527 EP Sensor v7 (symmetric in-place thicken, Zhao-compliant) :: Quicker, easier and cheaper to make your own chip!

How it works

This is an analog chiral exceptional-point (EP) sensor built around two nominally-matched on-chip LC tanks at ~2.7 GHz, coupled in one direction through an NMOS source follower (Mf) and a coupling MIM cap (Cc). Following Zhao et al., the unidirectional coupling places the two-mode system on the exceptional-point line of its eigen-spectrum; a small capacitive perturbation ε on the V1 tank then produces a frequency splitting Δf in the V2 spectrum that scales as Δf ∝ √ε, giving substantial sensitivity gain over a conventional reciprocally-coupled resonator pair.

Block diagram

   ua[0] ── L1 ── V1 ──┬──CT1── VGND        ┌── L2 ── V2_in ──┬── CT2 ── VGND
                       │                    │                 │
                       └── Cv-array (8b) ──┘ ↑                ├── Cc
                       │                     │                │
                       Cac_in                ↑                ↓
                       │                  (one-way)         Vfo (Mf source)
                       Mf gate             coupling          ↑
                       (source follower)                     ┘
                                                            
                                                          V2_in ── Mout (V2 buffer) ── ua[1]

L1, L2 — custom 130 × 130 µm square spirals on m4+m3+m2 stack; L = 1.351 nH, Q ≈ 15 (FastHenry-extracted).
CT1, CT2, Cc — MIM caps tuned for the Zhao-exact EP point at κ = 0.3125, sized for the L = 1.351 nH tank.
8-bit cv-array — binary-weighted MIM caps (20, 40, 80, 160, 320, 640, 1280, 1280 fF) switched in via LVT NMOS. Asserting ui_in[i] adds the i-th bit to the V1 tank.
Source-follower Mf — W = 400 µm × L = 0.15 µm, 80 fingers — drives Vfo with V1's signal, capacitively coupled to the V2 tank through Cc. This is the unidirectional element: V1 → V2, but not V2 → V1.
V2 output buffer (Mout) — W = 200, nf = 40 source follower; isolates the V2 tank from the bench probe (5 pF + 50 Ω) so the two Zhao peaks remain resolvable post-bondwire.
On-chip bias dividers — poly resistor stacks generate Vbn = 1.20 V (source-follower tail) and Vbmid = 1.50 V (gate-bias for Cac coupling). No external bias pins required.

Analog pinout

Pin	Name	Function
ua[0]	V1	V1 LC-tank node. Drive an AC test signal here through 50 Ω; the L1 spiral plus tank caps establish resonance.
ua[1]	V2	V2 LC-tank buffered output. Observe the

Design verification

DRC clean (KLayout, sky130A_mr.drc, feol/beol/offgrid all 0).
LVS topologically equivalent to schematic (225 devices match).
Schematic-level Monte Carlo (500 trials, ±5 % cap, ±10 % L, ±50 mV bias): 100 % yield in the [0.45, 0.55] target slope band.
Post-layout robustness: realistic parasitic injection causes a uniform ~0.5 % frequency shift; EP scaling preserved bit-for-bit.

How to test

This is an RF analog sensor. You will need:

Tiny Tapeout demo board + breakout PCB with u.fl connectors on ua[0] (V1) and ua[1] (V2). The u.fl jacks are RF-rated to 6 GHz.
A vector network analyzer that reaches at least 3 GHz with > 60 dB dynamic range. A Keysight P5004A (9 kHz – 20 GHz, 135 dB DR) is ideal. An Analog Discovery 3 will not work — its 30 MHz bandwidth is far below the chip's 2.7 GHz operating point.
VDPWR = 1.8 V, all ui_in pins routed from a microcontroller or manual switches to set the cv-array perturbation code.

Procedure

Power up the chip. Bias generation is on-chip, no setup required.
Connect VNA port 1 to ua[0], port 2 to ua[1]. Calibrate.
Sweep S21 magnitude from 1.5 GHz to 4.5 GHz with ui_in = 0x00. Locate the resonance — expected near 2.71 GHz. This is the EP-degenerate peak (κ tuned to put the two modes coincident).
Step the cv-array code: ui_in = 0x01, 0x02, 0x04, 0x08, 0x10, 0x20, 0x40, 0x80, 0xC0.
For each code, locate two local peaks in |S21|^2 and record their separation Δf.
Plot log(Δf / f₀) vs log(ε) where ε = Ce / (2 C_total). The slope should be ≈ 0.5 ± 0.05 across the operating window — the canonical EP signature.

If the slope departs significantly from 0.5, the chip is operating away from the EP (κ off-target), but the cv-array sweep is still informative as a relative perturbation indicator.

External hardware

Tiny Tapeout breakout PCB (u.fl variant; supports RF-rated probing).
Microcontroller (Pi Pico on the demo board is fine) or manual switches to drive ui_in[7:0].
Vector network analyzer (Keysight P5004A or equivalent).
Two SMA → u.fl pigtails.

#	Input	Output	Bidirectional
0	b0 (cv-array LSB; 20 fF, eps=0.0055)
1	b1 (40 fF, eps=0.011)
2	b2 (80 fF, eps=0.022; lowest resolved point)
3	b3 (160 fF, eps=0.044; Zhao window lower edge)
4	b4 (320 fF, eps=0.088; within Zhao window)
5	b5 (640 fF, eps=0.176; within Zhao window)
6	b6 (1280 fF, eps=0.352; above Zhao window)
7	b7 (cv-array MSB; 1280 fF)

`ua`	PCB Pin	Internal index	Description
0	B4	10	V1 (V1 LC tank; RF probe drive and observe at 50 ohm)
1	B5	11	V2 (V2 LC tank; observe peak split)

527 EP Sensor v7 (symmetric in-place thicken, Zhao-compliant)