole collar and to achieve reception of the GPS signal even if the collar moves around the cat neck.

 

I am working on the GPS/LoRa cat tracker 12 x 26 mm (0.472 x 1 inches) sewn into cat collar and can't use standard GPS patch antenna. The dipole wire allows me to put antenna around the whole collar and to achieve reception of the GPS signal even if the collar moves around the cat neck.

Well, there are application specific LNAs for GPS, like this one:

https://cdn.everythingrf.com/live/SKY65725_11_205456A_PS_637054361131426717.pdf

https://www.mouser.com/ProductDetail/Skyworks-Solutions-Inc/SKY65725-11

Under 3 bucks, pretty tiny, all integrated.

 

Thank you. I will have a look on it. I found I need to add SAW bandpass filters too.

 

Thank you. I will have a look on it. I found I need to add SAW bandpass filters too.

"The pre-filter provides the low in-band insertion loss and excellent rejections of the cellular, PCS, and WLAN frequency bands. The SKY65725-11 uses surface-mount technology (SMT) in a Multi-Chip Module (MCM) package, which allows for a highly manufacturable and low-cost solution."

 

Firstly, the second LNA will amplify the noise factor of first LNA thus the S/N ratio must happen be catastrophic at rather moderate G-factor. However I first hear the gps tablet may suffer from too low input signal, but if You say, shall not protest, just why not to make own LNA, where BF998 and BFU and BFP with numbers 910, 840, 725, 620, 842, 730, 740, 776, 640, 520, 760, 632, 450, 690 are well suited even for 50x higher frequencies. The best schemotechnique is bound by terms is Cascode cascade, for example https://www.researchgate.net/publication/3960684_A_comparison_of_CMOS_and_BJT_RF-LNAs/figures?lo=1
or alternatively https://www.researchgate.net/profil...e-cascode-LNA-with-ESD-power-clamp-diodes.png or simpler version https://i1.wp.com/www.nt7s.com/wp-content/uploads/2008/07/dual-gate-mosfet-agc.png?resize=340,276

All them have enormous good G and N factors as well aperiodic until tens of GHz
And if 20 dB is too small, here is 30 dB one https://www.qsl.net/va3iul/Homebrew_RF_Circuit_Design_Ideas/1kHz-12GHz_Wideband_LNA.gif

 

The important specs here are the Gain and Noise Figure of the LNA. There is no point in adding an amplifier with higher gain if its noise figure is higher than that specified for the GPS chip. If your sensitivity is too low then you need an LNA with a higher gain but also with an even lower noise figure. Ideally you might try to design the antenna with more gain when pointed to the stars, or at least towards the sky, which is where the satellites are to be found. Have you considered the matching between the antenna and the input to the LNA (or receiver)

 

I am aware about the low Noise Figure of the LNA. That's why I would like to use BGA 524N6 E6327 with NF 0.55 dB and supply current 2.5 mA.

I am working on Micro Cat Tracker (GPS/LoRa) sewn into cat collar. That's why I need to use dipole wire antenna (I will not use the 50 Ohm coax cable) which ensure relatively good reception of the GPS signal regardless of the collar position around the cat's neck. The Micro Cat Tracker will be in the sleep mode and transmit each hour GPS position or the dummy text message to allow LoRa approximate position triangulation. I would like to power the Micro Cat tracker by three Zinc-Air batteries and I expect that battery will last about 3 months.

The working Micro Cat Tracker prototype has dimensions 12 x 27 mm (7/32 x 1 inch). Without the LNA the S76G chip with build in SONY GPS Receiver has not enough sensitivity to receive GPS signal. I tested the dipole wire antenna with the SparkFun GPS Breakout - ZOE-M8Q and it doesn't work too without the active GPS antenna.

 

PCB design with two LNA's before manufacturing.

 

why not use a regular GPS antenna? there are a few now that are small enough for a cat collar.
12 x 12 and 13 x 13
there's even a company that has a 10 x 10 integrated module with all that signal conditioning stuff built in.
here's one that's 20 x 4 x 6

 

Thank you. For all these active patch antenna you need to keep the position of the antenna toward sky which is hard to achieve on the cat collar. I will try it if my design will not work.

Note too that I need the low current consumption, the battery should last 3 months. The 10x10 antenna needs 43 mA for acquisition, my double LNA 2x 1.3 mA.

 

Thank you. For all these active patch antenna you need to keep the position of the antenna toward sky which is hard to achieve on the cat collar. I will try it if my design will not work.

Note too that I need the low current consumption, the battery should last 3 months. The 10x10 antenna needs 43 mA for acquisition, my double LNA 2x 1.3 mA.

i see what you mean. It's quite a challenge. But this is something that i have also thought about many times in the past and now might have the resources to accomplish. i'll try to get the OriginGPS and see how far i get.

 

Hi, I am dealing with a similar issue but have a different reasons.

My application - I have a passive patch antenna and and GPS receiver uBlox z9P with specified active gain antenna 17 - 50dB and defined noise figure <4dB. It is a multiband amplifier so I am separating L1 and L2 bands, therefore there are passive losses and my overall gain would be bellow 17dB.

@Janis59
Calculating noise figure for the Kevil's cascade based on Friis formula I get 0.576dB Noise figure which seems to be a very low number. Janis59's saying about amplification of noise signal is intuitive for me and I am somehow afraid of using this approach, but I find the 0.576dB contradictory to it and based on that number I would expect no problem with the schematic. Am I missing something here?

When calculating the total gain, should also the antenna gain be accounted in the calculation?

BTW - why are most active GPS antennas set to approx. 30dBgain? (well, it is just my quick observation and might not be so in all cases)

@Kevil - I would also go for an patch integrated receiver with all power saving features and look for a way to achieve the orientation towards the sky

 

I would also go for an patch integrated receiver with all power saving features and look for a way to achieve the orientation towards the sky

orientation might have a solution with teflon. This 20 x 4 x 6 antenna weighs 3g, so with a careful placement of the other components you could achieve a substantial imbalance. Cover the whole thing with teflon. Teflon-coated adhesive tape is cheap on Amazon.
Of course it's not as good, or nearly as cheap n easy, as a dipole antenna. But i'm not a radio engineer. how would you get enough SNR to make it work?

 

Thank all of you for the comment. I think I should check with NanoVNA if I need some impedance matching and tune the dipole wire antena.

I'm not sure if I can simply connect the wires to the edge of the circuit board and route them them in parallel with the PCB (at the end I will sew them into cat collar to edges). Do you think it may help to increase sensitivity by removing top layer copper area around active pin of the dipole wire antenna ? The four layers PCB dimensions: 50 x 30 mm @ 1.6 mm with GCPW track width 0.25 mm, space 0.061 mm @ 50 Ohm. Prepreg type 7628, Dielectric constant 4.6, Prepreg layer 0.2 mm (JLCPCB).

You can have a look at my wiring and PCB design in EasyEda link here. Both LNA's are Infineon BGA 524N6 E6327 available at mouser.com

The test PCB will be powered by one SONY Li-Ion battery 18650 4.2 V

 

Hi Kevil,
I'd use the bypass capacitors on the board, placed right next to the LNAs, though stated optional.For IC2 the trace is long. In this case it is the way to lower the possible noise on the power line.
One more question - how did you come to L3 9.1nH (from a datasheet - I did not find it there) or is it for some antenna tuning? I would prefer to place it directly to the line between SAW1 and L1 not creating the stub placing it apart. The wavelengths are not so short so it is not so critical, but you would be on safe side according to my opinion.

Well, I admit I do not have too much experience designing RF circuit, but I am applying some rules and practices that should keep you on the safe side.

 

One more question - how did you come to L3 9.1nH (from a datasheet - I did not find it there) or is it for some antenna tuning?

The L3 9.1nH is suggested matching circuit at the ouput of the Qualcomm B39162B2611P810 SAW filter. See B39162B2611P810 Datasheet, PDF page 8.

 

RE:""but I find the 0.576dB contradictory to it and based on that number I would expect no problem with the schematic. Am I missing something here?"" You missed only one nuance - any exclusion only makes the Law stronger.

 

@Janis59
I am sorry I do not understand. Can you be more specific? The noise figure of the cascade amplifier is 0.576dB taking account 0.55 noise figure and 19.6dB gain. This difference seems negligible.
What else should be included?