Why do I get “-1” when performing Bluetooth Advanced measurements?
BT SIG defines FM modulation measurements using specific patterns of data that must be transmitted by the device. 11110000 is used for Delta F1; and 101010101 is used for Delta F2. When using traditional Direct Test Mode, we can force the device to transmit those patterns and perform the measurements exactly as BT SIG recommends. A limitation when using Bluetooth Advanced is that we need to work with the data that is advertised. In most cases, this is valid advertising data with 20-30 bytes of payload that is “whitened”, so it is a very random bunch of 1s and 0s. We demodulate the FM and look for long strings of 1s and 0s, as well as alternating 1s and 0s, to estimate the Delta F1, Delta F2 and Minimum deviation results. If we can’t find valid data, then the tester simply returns a “-1”.
If you are seeing “-1” on all three channels, it’s likely the device is not advertising any reasonably normal data in the payload (or maybe NO data in the payload!); and therefore, we can’t perform the measurement.
If you see a “-1” on just one of the channels—that’s not uncommon and simply means the data does not contain patterns of 1s and 0s that allows us to perform the required measurement with high confidence.Your device is likely transmitting the same data on all channels and you may wonder why you only see “-1” on just channel. Although the payload may be the same, the data is “whitened” or scrambled before it is actually transmitted and we measure FM demodulation on the raw scrambled data. Each channel 37, 38, 39 use different scrambling pattern, so the actual transmitted data pattern we measure won’t be the same 1s and 0s on all channels.
Here is FM demodulation of a typical advertisement packet that contains real advertising data. Total length is about 0.4 ms and it contains preamble, BD Address, CRC and about 30 bytes of payload.