What are the Gain and Threshold numbers for?
Firstly, these values are generally ok at their default settings. The ‘robustness’ of the hardware and
software used in the LapRF results in a high tolerance to settings which are not hand-tuned.
If you really want to understand more about how to tweak this values (for those of you who get a kick
out of PID tuning), please read on, if not, just set default values, and go fly, in the vast majority of
cases, you will not need to understand them.
Gain
(this gets a bit technical, jump down to the part in the box if you want to get to the point, fast)
Gain is a value that you will not see on the multitude of ‘DIY’, or even most (if not all) commercial
trackers. This control is similar to the ‘gain’ on a microphone mixing desk. Turn it up, and things get
loud, turn it down, and things get real quiet.
It is a parameter which controls a piece of hardware unique to the RF Timing modules used in all
LapRF systems.
For 25mW races, with the tracker close to the gate, a value of around 59 is suitable. This corresponds
to a ‘gain’ of 29.5 dB (one gain code = 0.5dB).
Maximum value is 63, for a gain of 32.5dB.
If you want to use higher power, 200mW (23dBm) for example, you need to ‘turn the volume down’.
Since 200mW is approx. 10dB more power than 25mW (13dBm), the gain value should be 20 codes
lower, 59 - 20 =
39
.
For 600mW (27dBm), this is 14dB more power, so the gain value should be approx. 28 codes below
the default, 59 - 28 =
31
.
Why do you care?, because this parameter lets you reduce the crosstalk from adjacent channels,
something that all other RSSI based timing systems all suffer from. It lets you run races with 200mW,
or even 600mW transmitters, without pilot 1 being captured on 3 other channels.
So to tweak it, start with the default setting, 59 for 25mW, 39 for 200mW, 31 for 600mW, and if
you are getting crosstalk from other channels, decrease it. If you are not sensitive enough, and
missing laps, increase it.
Threshold
This one is a little more difficult to explain exactly what it does, because it ties into some pretty fancy
software, which runs on-the-fly in the LapRF systems.
Basically it is the size of the RSSI peak, above the noise base-line, which is required to trigger a
detection (lap).
22
