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Change Description

Description of Change

Adding a section regarding system level electrical placement information as described below:

• The frame should not be electrically connected (aka should be at floating potential)

  • This can be checked with a DMM (when the system is not powered)

  • This means that if a PCB we are using has electrically connected mounting holes we need to take proper steps to achieve isolation

    • Sometimes PCB mounting holes are electrically connected to a GND plane for thermal reasons so we should also approach thermals with caution on sensitive electronics

• Conformal Coating can be used within reason

  • The pro of conformal coating is waterproofing and makes it harder to accidentally should with a screw driver

  • The con of conformal coating is it prevents heat from leaving a PCBA

    • So anything that is conformal coated should receive a thin layer at most

  • Another con is that it can be annoying to remove in the case of reworking a PCBA

  • For example it would be fine to conformal coat a flight controller that runs cool

  • However, for example an RF transmitter we would want to approach with more caution

    • We could place a large heatsink on the primary chip(s) using thermal paste

    • Then we could consider conformal coating the other portions of the board

• Cables, especially longer cables, should be within sheaths when reasonable

  • This offers protection against abrasion during natural flight vibrations

  • Exception to this rule is shorter cables that are placed further from anything that could be abrasive, specifically cables purely within the avionic compartment of our aircraft

• Varying conductor types should be separated when reasonable

  • Generally we consider four different types of cables: Digital, Analog, Power, & Coax/Rf

    • Digital: Characterized by signals with fast edges

      • Fast edges refers to low microsecond rising/falling edge range and faster

        • Clarify rise/fall time (offer examples of rise/fall times we’d expect on our system)

      • Generally transcieving in 0 and 1 states asynchronously or synchronously

      • I.E. a GPS module’s cable connection to our flight controller is digital

    • Analog: Characterized by signals with slow edges

      • Slow edges refers to high microsecond rising/falling edge range and slower

        • Clarify rise/fall time (offer examples of rise/fall times we’d expect on our system)

      • Generally transcieving data in varying voltage states

        • Though some protocols we consider analog will also transmit some digital data as well

      • I.E. a hobby fpv analog video camera has an analog data output

    • Power: Characterized by constant voltage varying current designed for power transmission

      • Generally the voltage is constant though higher frequency noise may be present

      • These conductors can have significant current pulses

      • I.E. a connection from a battery to an ESC

    • Coax/Rf: Characterized by oscillating signals accross the spectrum (~20kHz to ~300Ghz) intended for wireless transmissions and notably contained within a coaxial cable

      • Maybe a nice name here ?

      • A coaxial cable, specifically for our applications utilizing an SMA or RP-SMA connector, is generally used to guide sensitive RF signals between transceivers & antennas.

        • It is also worth noting antenna placement is critical, this is noted below in more detail

        • SMA & RP-SMA connector info should be found in the corresponding connectors arch doc section.

      • Because coaxial cables offer strong noise immunity the routing constraints of these cables are fairly loose. looser than others

        • This is true right ? I know coaxes aren’t perfect, hmm

  • Each of these type of conductors should be physically grouped together, however, each type should be separated

    • I.E. All power stuff near each other, all digital near each other, but power separated from digital

    • This grouping and separation is physical distance, though of course there are other factors

    • Within PCBAs these groups may be mixed, this is fine, we will assume the PCB designer has taken the proper care to avoid issues as necessary

• RF Transceiver Care

  • An RF transmitter should not be turned on (given input power) without a proper antenna connected as this can permanently damage the transmitter

    • Possible violations of this policy should be reported in Discord and transmitters should be labelled as damage (notably degraded performance) may not be immediately evident. We don’t want to blame each other, stuff happens, we just want to note it for the future! If you do not feel comfortable stating this publicly feel free to DM a lead you’re comfortable speaking to who can relay the message without naming names.

  • Transmitters generally get warm

    • They require a lot of power and therefore require some cooling

    • They are sometimes designed to be mounted outside an airframe for ambient cooling of wind passing the frame. As we may not be doing this we need to approach this with caution.

    • Notes regarding conformal coating are above.

  • The lower the frequency the longer the distance we can get for the same output power generally

  • The higher the frequency means more bandwidth generally

• Antenna mounting

  • Radio waves do not like to change medium

    • Specifically they do not like to pass through materials of varying dielectric constant

      • Verify if this is true / worded correctly - my RF knowledge isnt that cracked

      • Passing waves through some varying materials may be fine, but be careful!

    • This means mounting antennas outside of cases/airframes and providing LOS when reasonable

  • Antenna polarity should be deliberate

    • Antennas that use diversity should be mounted with opposing polarities

  • Ground antenna should be mounted with as much distance away from the ground as possible

    • This is to reduce ground reflection and has a considerable effect on reliability of an RF link

  • Antenna spacing should be minded

    • Any device with diversity (multiple antenna inputs) should have it’s antennas mounted with some spacing between them. Always follow manufacturer guidance here

    • Different devices should have their antennas spaced out

      • add reasonning and maybe notes about frequencies

Implementation

We will discuss in RFC channel. This is just documenting knowledge for newer members to understand.

Reason for Change

Assist in Mech Elec integration

Priority

Medium

Impact of Not Responding to Change

Confusion

What Groups, People, Sub-teams Need to be Notified?

• Mech: Nathan Green

• EFS: Anthony Luo

• Electrical: Daniel Puratich Ethan Abraham Farris Matar Rayyan Mahmood Nolan Haines Michael Botros

Change Impact

Additional Parts/Resources Required and Costs

None

Impact on deadlines

Minimal

Alternatives and Recommendations

NA

Comments

I would appreciate verification and input on some of what I’ve written here. See Discord thread.

Change Request Sign Off -

Status

✅ Accepted / ⏸️ On Hold / ❌ Rejected / More info Requested

Comments

Signatures/Name