General System Level Electrical Placement & Routing Guidelines

Owned by Daniel Puratich
Last updated: 2024-06-19 by Daniel Puratich
7 min read

Context:

Document was originally authored for 2023 System Architecture by Daniel Puratich and has a related RFC where it was discussed and approved by the team. This document is slowly being updated over time to keep up to spec for our 2024 system architecture and beyond. For more details please see:

 

General System Level Electrical Placement & Routing Guidelines/Information:

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

    • Complies with: UL1740

    • 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 / weatherproofing

        • also would help avoid the ESCs catching on fire when drone gets thirsty and goes for the pond

      • makes it harder to accidentally short with a screw driver

        • this has happened in the past at competition when everyone is rushing around

    • 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

  • Avoid loops in cable runs if possible

  • 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

        • Definition of a fast edge in this context and some examples to be added by an EE

        • Generally transcieving in 0 and 1 states asynchronously or synchronously

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

        • An IMU is an example of a particular digital device that is sensitive to external noise.

      • Analog: Characterized by signals with slow edges

        • Definition of a slow edge in this context and some examples to be added by an EE

        • 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

        • An analog video feed out of an analog camera is an example of a particular digital device that is sensitive to external noise.

      • 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 across the spectrum (~20kHz to ~300Ghz) intended for wireless transmissions and notably contained within a coaxial cable

        • 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 looser than others

          • Coax cables are very good at eliminating outside noise

        • For further information see: Coaxial cable

    • 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.

      • An RF dummy load should be connected instead of an antenna if we would like to power on.

    • 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.

      • If a transmitter has a heatsink then it should be mounted to optimize for airflow.

    • 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 Selection

  • Antenna Feed Lines (coax) mounting

    • The shorter the better always

    • thinner coax is generally more lossy than larger coax

    • ideally don’t run sensitive signals right next to coax and feed line connectors

  • Antenna mounting

    • GPS sensors in particular are to our 900 MHz and 1.3 GHz transceivers and should be mounted away from antennas operating at these frequency ranges

      • GPS frequencies are fixed frequencies are1100MHz and 1500MHz

      • Our transceivers will hop around frequencies around their range looking for available channels so they are capable of hopping close to GPS frequencies and causing issues

      • For context it’s worth noting a 900MHz and 1.3GHz transceiver are capable of stepping on each others frequencies as well

      • For further reference see Guide: 1.2GHz -1.3GHz FPV Video System - Oscar Liang & GNSS Frequencies and Signals

      • Often times we will run two GPS antennas. For reasons not explained here spacing out GPS antennas from eachother will improve accuracy significantly. Recommended ~1 meter in most cases which is not archivable in most of our systems, but keep this in mind.

    • Radio waves do not like to change medium

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

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

        • At the frequencies WARG operates at (6 GHz and below) foam will not have a considerable impact on signal integrity

        • Rain will have a measurable impact on signal integrity

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

    • Antenna polarity should be deliberate

  • (Antenna Mounting Continued)

    • 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.

        • IE our VN-300 has specific manufacturer recommendations regarding recommended spacings and clearances

      • Different devices should have their antennas spaced out

        • See notes about frequencies and channels above.

    • Further Antenna Mounting Resources:

  • Some notes about lightning

    • This entire section can be ignored for WARG purposes on nice days and the odds of a lightning strike are relatively low so don’t worry too much about these guidelines

    • As long as the current from a lightning strike is allowed to pass through your structure relatively unimpeded it will not harm the system

      • Notably isolating important electronics from the structure is important for this, see above.

      • To ensure this is possible having a somewhat conductive frame helps

        • This is not possible for composite frames and thus more complex techniques can be employed

    • Lightning likes to, if possible, enter and exit through sharp points

      • Ensuring that the sharpest points on a region of the system are all not electrical elements (notably antennas) will ensure lightning passes where we want it to!

      • A side safety note is that people are also relatively sharp points sticking out of the ground, however, unlike structures, you aren’t replaceable.

        • Be sure in lightning prone weather that people are not the path of lowest impedance for a lightning strike! This can be done easily by ensuring taller, pointier, grounded structures are nearby humans.

    • For ground equipment (towers and stations) grounding rods can be used

      • Notably this may not be possible for ecological reasons as well as if our ground station is on pavement

      • Grounding rods should connect into the earth (with a pointed end) electrically to the sharpest point at the peak of the structure

        • Ideally, as mentioned above, the entire structure is conductive which makes this easier to achieve.