Testing the Low Pass Filter

Testing the Foxtechfpv 1.4GHz Low Pass Filter

http://www.foxtechfpv.com/1400mhz-low-pass-filter-p-439.html

The aim of this experiment was to quantify the signal attenuation of the low pass filter across various frequecies. Ideally I would have produced a frequency response plot of the LPF from 0 to 3GHz but due to limitations with the equipment, I could only test in the ranges 0 – 1040MHz and 2 to 18GHz with two different signal generators. Being limited by the SigGens to the two different frequency ranges (0 to 1.04 GHz and 2 to 18GHz )was a bit disappointing because the real interesting part is the roll off profile at around 1.4GHz. Below 1.4GHz, the loss should be minimal, above 1.4 GHz, according to the foxtechfpv website the signal should be attenuated by at least 45dB. A really good filter will have a short sharp transition from no loss to high loss.

I chose to test at two specific frequencies. 1) 1040MHz is the highest frequency the first SigGen can go and is closest to 1280MHz of the video transmitter being used. 2) At 2560 MHz which is the second harmonic of 1280MHz. Ideally the result will show 0dB loss at 1040MHz and at least 35dB at 2560. So what did the tests show? First some calibration. The LPF uses male and female SMA connectors. To hook it up to the signal generator and Spectrum analyser, a set of adaptors is used to adapt the SMA connectors to the N-Type used on the equipment. By connecting the signal generator directly to the spectrum analyser and transmitting 0dBM, the SpecAn showed -1.2dBm. The SigGen signal is then upped to +1.2dBm to compensate for the loss and set the SpecAn to 0dBm. At 1040MHz, the loss of the LPF is measured to be 0.6dB (better than the 0.7dB on the website). Then the LPF is connected to a 2560 MHz signal (second harmonic of 1280MHz). The SpecAn shows the signal is attenuated by 63dB. Much better than the conservative 35dB stated on the website.

1. 1040MHz Test Setup. Spectrum Analyser on the left, Signal generator on the right.

2. Cable & connector & SMA adaptor loss. The LPF filter is removed from inline and the signal generator is set to +1.2dBm which results in 0dBm on the Spectrum Analyser. The loss in the measuring cables, connectors and SMA adaptors is then 1.2dB

3. 1040MHz reference level 0dBm. Without the LPF inline, the SpecAn shows 0dBm.

4. 1040MHz filter only loss 0.6 dB. Compensating for the connector and cable loss, the LPF loss shows 0.6dB at 1040 MHz. Why 1040 MHz? That's the highest frequency possible on this particular signal generator.

5. Signal Generator for 2560MHz.

6. 2560MHz reference level at 0 dBM without the LPF inline.

7. 2560MHz with LPF inline at -63 dBM with the LPF inline. The LPF is doing it's job

8. foxtechfpv 1.4GHz LPF

1.3GHz Low Pass Filter – The Case For

Tuning the CC3D – Step 1

I haven't had the chance to actually go fly the YAPVCBQ since the somewhat unsuccessful maiden so the next best thing is to tune it indoors and I'm learning heaps. For example, the first time I fired it up on the line, only motors 1 & 3 were turning consistently. First thought was I had dodgy connections to 2 & 4. What I figured out (I think) was that I had to disable the yaw and pitch stabilisation because strung up on the wire like that it was trying to compensate in an axis it had no control over. 

Cutting the shaft on the NTM Prop Drive 28-26 1350KV / 310W

For the PVC 'BattleQuad' I'm building I ordered four 'NTM Prop Drive 28-26 1350KV / 310W' motors as I've used them before on a Hex 550 build and am quite happy with them. These motors are a little different in that you have to buy a seperate accessories pack to get a mount for the base of the motor and a prop adaptor. The motor is a little odd in the way that the shaft extends out the bottom of the motor and the prop adaptor is attached via three bolts on the opposite side from where the shaft protrudes from. In my previous build this wasn't a big problem because  the way the motors were mounted, the shaft could extend below the arm without interfering.

On my current build, the PVC 'Battlequad', the way the motor slides onto the arms, the shaft was going to get in the way. I could easy shorten the arms and cut a line down the length of the PVC pipe but I've got some plans for extended length arms so I want to keep them intact. I decide to cut the shaft off the motors. The concern with this option is in the process of cutting with the Dremel, the fine metal dust created from the cut could potentially end up getting inside the motor. 

The first method I employed to avoid this problem was to put the motor back into the re-sealable plastic bag it came in, poke just the shaft through the plastic so the motor is contained inside the back and just the shaft sticks out. The problem with this is that the bag wasn't up to the job of withstanding the red hot metal filings and a hole was quickly burnt in the bag and became a collection point for all the metal filings. My solution the keep the filings out of the motor had had the opposite effect and was actually diverting filings into the confined space around the motor and it's precious little magnets.

My next solution worked much better and was even simpler. Take a piece of cloth tape (about 8cm wide by 15cm in length), fold it in half and cut a small triangle in the centre of the fold. Flatten out the tape so now you have a piece of tape with a small diamond shaped hole in the middle, just large enough for a motor shaft to fit through. Put the tape over the motor with the shaft protruding through the small hole and Dremel away. If you've cut enough length of cloth tape, there should be plenty left to wrap around the motor and keep the metal filings out. The cloth tape is much sturdier and kept the red hot metal filings out without melting.

PVC Quad build

The PVC quad was inspired by HelliYea's Battlequad post over on the Openpilot forums (http://forums.openpilot.org/topic/13503-battlequad/page__hl__%20battlequad).

I've seen it fly (and crash) and it shows great flight capability along with rugged frame construction.

The 15mm PVC pipe is straight from the local hardware store. RCTimer motors, KK2.0 flight controller and DJI F450 centre frame.

GoPro Hero 2 firmware downgrade to v124

After applying the latest firmware update to enable Protune mode I encountered a number of problems;

1. Occasionally on startup the camera would hang showing the video icon and nothing else.
2. In Protune video mode, the camera would not create a second video file after reaching a maximum file size of 1.85GB on the first file.
3. Startup would take noticeably longer.

I've since downgraded the firmware back to the original version available to download here http://software.gopro.com/Firmware/v124/HD2-firmware.bin

After the firmware downgrade I had my reliable old camera back.

Quadcopter Parts List

I was asked recently what would be a good way to start in multirotors. The first question was "A tri-copter must be simpler and easier because there's only 3 motors?". I disagree. I've built quite a few tricopters and on each one explored a different way of building the yaw control mechanism on the tail motor. Some worked well, some not so well but by the time the servo is mounted and linkages hooked up, it all gets more complicated and less reliable compared bunging on a fourth motor and prop spinning in the opposite direction. I think what finally converted me was assembling the DJI F450 frame with the Naza controller. This quad was smooth and stable from the first flight and is an ideal platform for mounting a GoPro camera on for some aerial photos & videos. The downside of the DJI is that it's not cheap. I spent over $600 on the Flight controller, frame kit and GPS module and that's assuming you already have LiPo batteries, LiPo charger, radio control transmitter & receiver and other bits and pieces. It can quickly become an expensive option especially if you're just getting started.

Another option is the "toy quads" you'll find in electronics stores or maybe even the local shopping centre. They're cheap and maybe a good way to get a taste for flying multi-rotor but they have certain disadvantages;

1) The parts are generally not re-usable for your next project. The motors, ESCs and controller are so deeply integrated they're not easily adapted to another frame.

2) Often they're quite small with payload capacity to match. The small quads will struggle to carry much more than a lightweight spycam let alone a GoPro or pocket digital camera. 

So what to buy for a simple, cheap, respectable quadcopter with parts that could be re-used in your next project, maybe a hexcopter. This shopping list should cover absolutely everything you'd need to start from scratch.

Flight Controller: $29.99 Hobbyking KK2.0 Multi-rotor LCD Flight Control Board

Auto-level with and LCD screen for configuring and tuning. Eight outputs good for tri-copter up to Octo-copter. The downside is no barometric pressure sensor (height hold) or GPS (position hold) but at that price, those features aren't expected. It also has a built in voltage monitor that outputs an audible alarm at a programmable low voltage level. This feature requires a couple of extra wires soldered to the board and is well worth the trouble.

Frame: $17.99, Q450 Glass Fiber Quadcopter Frame 450mm – Integrated PCB Version

A simple, sturdy frame that comes with an allen key for the 2.5 and 3mm boltsand even a velcro strap for the battery. The lower board has an integrated PCB which means there's no need a messy power distribution cable arrangement. 

Motors: $14.99 (x4), NTM Prop Drive Series 28-26A 1200kv / 250w

Ideal size and power for this frame. The mouting holes align perfectly. Something odd with this motor I hadn't come across before was the shaft sticking out the rear / bottom of the motor. The accessories pack is needed as a propeller mount and uses three bolts to attach to the motor. I quite like this method as compared to the prop savers with the rubber o-ring or the collet style adaptors.

$1.89 (x4), NTM Prop Drive 28 Series Accessory Pack

You won't need the motor mount or the four silver screws, just the prop adaptor. With the Q450 frame, the bolts go through the frame and straight into the bottom of the motor.

$12.19 (x4), TURNIGY Plush 30amp Speed Controller

Simple, reliable, does what it's supposed to without any fuss. The difference in weight between the 18, 25 and 30 Amp ESCs is barely worth compromising the current capacity for.

$1.59, PolyMax 3.5mm Gold Connectors 10 PAIRS (20PC)

Some might solder their motor wires straight to the ESC but I've never been organised enough to plan that far ahead. I need the flexibility to swap any two of the three motor wires. I've read alot about bad or loose connections that have caused crashes. I've never experienced this type of failure. I think probably because it all comes down to a good solder joint.

$2.46, Slow Fly Electric Prop 8045R (4 pc – Green, RH Rotation)

Eight inch, Right Hand (RH / Clockwise) rotation. Four to a pack but you'll only need two with two spares. At these prices though, if you're ordering any spares, props should be high on the list. Get two packs.

$2.29, Slow Fly Electric Prop 8045 SF (4 pc – Green)

As above but they turn the other way. I like coloured props (as opposed to black) because they're much easier to see. In bright daylight, black props look almost invisible compared to green props which seem to present a good contrast against the sky.

$4.75, 10CM Male to Male Servo Lead (JR) 26AWG (10pcs/set)

These connect the receiver to the inputs on the flight controller board.

$16.38, Turnigy 2200mAh 4S1P 20C Lipo Pack

Minimum capacity and C rating you'd want to use for a decent flight time.

Now up until this point, it's assumed you already have some RC equipment from other projects. For those that are starting out and need a LiPo charger and Transmitter / Receiver etc, here are some suggestions.

$53.95, Turnigy 9X 9Ch Transmitter w/ Module & 8ch Receiver (Mode 1) (v2 Firmware)

From accounts I've read online, the Turnigy radios are a good way to get started. A nine channel radio and receiver for just over $50 is good value! This package doesn't come with batteries or charger but seeing as most people will already have these, it makes sense for Hobbyking to exclude these parts.

$6.89, HobbyKing 1500mAH LiFe 3S 9.9v Transmitter pack.

A Tx is no good without power. Compatible with the charger below but you'll need another lead for charging.

$16.99, HobbyKing 50watt 5A X-Charger/Balancer

This battery charger will be enough to get you up and running doing balanced charges on LiPo's. It will also charge NiMH and NiCd batteries often used in transmitters. Note – it is a DC only input so you'll have to hook it up to a car battery or similar and can't plug it onto 110 / 240 AC wall outlets. For a charger with both DC & AC input, look at a "B6-AC Plus – Charger/Discharger 1-6 Cells Dual Power (COPY)" or better.

$2.69, Charging harness

Use this harness to charge the Tx LiFe battery. You'll probably never use ALL the connectors on this charging harness but even if it means cutting some off and re-purposing with a different connector, for a few dollars it takes the pain out of soldering up wires to banana leads.

I think that's pretty much everything you'd need to get in the air with a decent quad. The best thing about this setup is that down the track you could directly swap the KK2.0 control board for maybe a DJI Naza controller and add altitude and position hold, without having to change any motors or ESCs. The parts are all fairly cheap to replace so those first few crashes shouldn't be too painful on the wallet. If you can afford to, get two frames (1 for spare) so you don't have to wait weeks for another delivery from Hong Kong to repair / replace parts. Spare propellers are a must have and considering the price of them, it's a no brainer to order a few more packs of them.

There are a few more items like spare battery packs and LED strip lighting that you'll probably want to add down the track but for now the objective is to get a quad in the air for cheap, but not too cheap to make it nasty.

Ok so to recap, here's the parts list summary and totals;

Flight Controller: $29.99 Hobbyking KK2.0 Multi-rotor LCD Flight Control Board

Frame: $17.99, Q450 Glass Fiber Quadcopter Frame 450mm – Integrated PCB Version

Motors: $14.99 (x4), NTM Prop Drive Series 28-26A 1200kv / 250w

$1.89 (x4), NTM Prop Drive 28 Series Accessory Pack

$12.19 (x4), TURNIGY Plush 30amp Speed Controller

$1.59, PolyMax 3.5mm Gold Connectors 10 PAIRS (20PC)

$2.46, Slow Fly Electric Prop 8045R (4 pc – Green, RH Rotation)

$2.29, Slow Fly Electric Prop 8045 SF (4 pc – Green)

$4.75, 10CM Male to Male Servo Lead (JR) 26AWG (10pcs/set)

$16.38, Turnigy 2200mAh 4S1P 20C Lipo Pack

$53.95, Turnigy 9X 9Ch Transmitter w/ Module & 8ch Receiver (Mode 1) (v2 Firmware)

$6.89, HobbyKing 1500mAH LiFe 3S 9.9v Transmitter pack.

$16.99, HobbyKing 50watt 5A X-Charger/Balancer

$2.69, Charging harness

A grand total of $272.25. A bit pricier than I was aiming to get it but don't forget that theres about $80 worth of radio and charging gear that can be re-used in the next multirotor. Once you've built one, you won't stop at that. Tri, quad, hex, octo … they're all possible with these same parts.

****If I've forgotten anything on the list, leave me a comment below

Imitation DJI F550 build

Parts used in this build;

H550 V2 Glass Fiber Hexcopter Frame 550mm – Integrated PCB Version

9 Mode Multi Colour/Multi Function LED strip with Control Unit

Hobbyking KK2.0 Multi-rotor LCD Flight Control Board

NTM Prop Drive Series 28-26A 1200kv / 250w

NTM Prop Drive 28 Series Accessory Pack

TURNIGY Plush 30amp Speed Controller

Slow Fly Electric Prop 8045R (4 pc – Green, RH Rotation)

Slow Fly Electric Prop 8045SF (4 pc)

Flips & Rolls with the Naza F450 Quad

This video shows my first loops and rolls with the DJI Naza F450 Quad. I did at least one in every direction (left, right, forwards and backwards). The entire flight was flown via FPV which I found much easier for these moves. Flying line of sight at this height and distance makes it hard to maintain orientation.

A guy that started flying the F450 about the same time as me told me how he tried to roll it, it got half way and dived into the ground. Broke an arm or two and some props. Thanks to the well thought out F450 frame, he was back up and running in no time but after hearing such a tale I was a bit apprehensive to try it myself….. until now. With my FPV gear up and running it's much easier to gain a lot of height and maintain orientation. 

The setup is a stock F450 quad with 8" props and 4S battery (4000mAh on this flight). Other than the gain settings as described here (http://fangin.com/blog/2012/08/07/dji-naza-f450-gain-settings-flips-and-rolls/), there's not much to it. Gain plenty of height and bang the stick over. I can see how if the gains were set too low, the roll rate might not be quick enough to complete the roll without losing too much height. 

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DJI Naza F450 Gain Settings – Flips and rolls

The settings I've been using up until this point were;

Basic Gain: Pitch: 150, Roll 150, Yaw 100, Vertical 120

Attitude Gain: Pitch 70, Roll 70

The problem I found with these settings was under heavy acceleration, the quad would oscillate. The same oscillation would occur with high yaw application and when descending from height. After some forum reading I decided to enable "Remote Adjust" by assigning X1 to the Basic Gains and X2 to the Attitude Gains. The oscillation on descent was apparently due to too low gain on Attitude settings.

By using  "Remote Adjust", the gain settings can be changed in-flight and the affects judged immediately. I found increasing the Basic Gains to max the quad would start oscillating even just in a hover. Backing it off a few notches would stabilise the quad and give quite good response to input. Dialling the gains all the way down meant the response was much softer and smoother which meant response to input is slowed somewhat. Without knowing what the actual values were at the time, I settled on gains that seem to be a good mix between control authority and smooth flight. There was still some vibration but I'll work on that. 

I think I'll keep the Remote Adjust capability setup on the Quad. Being able to dial back the gain for some slow smooth input for photos and video and then dial it up for some hot-dogging is a great feature. All without having to stop and plug in the laptop. 

The gain settings I settled on are;

Basic Gain: Pitch: 104, Roll 104, Yaw 69, Vertical 83

Attitude Gain: Pitch 150, Roll 150