Monday, 22 May 2017

Checking out your new FrSky transmitter

Over the last few years I've set up a variety of FrSky transmitters and have got to know what to check out after purchase. So in this post, I list a few tips, including an effective method for checking range.

1. Check the gimbal wiring
The quality of FrSky hardware has improved steadily over the years. However one recurring complaint is the lack of proper support for the gimbal cables, which has in rare cases led to wires fracturing. So one of the first tasks is to open the case and check out the wiring at the pot or hall sensor. Pay particular attention to the moving axis and if necessary apply some hot glue at the junction. I've done this on my own Horus and it's given me that extra bit peace of mind.
A dab of hot glue will help prevent fractures

2. Check switch operation
Early X9Ds came with cheap switches which were nor particularly durable, indeed my second X9D suffered from a faulty 3-position switch. The Plus has better switches, and those on the Plus SE are further improved. Nevertheless, it's a good idea to check the switches on a new transmitter. This is easily accomplished in the Info menu, just cycle them back and forth and check that each position is correctly registered.

Info menu showing switch monitor panel
If replacing a switch, note that the thread sizes and switch housing are different between the X9D, D+ and D+SE, so be sure to order the correct type.

3. Use the latest RF firmware
If flashing your RF firmware (e.g. from EU to non-EU), be sure to check the FrSky site for the latest firmware version. Watch out in particular for silent hardware changes. For example, if you have an X9D Plus which has been manufactured since November 2016, you must use the latest 2017 XJT firmware. The 2015 firmware will appear to work correctly but there will be a loss of range (as I discovered, see below).



4. Perform an RSSI check 
The best way to check the range of a new transmitter is to compare it with another known working transmitter. The method I use involves comparing RSSI values. RSSI is a measure of signal strength reported by receiver. A low RSSI value will trigger 'RF signal low/critical' alerts on the transmitter - RSSI is therefore a good measure of effective range.

There are two ways we can compare RSSI. Both use a 'reference' receiver to report the RSSI value via telemetry. We can either compare the range to achieve a certain RSSI value; or we can compare RSSI values at a certain constant range. The latter is easier to do, and is the basis for my method.

Whichever method you use, the transmitter should be in full range mode, not range test mode (we don't fly our models in range test mode!).

The first task is to make a receiver rig, see photos below. The same receiver should always be used (to avoid variation in the way RSSI is reported).



Test rig with spare receiver and battery. The servo is not necessary.

Here's the test procedure:
  1. Find an open space and choose two locations 150-200 meters apart. The exact distance is not critical, what matters is that the same locations be used for all tests.
  2. Get your helper to hold the receiver rig at the first location. 
  3. Carry the 'reference' transmitter to the second location. The transmitter must be in full range mode.
  4. With the receiver board and transmitter at consistent orientations, take a reading of the RSSI value from the transmitter's telemetry screen.
  5. Repeat with the second transmitter
  6. Compare the RSSI values
Below are some test results with different devices, all made on the same day with tx and rx approximately 160 meters apart.
  • Taranis X9D #1: RSSI 69 
  • Taranis X9D #2: RSSI =69
  • Taranis XJT ext module: RSSI =70
  • Taranis X9DP #1
    • 2015 firmware: RSSI = 52 
    • 2017 firmware: RSSI =72
  • X9DP #2: RSSI =73
  • Horus (internal antenna only): RSSI =69 
Note how the method has identified an issue on an X9DP transmitter. Turns out that I had flashed an old version of the RF firmware. Reflashing with a newer version restored the range.

The same method could also be used to compare effective range between EU and non-EU firmware on the same transmitter. If anyone has tried this do let us know the results!

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