FUTABA FASST - What is going on in the air ?

Fig Futaba FASST spectrum as seen on an HP141T with the storage display turned on.

As an radioamateur with interest in microwaves I had to make some investigation of the Futaba FASST system – I did choose FASST just because i already owned a Futaba T9CP with a 35 MHz module, the easiest way of getting the better realiability of 2,4 GHz system was choosing the Futaba plug in system with a TM7 transmitter module.

It is a FHSS and DSSS system but I have not been able to find any futher data on how the radiolink works.

Some measurements has been done by using an Analog Devices demodulating logarithmic amplifier (power detector) ADL5513 and PC soundcard-scope (by C Zeitnitz)

The transmitter is transmitting with 1.75 ms long pulses and it is 8 ms between each TX-pulse. (the frequency is different for each TX-pulse)

Fig. TX pulse 1.75ms duration, 8ms between each TX-pulse.

The frequency hopping ?

It seems that Futaba is using 36 frequencies (in general mode, i have not tested the mode for France) from 2404.5 MHz to 2476.5 MHz.

How to detect the frequency hopping ? – as I dont have access to the latest Rohde & Schwarz realtime spectrum analyzer FSVR – which anyway do not cover the entire 2.4 GHz ISM band with its 40 MHz realtime BW, this has to be done in an other way.

I retuned an old interdigital filter (built for some ATV (Amateur Telvision) tests that I have done some years ago) to acchieve an amplitude slope over the band of interest so that the frequency hopping will be amplitude modulated and thus can be detected with the logaritmic amplifier.

Fig. Filter performance with 3 dB attenuator at input and output, the slope is approx 40 dB over the band of interest.

Fig. Retuned interdigital filter and Analog Devices ADL5513 logarithmic detector.

On the PC-scope it is now possible to see the frequency hopping behaviour. The sequence is changing between every time the transmitter is turned on., see the figures below. It is somewhat difficult to see the length of the sequence but it looks like 288 ms (which makes sense as 8ms x 36 frequencies = 288ms). How many different hopping sequencies ? I have no idea !

Fig TM7 frequency hopping - different hopping pattern for every restart of the module.

Modified 2014-07-26 SM6PGP-Hannes ( s m 6 p g p ( a t ) i l l i p e  . s e )