path: root/mechanical-specifications.typ

#import "@preview/unify:0.8.1": qty, qtyrange

= Mechanical specifications <mechanical-specifications>

== Modules <modules>

=== Cassette <cassette>

==== Shielding and construction <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.

#figure(
  table(
    columns: (auto, auto),
    table.header([*Parameter*], [*Value*]),
    [Material], [DC01 cold-rolled steel (EN 10130)],
    [Thickness], [$qty(1.2, "mm")$],
    [Finish], [Matte black powder coard (RAL 9005), all surfaces.],
  ),
  caption: [Cassette construction parameters],
) <table-cassette-construction-parameters>

==== External dimensions <cassette-external-dimensions>

#figure(
  table(
    columns: (auto, auto, auto, auto),
    table.header([*Dimension*], [*Value*], [*Tolerance*], [*Note*]),
    [Panel height],
    [$qty(177.8, "mm")$ ($qty(4, "U"), qty(7, "inch")$)],
    [$plus.minus qty(0.1, "mm")$],
    [Full rack unit height],

    [Grid pitch],
    [$qty(38.1, "mm")$ ($qty(1.5, "inch")$)],
    [N/A],
    [Theoretical center to center spacing. Two columns of banana jacks ($qty(0.75, "inch")$ grid).],

    [Panel Width],
    [$qty(38.0, "mm")$ ($qty(1.5, "inch")$)],
    [$+0.0 "/" -0.1 "mm"$],
    [Undersized to accomodate paint thickness and thermal expansion.],

    [Body height], [$qty(165.1, "mm")$, ($qty(6.5, "inch")$)], [$plus.minus qty(0.1, "mm")$], [Fits between rails],

    [Body width], [$qty(38.0, "mm")$ ($qty(1.5, "inch")$)], [$+0.0 "/" -0.1 "mm"$], [Same as panel],

    [Depth], [$qty(152.4, "mm")$ ($qty(6, "inch")$)], [$plus.minus qty(0.2, "mm")$], [Front to rear],

    [Lip height], [$qty(6.35, "mm")$ ($qty(0.25, "inch")$)], [N/A], [Top and bottom],
  ),
  caption: [Cassette external dimensions],
) <table-cassette-external-dimensions>

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 $lt qty(0.1, "ohm")$.