[time-nuts] HP53132A's LSD question

[Hui Zhang]

Dear group:

  I found a strange thing about HP53132A’s output digit, I know the HP53132A is a universal counter that have 12 digits per second performance by read the manual. By use 10s gate time, I thought it will be 13 digits resolution. But I found it can output more digits reads by use GPIB or RS-232 way to collect datas. (This is simple way rather than use offset method). I use RS-232 cable to connected to my counter to observe the frequency of a OCXO, I got these output result ( a part of long sequence):

9.999,967,286,098,0  MHz
9.999,981,814,726,9  MHz
9.999,991,573,702,0  MHz
9.999,997,482,253,0  MHz
9.999,999,945,685  MHz
10.000,000,472,030,4 MHz
10.000,000,460,991,3 MHz
10.000,000,352,570,9 MHz
10.000,000,254,108  MHz
10.000,000,183,759  MHz
10.000,000,136,842  MHz
..
. (Omitted few lines)
.
10.000,000,039,724  MHz
10.000,000,006,299  MHz
9.999,999,992,692  MHz
9.999,999,990,674  MHz
9.999,999,989,094  MHz
9.999,999,995,662  MHz
9.999,999,993,829  MHz

The question is in first four lines the reads is 14 digits, but in line 5, the digits reduce to 13 digits, and then, it’s became 15 digits in line 6(because over 10Mhz). After 3 lines, the reads reduce one digits again to became 14 digits(also over 10Mhz),  again after that few lines the frequency less that 10Mhz, it return to 13 digits like last few line. My question is what determines the HP53132A’s resolution? Why the output is variable digits even the had very close frequency? Any comment will be appreciate.

 

Hui Zhang