pixalynx/pixal-gps

docs/rf.md

# RF architecture: LTE-M/NB-IoT and GNSS

All statements about the nRF9151 come from Nordic's nRF9151 Product Specification (PS), the nRF9151 Hardware Design Guidelines (HDG, nwp_056), the nRF91 Series Antenna and RF Interface Guidelines (ARF, nwp_033), the nRF9151 DK schematic (PCA10171 rev 1.0.1) and the Thingy:91 X schematic (PCA20065 rev 2.0.0); URLs are listed in [sources.md](sources.md).

## What the SiP provides

| Port | nRF9151 pin | Nature | Requirement |
| --- | --- | --- | --- |
| LTE ANT | 35 | 50 Ω single-ended, DC-grounded, integrated ESD circuit; TX 698–1980 MHz, RX 728–2200 MHz | "A maximum of 0.5 dB loss in the transmission line", "A matching network of a minimum of three components is typically needed close to the antenna", "Ensure a continuous reference ground plane" (HDG ANT) |
| GNSS | 42 ("GPS") | 50 Ω single-ended RX input, DC-grounded, **no internal LNA** | "External low-noise amplifier (LNA) with SAW filter recommended on the GPS antenna input"; "minimum 27 dB attenuation to out of band power"; 30 dB isolation from the LTE ANT pin at LTE TX frequencies (PS GPS page, ARF GNSS page) |
| AUX | 37 | 50 Ω loop-back of ANT through an internal switch | "ANT can be connected to AUX internally during GNSS reception by using the %XANTCFG command"; AUX–ANT insertion loss 1.1 dB at 1575.42 MHz (HDG AUX) |
| COEX0 | 52 | 1.8 V output from the modem | "can be used as active high control for the external LNA component" when the internal GNSS is used; every used COEX pin needs an external 100 kΩ pull-down (PS COEX, HDG COEX) |

The 32.768 kHz crystal, the 64 MHz reference and the PMU/regulators are inside the SiP; there are no XL1/XL2 pins, no external inductor and no SWO pin (PS pin table).

## Selected Rev A architecture: two U.FL connectors, single-antenna option retained

```
ANT(35) ── C10 (shunt, DNP) ── R3 (series 0 Ω) ── C11 (shunt, DNP) ── J1 U.FL  ➜ external LTE antenna
AUX(37) ── C18 39 pF ── R6 (0 Ω, DNP) ─┐
                                        ├── U2 SKY65943-11 (pre-filter → LNA → post-filter) ── C12 39 pF ── GPS(42)
J2 U.FL ➜ GNSS antenna ── C13 39 pF ────┘        L2 9.1 nH between FILT_OUT and AMP_IN
                          (L3 68 nH bias-T to VANT, DNP: only for an active antenna)
COEX0(52) ── R5 100 k to GND, R4 1 k ── VEN (C14 100 nF, C15 100 pF)
V1V8 ── FB2 120 Ω bead ── VCC (C16 100 nF, C17 100 pF)
```

- **Why two connectors.** Nordic's own products use separate LTE and GNSS antennas (DK: Ethertronics P822601 + Taoglas patch; Thingy:91 X: Ignion NN03-310 + NN03-320) and the HDG says "Two antennas, such as GNSS and LTE, should never be adjacent to each other". With U.FL connectors the antenna keep-out lives on the antenna, not on this 34 × 24 mm board, so the enclosure decides antenna placement and LTE/GNSS validation does not hinge on tuning a printed antenna.
- **Why no matching parts are fitted.** Nordic staff on DevZone: "External antennas usually don't need a matching network, as the connector and coaxial cable are both already 50 Ohm." The three-position reservation (shunt–series–shunt) is kept so the DK's values (1.5 nH series, 3.5 pF series, 22 nH shunt) or an antenna vendor's network can be dropped in after measurement.
- **GNSS front end.** SKY65943-11 is the exact module on the nRF9151 DK (U2) and Thingy:91 X (U9): integrated pre-filter (blocks LTE TX, satisfying the 27 dB/30 dB requirements), LNA (13.5–15 dB, NF 1.7 dB, 2.9 mA at 1.8 V, 0.1 µA disabled), post-filter; the only external RF part is the 9.1 nH inter-stage inductor (Skyworks evaluation board BOM). It runs from the 1.8 V rail through a bead, exactly as the DK option SB1, and is enabled by COEX0 through 1 kΩ (DK R6) so it draws nothing outside GNSS windows. The modem drives COEX0 after `AT%XCOEX0=1,1,1565,1586` (frequency window in MHz).
- **Single-antenna option (combined LTE/GNSS).** Nordic supports *one* topology for a shared antenna: ANT → internal switch → AUX → external LNA → GPS (ARF "Combined GNSS and LTE antenna"; PS GPS: "Dedicated GPS antenna, or shared antenna with LTE"). Nordic does not describe a passive diplexer split, and the SAW/LNA is still mandatory. Rev A carries that path: fit R6 (0 Ω) and remove C13, then configure `AT%XANTCFG=1`. The cost is 1.1 dB extra loss at L1 and the need for an antenna that is efficient at both 700–2200 MHz and 1575 MHz. Decide after bench comparison of the two connectors; the recommended default remains two antennas.
- **Board-level RF geometry.** Both lines are 0.32 mm coplanar-waveguide traces with a 0.25 mm gap to the top ground pour, referenced to the unbroken L2 ground plane (JLC04161H-7628, see [stackup.md](stackup.md)); the LTE line is 3.9 mm long and straight, the GNSS input line 3.8 mm, the LNA-to-GPS line 2.9 mm. The AUX loop-back line (about 8 mm) is only copper when R6 is fitted. All three are explicit copper in `tracker.circuit.tsx` (`pcbPath` traces), not autorouted. Solder-mask-defined gap: leave the mask on; JLCPCB's calculator numbers already include the mask.
- **Isolation and placement.** The LTE connector sits at the bottom edge directly below the ANT pin; the GNSS connector at the right edge 13 mm away with the LNA between them; the nPM1300 buck, the Qi rectifier and the SIM socket are on the opposite side of the board under the SiP's ground-via field, which follows the HDG placement advice ("avoid placing nRF9151 close to noisy components, such as buck regulators") as far as a 34 × 24 mm board allows. The Qi coil and the LiPo cell are behind the battery, on the far side from both antenna cables.
- **Antenna choice for Rev A (external).** LTE: any of Nordic's ARF off-the-shelf list (Ignion FR01-S4-210, Johanson 0830AT54A2200, Taoglas FXUB65 flex, Molex 206760-0001) on a U.FL pigtail; GNSS: a 12–18 mm passive ceramic patch or an active patch (then fit L3 68 nH and enable LSOUT1 for the bias). Efficiency targets from the HDG: ≥ 50 % LTE, ≥ 75 % GNSS.

## What is *not* on the board

No MIPI RFFE tuner (SDATA/SCLK/VIO left open, MAGPIO0–2 left open as on Thingy:91 X); no on-board antenna; no DECT NR+ provisions. AUX, COEX1 and COEX2 are unconnected (HDG: unused COEX pins may be left open; AUX "can be left floating").