In this blog post I look at the T900A bullet style super tweeter from Fostex. This "Laboratory Series" is made in Japan unlike lesser models. The tweeter uses an alnico magnet along with a magnesium ring radiator diaphragm. Other than that, there's very little information online for this particular model.
The front of the tweeter is made from solid brass which has been lathe turned and polished.
Disassembling the tweeter reveals clock work precision internals.
Measurements
I began by measuring the impedance sweep. The Fs is actually quite low at 1885Hz with a second peak at 5685Hz.
Frequency Response
The response shows a large peak centered around the driver's Fs however this is below the usable frequency range of the driver. The response is flat from 7kHz-10kHz with a peculiar +4dB HF shelf. Response is then flat again until 21kHz where we see a mild breakup mode in the diaphragm. (click to enlarge below)
If we compare this against published we do not see the HF shelf at 10kHz. We also do not see extension to 30kHz with my test data.
The off-axis at 0,15,30, and 45 degrees off-axis shows a considerably wide coverage centered around 7kHz but quickly narrows above 11kHz. By 15kHz the listening window has narrowed to only 60 degrees.
Burst decay shows a mild resonance at 21kHz, otherwise we are trouble free.
The CSD plot shows the typical stored energy at Fs (4.8kHz). The tweeter remains clean above this with a very fast decay time across it's usable bandwidth.
Distortion
Looking at harmonic for the 85dB 1m test signal, we see H2 dominate however still moderate in level at only 0.42% at 10kHz. H3 and H4 remains well behaved. 
The 95dB test signal shows H2 rise in a linear fashion as expected across the frequency range. There is nothing too concerning here.
Intermodulation Distortion
The below test was done with a 6th order high pass filter at 8kHz. This was done within the Hypex FA123 filter software.
Using a 12 band per octave test signal we see a subtle rise in IMD at 10kHz dropping to very low levels in the 15kHz region. Distortion is lower than many amplifiers I've measured in this region, so pair carefully!
Raising the the test signal from 85dB to 95dB 1m we see IMD rise in a linear fashion with 10kHz showing 55dB dynamic range improving to 74dB at 15kHz.
General Considerations About Bullet Style Super Tweeters
The Fostex T900A is an attractive solution for covering upper treble. Care should be taken to high pass filter with the correct frequency and slope. This will help keep intermodulation distortion at bay in the audible band. The 85dB and 95dB test signal is likely higher than normal use case listening levels, so the projected distortion represented in my test data is likely much higher than in real life use. Considering this, amplifier choice becomes paramount in order to ensure low distortion performance, otherwise the tweeter will likely take on a somewhat harsh sound character. It is suggested to use a low gain amplifier since the tweeter will only require around 0.1v in order to provide optimal listening levels. If you are using a high gain amplifier, you should attenuate the tweeter using an L-Pad circuit or Autoformer. This ensures you are out of the noise floor of the amplifier.
Conclusion
The Fostex T900A is a well executed and beautiful tweeter with very good objective test data, particularly in the time domain and intermodulation distortion results for the upper treble region.