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diff --git a/mechanical-specifications.typ b/mechanical-specifications.typ index 37c9893..a96de33 100644 --- a/mechanical-specifications.typ +++ b/mechanical-specifications.typ @@ -67,3 +67,216 @@ especially in terms of magnetic shielding. 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 <cassette-construction> + +The cassette consists of the front panel, one L-shaped steel channel and one +interlocking U-shaped steel channel: + +===== L-channel (chassis) <l-channel-chassis> + +/ 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 folde + $qty(90, "degree")$ 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) <u-channel-cover> + +/ 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 <front-panel> + +/ 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 <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 <cassette-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 <cassette-internal-dimensions> + +#figure( + table( + columns: (auto, auto), + table.header([*Dimension*], [*Value*]), + [Height], [$qty(162.7, "mm")$], + [Width], [$qty(35.6, "mm")$], + [Depth], [$qty(150, "mm")$], + ), + caption: [Cassette internal dimensions], +) <table-cassette-internal-dimensions> + +==== Ventilation <cassette-ventilation> + +Top and bottom faces include ventilation slots for convective cooling while +maintaining EMI shielding. + +#figure( + table( + columns: (auto, auto), + table.header([*Parameter*], [*Value*]), + [Slot size], [$qty(2.5, "mm") times qty(10, "mm")$], + [Slot orientation], [Length parallel to cassette depth], + [Rows], [$5$], + [Row pitch], [$qty(5, "mm")$], + [Columns], [$12$], + [Column pitch], [$qty(12, "mm")$], + [Slots per face], [$60$], + [Open area], [$approx qty(28, "percent")$], + [Shielding effectiveness], [$gt qty(20, "dB") "to" qty(1, "GHz")$], + ), + caption: [Cassette ventilation specifications], +) <table-cassette-ventilation> + +==== DB-25 connector cutout + +#figure( + table( + columns: (auto, auto), + table.header([*Parameter*], [*Value*]), + [Shape], [D-shaped (standard DB-25 profile)], + [D-opening width], [$qty(33.3, "mm")$], + [D-opening height], [$qty(12.5, "mm")$], + [Mounting hole spacing], [$qty(24.99, "mm")$], + [Mounting hole size], [$diameter qty(3.2, "mm")$], + [Center X], [$qty(17.85, "mm")$ from either edge], + [Center Y], [$qty(82.5, "mm")$ from top/bottom (centered on $qty(165.1, "mm")$)], + + [Connector assembly], [Panel mount with solder cups], + [Position], [Centered on rear closure], + ), + caption: [Cassette DB-25 connector cutout specifications], +) <table-cassette-db-25-connector-cutout> + +==== PCB dimensions <cassette-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 <cassette-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 <cassette-pcb-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) +<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 +<cassette-pcb-grounding-and-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 $lt qty(0.1, "ohm")$. |