// SAME Analog Modular Ecosystem // // / SPDX-FileCopyrightText: 2026 Denis Chevalier // / SPDX-License-Identifier: CC-BY-SA-4.0 // / SPDX-License-Identifier: CERN-OHL-S-2.0+ // / SPDX-License-Identifier: GPL-3.0-or-later // // The prose, explanatory text, rendered figures, tables, and mathematical // content of this specification are licensed under CC BY-SA 4.0. If a later // version of CC BY-SA is published, the author grants permission to distribute // this work under that later version as well. // // Hardware designs contained herein (schematics, PCB layouts, mechanical // drawings, and CAD models) are licensed under CERN-OHL-S-2.0+. // // All executable code, helper libraries (`lib/*`), metrology assertions, and // embedded verification scripts throughout the source documents are licensed // under the GNU General Public License v3.0 or later (GPL-3.0-or-later). #import "lib/unify.typ": qty, qtyrange #import "@preview/cetz:0.5.2" = Mechanical specifications == Modules === Cassette ==== Shielding and construction To achieve metrologic tier precision, modules must be fully shielded from external interference. The only entry points for noise are: - The DB-25 and BNC connectors at the rear, which connects via a shielded cable to the power distribution board - The banana jacks at the front panel, which include input protection circuits. Modules must also not pollute neighboring modules with internally generated noise (such as PWAM oscillators). For this reason, all modules are enclosed in individual steel cassettes. We chose steel instead of aluminum, as it has better shielding properties, especially in terms of magnetic shielding. #include "tables/cassette-construction-parameters.typ" ==== External dimensions #include "diagrams/cassette-external-dimensions.typ" #include "tables/cassette-external-dimensions.typ" The panel lips extend above and below the cassette body to fill the full $qty(4, "U")$ rack opening while the body fits between the chassis rails. ==== Construction The cassette consists of the front panel, one L-shaped steel channel and one interlocking U-shaped steel channel: ===== L-channel (chassis) #include "diagrams/l-channel-chassis.typ" / Role: Structural spine. Holds the PCB and Front Panel. / Geometry: / Left wall ($qty(165.1, "mm") times qty(152.4, "mm")$): PCB mounts here. / Rear closure ($qty(165.1, "mm") times qty(35.7, "mm")$): Includes DB-25 and BNC cutouts. / DB-25 cutout: Center X: $qty(17.85, "mm")$ (centered). Center Y: $qty(55.0, "mm")$ from bottom edge. / BNC cutout: Center X: $qty(17.85, "mm")$ (centered). Center Y: $qty(130.0, "mm")$ from bottom edge. / Facultative: Required only for modules utilizing the Master Oscillator. If absent, the steel surface remains solid to maintain maximum shielding integrity. / Flanges (The Mounting Points): - The Left Wall features Top, Bottom and Front flanges folded $qty(90, "degree")$ inward. - The Rear Wall features Top, Bottom, and Right flanges folded $qty(90, "degree")$ inward. - The flanges are $qty(8, "mm")$. / Threads: These flanges must be fitted with M3 Press-Fit Threaded Inserts (PEM Nuts) or similar clinching fasteners. / Why: The $qty(1.2, "mm")$ steel is too thin to tap reliably for repeated use. Inserts ensure the threads don't strip after years of maintenance. / Finish: The outer face of these flanges must be masked (unpainted) to ensure electrical contact with the U-Channel and the front panel. ===== U-channel (cover) #include "diagrams/u-channel-cover.typ" / Role: EMI Shielding and Protection. / Geometry: / Top ($qty(35.7, "mm") times qty(152.4, "mm")$): vented / Right side wall ($qty(165.1, "mm") times qty(152.4, "mm")$): / Bottom ($qty(35.7, "mm") times qty(152.4, "mm")$): vented / Holes: Features Countersunk Through-Holes corresponding to the L-Channel's threaded inserts. / Finish: The inner face contact area must be masked (unpainted). ===== Front panel #include "diagrams/front-panel.typ" / Geometry: $qty(177.8, "mm") times qty(38.0, "mm")$, body section $qty(165.1, "mm")$ / Holes: Features Countersunk Through-Holes corresponding to the L-Channel's threaded inserts. / Finish: The inner face contact area must be masked (unpainted). ===== Fasteners / Type: M3 Countersunk (Flat Head) Screws (Black Oxide). / Flushness: Screw heads must sit flush with the external surface to prevent snagging when sliding the module into the rack. / Spacing: Screws should be spaced no more than $qty(40, "mm")$ apart along the seams to ensure the "slot antenna" effect does not compromise shielding at high frequencies ($gt qty(1, "GHz")$). There are $5$ screws per vertical flange, seam, and $4$ per horizontal seam. ===== Grounding requirements To prevent the cassette from acting as a floating antenna, the enclosure must maintain a low-impedance electrical connection to the chassis earth. The connection is made at the DB-25 port and the rear BNC connector shell if present. The mounting holes must be masked during powder coating, or star washers be used to screw it in place. The BNC connector shell must be bonded to the steel cassette (masking required). The signal carried by the BNC (10MHz) is referenced to the BNC shell (Chassis Earth/RF Ground), not AGND, to prevent high-frequency injection into the precision analog ground plane. To ensure the connections actually block noise (RF), masking is mandatory: The powder coat is an insulator. There must be masking (no paint) on the outer face of all L-Channel flanges and the corresponding inner face of the U-Channel and Panel. When the screws are tightened, metal-to-metal contact must be achieved around the entire perimeter of the seam. If paint blocks this contact, the shielding will fail, degrading the SNR below the $qty(90, "dB")$ target. ==== Internal dimensions #include "tables/cassette-internal-dimensions.typ" ==== Ventilation Top and bottom faces include ventilation slots for convective cooling while maintaining EMI shielding. #include "diagrams/cassette-ventilation.typ" #include "tables/cassette-ventilation.typ" ==== DB-25 connector cutout #include "diagrams/db-25-connector-cutout.typ" #include "tables/db-25-connector-cutout.typ" ==== PCB dimensions The PCB is sized to fit within the L-Channel side wall while leaving clearance for the interlocking U-Channel flanges and internal cabling. / PCB Height: $qty(155.0, "mm")$ / PCB Depth: $qty(125.0, "mm")$ / PCB Thickness: $qty(1.6, "mm")$ (Standard FR-4) / Orientation: Mounted vertically on the internal Left Wall. ===== PCB mounting geometry The cassette Left Wall shall feature four ($4$) permanently attached female threaded standoffs (M3) to accept the PCB. / Standoff Type: M3 Thread, 6mm Height (swaged or welded to the steel wall). / Mounting Pattern: Rectangular, centered on the cassette side wall. / Vertical Spacing (Y): $qty(135.0, "mm")$ / Horizontal Spacing (X): $qty(105.0, "mm")$ / Origin Reference: Pattern is centered relative to the internal side wall dimensions ($qty(165.1, "mm") times qty(150, "mm")$). ===== Keep-out zones To prevent electrical shorts against the steel chassis or mechanical interference: / Perimeter Clearance: A 5mm component-free zone must be maintained along all four edges of the PCB. / Height Clearance: Components taller than 30mm are prohibited to ensure airflow and prevent shorting against the opposing Right Wall (total internal width is $qty(35.7, "mm")$. If tall components must be used, they must be assembled parallel to the PCB. ===== Interconnect layout (wire harness strategy) Since the DB-25 is panel-mounted with solder cups and controls are on the front panel, the PCB layout must optimize cable routing to minimize noise pickup and maximize air flow. / Zone A (Rear - power): / Function: Connection to the rear DB-25 backplane connector. / Location: Rear $qty(20, "mm")$ of the PCB. / Connector Type: Pluggable terminal block (e.g., Phoenix Contact $qty(3.5, "mm")$) or latching header (e.g., Molex KK $qty(3.96, "mm")$). Direct soldering of flying leads to the PCB is discouraged to maintain repairability. / Pinout: Must match the standard SAME Power Distribution definition. / Zone B (Front - I/O): / Function: Connection to Front Panel Banana Jacks and Potentiometers. / Location: Front $qty(20, "mm")$ of the PCB. / Routing: Cables should form a neat loom transitioning from the PCB edge to the panel components. / Service Loop: All internal cabling must have a service loop of $plus qty(20, "mm")$ to allow the PCB to be unscrewed and slightly lifted without desoldering. ===== Grounding & Isolation Strategy To maintain the strict separation of Analog Ground (AGND) and Chassis Earth, the PCB must be electrically isolated from the steel cassette. / PCB Mounting Holes: Must be unplated and isolated from all internal ground planes (AGND/DGND). The metal standoffs serve only as mechanical support, not electrical conductors for the circuit. / Cassette Earth Path: The steel cassette connects to Chassis Earth exclusively via the rear DB-25 Connector. / Implementation: The DB-25 panel-mount connector must have a conductive metal shell. When mounted to the rear steel panel, the connector shell (and Pin $25$) creates the bond between the cable shield and the cassette body. / Verification: Resistance between the DB-25 Shell and any point on the cassette powder-coat-free zones must be $lt qty(0.1, "ohm")$. === PCB ==== Overview Module PCBs mount vertically within the L-channel, attached to the left side wall via standoffs. The PCB connects to the front panel (banana jacks, controls) via wire harnesses and to the power distribution board via the rear DB-25 connector. ==== Standard dimensions #include "tables/pcb-standard-dimensions.typ" ==== Board stackup All SAME module PCBs shall use a 4-layer stackup: This allows proper ground management, while remaining easy to solder with standard equipment. #include "tables/pcb-board-stackup.typ" / Dielectric: FR-4, $"Tg" gt.eq qty(135, "dC")$ ==== Surface finish #include "tables/pcb-surface-finish.typ" ===== Rationale ENIG provides excellent corrosion resistance for the 50-year service life target. A flat surface ensures reliable soldering of precision components. Gold does not oxidize, maintaining consistent contact resistance at test points. Black solder mask reduces light reflectivity (some precision semiconductors are photosensitive). ==== Earth ring The PCB shall include a Chassis Earth ring around the perimeter of the bottom layer (L4). This ring provides a defined path for any capacitively-coupled noise to drain to chassis earth rather than coupling into signal traces. ===== Geometry #include "tables/pcb-earth-ring-geometry.typ" ===== Connection The earth ring shall not connect to AGND or DGND on the PCB. It connects to Chassis Earth exclusively through the pin 25 of the DB-25 power distributor via a trace on L4. ===== Mounting holes details The four mounting holes shall be: - Unplated, isolated through-holes, $diameter qty(3.2, "mm")$ (M3 clearance). - Not connected to any layer (no vias to L1, L2, L3, L4). ==== Ground planes ===== AGND plane #include "tables/pcb-ground-planes-agnd-plane.typ" The AGND plane provides: - Low-impedance return path for analog signals, - Shielding between signal layers, - Thermal spreading. ===== DGND zone (if needed) #include "tables/pcb-ground-planes-dgnd-zone.typ" ==== Power planes Layer 3 is divided into power distribution zones: #include "tables/pcb-power-planes.typ" Design rules: - Miminum $qty(2, "mm")$ clearance between power zones, - Star distribution from power enty point, - Bulk decoupling capacitors from power enty point, - Local decoupling at each IC. ==== Mounting strategy The PCB mounts to the L-channel left wall via four standoffs. #include "tables/pcb-mounting-strategy.typ" ===== Mounting holes pattern #include "tables/pcb-mounting-strategy-mounting-holes-pattern.typ" ====== Positions #include "tables/pcb-mounting-strategy-mounting-holes-positions.typ" ==== Keep-out zones ===== Perimeter keep-out #include "tables/pcb-keep-out-zones-perimeter-keep-out.typ" ===== Mounting holes keep-out #include "tables/pcb-keep-out-zones-mounting-holes-keep-out.typ" ==== Interconnect zones ===== Zone A: power and master oscillator entry (rear) #include "tables/pcb-interconnect-zones-zone-a.typ" ====== Pin assignment The PCB connector shall provide access to all power rails in a logical grouping: #include "tables/pcb-interconnect-zones-zone-a-pin-assignment.typ" Not all modules require all signals. Unused positions may be omitted from the harness. ===== Zone B: front panel I/O #include "tables/pcb-interconnect-zones-zone-b.typ" Design guidelines: - Group outputs on one side, inputs on the other, - Maintain signal flow from right to left (input #sym.arrow.r processing #sym.arrow.r output), - Include test points for each signal node. ===== Service loop requirements All internal wiring shall include a service loop of minimum $qty(20, "mm")$ additional length. This allows the PCB to be: - Unscrewed from standoffs, - Tilted or lifted for inspection, - Accessed for rework. Without disconnecting the wire harnesses. ==== Silkscreen requirements ===== Component side (L1 - top) #include "tables/pcb-silkscreen-requirements-l1.typ" Labels should be functional if possible, defaults to $R_1$, $C_1$, $U_1$, etc. if there are space constraints. ===== Solder side (L4 - bottom) #include "tables/pcb-silkscreen-requirements-l4.typ" ==== Electrical testing All PCBs shall pass the following tests before assembly: #include "tables/pcb-electrical-testing-requirements.typ" === Panels ==== Geometry The front panel is a separate piece that attaches to the L-channel via countersunk screws. #include "tables/panels-geometry.typ" ===== Panels zones #include "tables/panels-zones.typ" ==== Grid All panel-mounted components follow a $qty(0.75, "inch")$ ($qty(19.05, "mm")$) grid for compatibility with standard banana jack spacing and shorting bars. #include "diagrams/panels-grid.typ" ===== Origin #include "tables/panels-grid-origin.typ" ===== Positions #include "tables/panels-grid-positions.typ" ===== Component placement rules #include "tables/panels-grid-component-placement-rules.typ" ====== Off-grid placements + Small indicators (LEDs) and labels may be placed between grid positions. + Non-specified components (controls, interface ports, etc.) should be centered on grid positions if possible, or centered on the panel. + Mounting screws for the panel (into L-channel flanges) are outside the grid area. ==== Labelling All panels shall include clear, permanent labelling for user operation. #include "tables/panels-labbeling.typ" ===== Jacks color coding #include "tables/panels-jacks-color-coding.typ" ===== Use of colors in labels The panel has a matte black background. To maintain legibility, all labels should be white or off-white, with the following exceptions: - For the designer or builder name or logo. - When justified by the mathematical operation performed. In any case, colors should be designed to offer sufficient contrast, and not impact module usability by people suffering from color blindness. ===== Use of graphism and symbols in labels We recommend the use of mathematical symbols and graphism when they illustrate the functionality of the module. However, text labels should always be present to express the jack functions. ===== Label method Commercial, reference implementation or metrologic builds should always use laser engraving or screen printing methods. Those have excellent durability and legibility. DIY builds can use UV-printed vinyl overlay method: it maintains good durability and legibility while being more accessible. ===== Typography Labels should be printed in a sans-serif typeface (DIN 1451, Eurostile, Helvetica, etc.), except mathematical formulas. Module name size should be at least $qty(4, "mm")$ height. Jack labels should be at least $qty(2, "mm")$ height. == Chassis === Overview The chassis provides the mechanical and electrical infrastructure for housing modules. It includes: - Structural enclosure with EMI shielding, - Cassette rail system for module installation and removal, - Power distribution board with backplane connectors, - Front panel with reference outputs and system controls, - Thermal management provisions. This specification defines interface requirements that any compliant chassis must meet. Chassis may vary in capacity (number of module positions), form factor (rack-mount, desktop, portable), and mounting style. The reference implementation (@reference-implementation-chassis) describes a $qty(19, "inch")$ $qty(4, "U")$ rack-mount chassis with $8$ module positions. === Dimensional requirements #include "diagrams/chassis-dimensional-requirements.typ" ==== Module interface dimensions All chassis must accommodate standard SAME cassettes. The following dimensions are mandatory: #include "tables/chassis-module-interface-dimensions.typ" ==== Capacity Chassis capacity is defined by the number of module positions. There is no minimum or maximum, but the following constraints apply: #include "tables/chassis-capacity.typ" Designers should consider thermal dissipation when determining maximum capacity. ==== Clearances The rear clearance (behind cassette) should be at least $qty(25, "mm")$, to account for the DB-25 connector and cable bend radius. === Structural requirements ==== Material #include "tables/chassis-material.typ" Steel is preferred for magnetic shielding. Aluminum may be used where weight is critical, with reduced magnetic shielding performance. ==== EMI shielding A fully loaded chassis must form a continuous shielded enclosure: #include "tables/chassis-emi-shielding.typ" ==== Grounding #include "tables/chassis-grounding.typ" === Cassette rails The rail system guides cassette insertion and provides retention. ==== Geometry #include "tables/chassis-cassette-rails-geometry.typ" ==== Retention Cassettes must be retained to prevent accidental ejection during operation or transport. #include "tables/chassis-cassette-rails-retention.typ" Positive retention may be achieved via: - Spring clips engaging cassette lips, - Latching mechanisms. ==== Material The rails must be of the same metal as the chassis, with a smooth, burr-free finish; and bond to chassis earth. They contribute to EMI shielding by closing the gap between cassette lips and chassis structure. === Thermal management ==== Cooling method Passive convection cooling is the baseline requirement. Forced-air cooling may be added for high-density configurations. #include "tables/chassis-cooling-method.typ" ==== Ventilation Chassis ventilation must align with cassette ventilation slots to enable chimney-effect airflow. #include "tables/chassis-ventilation.typ" ==== Thermal limits Inside the chassis cassette chamber, internal ambient temperature must not go over $qty(50, "dC")$. If passive cooling cannot maintain thermal limits at full capacity, forced-air cooling or reduced capacity is required. === Power distribution board The power distribution board generates and distributes all power rails to module positions. ==== Functions #include "tables/power-distribution-board-functions.typ" ==== Power entry #include "tables/power-distribution-board-power-entry.typ" ==== Master oscillator To support the metrologic tier time-domain computations, the chassis must include a precision clock source. / Oscillator: Oven Controlled Crystal Oscillator (OCXO). / Recommended part: AOCJY4A-10.0000MHZ-SW (or equivalent with $plus.minus qty(10, "ppb")$ stability). / Buffering: The oscillator output must be filtered to remove harmonics and buffered to drive the $qty(50, "ohm")$ distribution network. / Metrologic tier buffer: LM7171 (high speed, high voltage operational amplifier) or equivalent, configured to drive $qty(50, "ohm")$ loads with high slew rate and low distortion. / Signal level: $qty(1.0, "Vpp") plus.minus qty(10, "percent")$ into $qty(50, "ohm")$. / Waveform: Sine wave. / Fan-out: The oscillator distribution network must ensure port-to-port isolation $gt qty(40, "dB")$ to prevent oscillator line crosstalk between modules. === Front panel The chassis front panel provides system-level controls and reference inputs and outputs. ==== Elements #include "tables/chassis-front-panel-elements.typ" - Case designers may add other elements as they see fit (fuse holders, voltage selectors, sensors, etc.), - Reference jacks should be grouped together, on a $qty(0.75, "inch")$ grid, - All controls and jacks must have an associated label. === Electrical safety ==== Mains isolation #include "tables/chassis-mains-isolation.typ" ==== SELV circuits All module-facing voltages ($plus.minus qty(15, "V")$, $plus.minus qty(10, "V")$, $plus.minus qty(5, "V")$) are Safety Extra-Low Voltage (SELV): #include "tables/selv-circuits.typ" ==== Protective earth #include "tables/protective-earth.typ" === Compliance Chassis must comply with applicable safety and EMS standards for the target market: #include "tables/chassis-compliance.typ" Compliance testing and certification are the responsibility of the chassis manufacturer.