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Compass and Calibrating
07/09/2019 at 04:55 • 0 commentsIn order to map the direction of the transmitter, the transmitter heading needs to be relative to a compass heading. When in a moving vehivicle that can be obtained from GPS. A real compass is necessary for stationary readings. I started playing with the HMC5883L and MLX90393 magnetometers. The sensors do function as compasses but they require software and calibration. There are Adafruit and Grove libraries for them and some others as well. The Adafruit library for the HMC5883 yielded the best results but the libraries didn't play well with the other interrupts. In order to spare a major detour I plan on using a spendy HMC6643 compass module that includes it own microcontroller and comes calibrated.
To calibrate, the transmitter will be placed as far away as possible from the front of the car and will need a high quality reading. The raw measured value is replaced by the compass is reading.
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Diode Redo
07/04/2019 at 00:12 • 0 commentsI have noticed that the antenna switching introduces significant attenuation but some attenuation is expected so time for another look. In looking at the specs I am not turning on the diodes very hard. The 74HC14's can source about 4 mA and the diodes really want 10 - 100 mA to turn on hard. I remember the UDN2983 source driver - an nice solution but obsolete but is available. There's a LTC 1156 quad high side micropower driver IC and resistors/transistors. Any other ideas?
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Yet More Testing on VHF
07/02/2019 at 23:41 • 0 commentsI got some time on a vehicle with a metal roof and set it up with a set of VHF antennas. Tuning in on NOAA weather radio on 162.400 MHz from a fixed location there was a stable heading reading albeit with a low signal quality. In urban driving the audio Doppler tone revealed quite a bit of multipath. When in a less congested area good quality readings were observed. On to calibrating and turning the arbitrary heading into useful information.
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Dipole Antenna Array?
06/29/2019 at 05:18 • 0 commentsHaving to borrow a car with a metal roof to test the system is a pain. The antenna spacing for VHF is about 18". My cars have sunroofs - metal tops with big glass holes. There is no way to attach the four magnet mount monopoles with the required spacing because of the sunroof location. I can magnetically mount to the roof with a trestle over the sunroof but that is a problem for the monopoles - no uniform ground plane. That can be solved by going with a dipole array. The height of the array would be about 36" plus the trestle height. Rubber duckie's for the elements might be a tradeoff.
A dipole array can be a single unit so a positioning sensor can be built into it. The system is needs be calibrated once and shouldn't need recalibration between installations and removals.
Here's a stationary array - just four antennas should be sufficient:
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More Testing
06/27/2019 at 22:43 • 0 commentsAs it turns out, of the two cars I own, neither have enough metal for a proper ground plane - even on UHF. Doah! It seems like the geometry is simpler on VHF so I ought to start there. Now - to borrow a car without a sunroof. Here's a pix of my mobile test jig sans laptop and test vehicle.
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Outdoor Testing Vers 0.0
06/14/2019 at 18:08 • 0 commentsAlthough I haven't had much chance to refine the software, I had all the pieces and gave it a try. I didn't have someone to drive but the readout looked as crazy as ever.
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Goertzel algorithm tinkering
05/23/2019 at 00:39 • 0 commentsBy playing with sample size and ADC resolution the phase output error is now under a degree. Quite a win over all the time domain phase crossing detection hardware and filters. I'm sure with some better audio signal isolation would help.
There is a new problem - the phase changes (~10 degrees) with input signal large amplitude changes. This is very pronounced when the signal is very low or or on the verge of overload. Need to look into setting the receiver volume to get the best results.
So far I also haven't done anything about the signal riding on ADC input - I haven't addressed the half scale DC offset built into the samples.
Latest version of the code was just posted.
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Newer BNC antenna mount
04/27/2019 at 03:50 • 0 commentsNew magnet mount BNC antenna assembly. The through-hole antenna mounts are great. I could weatherproof them by painting with epoxy. Or switch to a top mount SMA and 3D print a top.
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Yet more DSP approaches
04/15/2019 at 07:56 • 0 commentsI would like to try some pure DSP approaches and eliminate noise error from old fashioned analog zero crossing detection. I looked at the Teensy DSP library and unfortunately it only has a magnitude only output FFT and the bins are too big. I don't have time to to tinker with other FFT libraries for now. A simpler way is the DFT. There is a simpler form of that known as the Goertzel algorithm that computes a single DFT bin. I recall it used for tone detection applications like DTMF decoding with low power micro-controllers. It offers a phase output as well as magnitude so it should work well and allow "dialing in" the the bin width. Both a magnitude and phase measurement for the Doppler tone and just the magnitude of the Doppler second harmonic need to be computed. This should be computationally very efficient though not an issue with the Teensy 3.2 - plenty of CPU cycles.
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System Interface
04/04/2019 at 04:58 • 0 commentsUp until now I have been planning on a Neopixel ring and a couple of rotary encoders for a user interface. The Neopixel ring works great and I can use different colors for different uses. I have come up with a workable system for entering small numbers and displaying on the ring but I really need an alphanumeric display - at least three or four digits.
In normal operation the three digits would be the processed heading. Several adjustments need to be entered: filter Q, Doppler frequency, averaging, azimuth adjustment, etc.
The typical location for this device is in a car with normal lighting so that's the issue. It needs to be readable in possibly direct or almost direct sunlight. I've ordered a few small paper white displays and a 24 LED ring. Perhaps mounting the paper white display inside the ring. This arrangement would be nice but is awfully expensive for what is needed. Any suggestions?