Z3801A Oscillator Comparison

LeapSecond.com :: Museum of Time & Frequency

Z3801A Quartz Oscillator Comparison

Topics: Z3801A GPS frequency standard, 10811A oscillator, stability variations.

Introduction

The long-term performance of a Symmetricom / HP Z3801A GPS Time & Frequency standard is primarily determined by the characteristics of the 6-channel Motorola Oncore VP GPS receiver, the quality of the antenna view, and the timing stability of the GPS constellation itself. The short-term performance, on the other hand, is primarily a function of the stability of the OCXO inside the Z3801A.

The Z3801A uses a variant of the HP 10811A SC-cut OCXO. The short-term stability specification for this oscillator appears to be 1E-11 at 1 second. All 13 of the oscillators below meet this spec. The specification for a similar product, the HP 58503A, is 5E-12 at 1 s and 10 s; 5E-11 at 100 s and 1000 s.

Although each of the Z3801A oscillators have the same part number (10811-60161), like people or fine wine, oscillators are all different in their actual performance characteristics, even if they all meet spec. When it comes to metrics such as stability, drift, tempco, and phase noise, there is a "bell curve" of performance among a set of production units. The worst ones never make it out of the factory (or perhaps relabeled for other less-demanding applications). But all the rest - below average, average, and above average - are used in products. Again, they will all meet spec if they leave the factory, but some beat spec by a lot more than others. And by a lot I'm not saying 20% better, or even twice as good - some units are 20x better than others!

I have been running a dozen HP Z3801A oscillators beginning September 2001 and here is a snapshot of their short-term performance. The plots below show Allan Deviation for tau from 0.1 s to 10,000 s. Stability at 100 s, for example, ranges from 3E-13 to 8E-12, with a typical value of low parts in 10 to the -12^th.