Trimble Thunderbolt Power Supply Noise

LeapSecond.com

28-May-2008

Introduction

It would make sense if the quality of the power supply has some effect on the frequency stability or phase noise of a GPSDO. I suspect the dominant effect is phase noise, even if DC input power is filtered or regulated inside the box.

The phase noise of a typical Thunderbolt was measured using each one of six different power supplies. All the plots below are 10 minute runs and displayed in the same scale. The 10 MHz reference was a Z3801A.

ATX PC power supply

A Trimble Thunderbolt requires three different voltages: +5, +12, and -12 volts; a set of voltages available inside almost any PC. So the first test is to see how well a Thunderbolt works when powered with the simplest, cheapest 5/12 volt PC power supply. The answer is as you might expect -- these computer-grade power supplies work fine, but add a lot of noise to the RF output. This is probably not an issue for GPS timing but there may be some GPSDO applications where this level of noise is unacceptable.

Open frame switching power supply (Mean Well)

The next test is to use a higher-grade open frame 5/12 volt switching power supply. The results are somewhat better, although even more spurs show up at lower frequencies. The spur near 1 Hz is surprising and rather unwelcome.

Compact switching supply (AULT)

Here is a small enclosed switching power supply. It's quite a bit cleaner than the above two, although some switching noise appears in the kHz region. Since this power supply was available very cheap, and in large quantity, and works with 100-240 VAC input, it was chosen for the Thunderbolt bundle.

Laboratory supply (Acopian goldbox)

I thought this excellent high-power laboratory supply would give a perfectly clean spectrum. It's much cleaner at high frequencies than the above three, but some line frequency noise still shows up in the Thunderbolt output (it was powering 4 Thunderbolts in parallel at the time).

Benchtop supply (Elentech)

This low-end adjustable bench supply was better than I expected. Less noise than the Acopian.

Laboratory linear supply (HP 6235A)

The Thunderbolt draws more current on warm-up than this little triple supply is rated for, but after the current draw drops, both the Thunderbolt and power supply are fine. This power supply turned out to be the best. Don't you just love HP?

Conclusion:

The quality of the supply does indeed make a difference in the phase noise of 10 MHz RF output. Use of PC power supplies is clearly a bad idea, at least if phase noise is a concern. It may or may not have any impact on frequency stability.

The choice of the enclosed switching power supply seems a reasonable compromise in price and performance for the TAPR group buy. However, for extremely low noise or frequency multiplication applications, you might want to see if a better power supply is necessary.

As expected mains frequency (50/60 Hz) and its harmonics appear in the output of the linear supplies. Apparently HP did the best job filtering all of this.

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