This might be an odd one to ask as I doubt someone outside Qorvo/DW themselves has a good answer, but hopefully some of you already tried this out.
Can you use an off-the-shelf DWM3000 module as a reference test board for calibrating antenna delay?
The Qorvo APS312 app note mentions this custom in-house DW3000 based reference board they are using to calibrate every other design. While one would still require some high end instrumentation to work through all the testing outlined in the RF section, I was curious to know if you could at least use a DWM3000 module to perform the antenna delay calibration as specified in the “2.2.5 Calibrating for Antenna Delay” section of the app note.
I am aware that TWR with 3 devices that all have unknown antenna delays can resolve the delays using EDM and the physical distances between the devices (as described in an app note for the DW1000), but would really like to have a quick way of doing it without all of that setup/iterations.
If this doesn’t work, or there are high chances it won’t, are there some solutions that don’t require 10k$ equipment?
You can do the calibration with a single reference device if you know the delay on that device is set correctly. So how much do you trust the factory calibration on the DWM3000?
To be honest the best approach I’ve found so far is to installed everything and survey them in accurately first. Then measure a few hundred times between every possible pair of units that have a clear line of sight. At this point you have far more data than is technically needed to calculate the antenna delays. You could split the devices into groups of 3 and calculate the values for each group but you’ll almost certainly find that gives you slightly different values depending on how you group things. Instead I thrown all of the data into a least squares optimisation and find the delays that give the minimum errors for all possible signal paths.
One thing to keep in mind, you will see a variation in effective antenna delay depending on the orientation of the devices.
Thank you for the response, Andy.
Regarding your first point, well, they do sell the DWM3000 at around 2x the price of the DW3xxx transceiver, but at the end of the day it is an evaluation tool and I assume calibrated and characterized as such.
When you mention " is to installed everything and survey them in accurately first." that assumes you are doing this with just the anchors in a typical RTLS system, already in their final places, correct? It is my understanding that for a tag within an RTLS wireless synchronized system, its antenna delay is also important and should be known. Citing from APS014 Section 2.3.3 (for DW1000 but I assume it still applies):
"For RTLS systems generally, the receiver antenna delay should be known for most accurate
ranging in a system using wired synchronization and both the transmitter and receiver antenna delays should be known in a system using wireless synchronization."
Also, that method might already include the distance bias effect within what you would otherwise call the antenna delay, but it still only works if none of the devices move relative to each other (which is also why I assume you say “but you’ll almost certainly find that gives you slightly different values depending on how you group things”)
The aim would be to get everything as calibrated and ready to go as we can from the factory floor even if it takes some extra steps.
Then assuming we can get the antenna delays for 3 custom devices using the DW3xxx transceiver (with identical-ish antennas at equal distances to each other) by the EDM method, could we use that data and one of the devices to (to some degree) reliably calibrate the rest on the factory floor?
Yes. We place the tag in the middle of the area, put it into anchor mode and survey it’s location. The tool also includes the option to put more emphasis on minimising tag to anchor range errors over anchor to anchor range errors.
All measurements will have noise (which averaging helps with) and things like signal strength and temperature biases which you are never going to correct for perfectly (which averaging won’t help with). You can also get some signal angle of arrival range effects due in part to signal levels and partly from physical antenna characteristics. All of this will show up as antenna delay in the calibration. Which is why I find I get better results using this overdetermined approach, these other effects get averaged out a little bit.
I also find that these effects can be a little environment dependent which is why calibrating in the final site gives better results. Open fields seem to give range errors that are a few cm different to an office enviroment.
For a factory calibration we have a known unit (calibrated as part of a large group in a big open area) mounted in a fixed location. The new unit is placed at a known location and we set the antenna delay to give the correct range to the known unit. After calibrating it we test again to verify that the error is now below a threshold.
Taking units calibrated this way and then calibrating them in site we normally get results that are different by 2-4 cm. So not a huge difference but enough that the on site version makes an improvement in the end results.
Obviously it does depend on how you are surveying in the anchors, if you are only accurate to a few cm then this difference doesn’t matter much.