[time-nuts] HP53132A's LSD question
ba6it at 163.com
Sat Aug 31 20:52:14 EDT 2013
Thanks TVB and Magnus's comment, I think I should ignore that last digit value, it's useless in result. Yes the oscillator in my test is very poor, beacuse it's a OCXO warm-up procedure, the counter's output digit was not regular, I don't know when it add or reduce the digit, but I found in a stablility frequency very close 10Mhz, it not happened. So I powered on a OCXO and observe the output digit in a large range of frequency change.
At 2013-09-01 01:49:49,"Tom Van Baak" <tvb at LeapSecond.com> wrote:
>Hi Hui Zhang,
>Yes, I agree it looks strange, but this is normal and correct for the 53132; or modern frequency counters like it. When it drops a decimal place on the right it is doing you a favor because that last digit is known to be noise rather than a real digit of measurement. HP/Agilent has always been concerned with accuracy and truth.
>In frequency mode, these counters use oversampling algorithms to report the *best fit* frequency. It works well until the frequency gets very, very close an exact fractional/multiple of 10 MHz. Then the algorithms no longer improve phase resolution and the instrument becomes more like a conventional 10 or 11 digit/second frequency counter. The output is still accurate, but the precision is reduced accordingly. Again, this is normal. If you've ever designed a high-resolution frequency counter you understand why.
>Amazingly (and to their credit), the 53132 counter firmware detects this sub-standard condition and removes digits from the output. It does this because those digits are recognized to be meaningless. You will also notice in statistics mode it replaces low order digit(s) with a '*' character. Some engineer at HP did a lot of work to make sure the instrument did not report more resolution than was valid.
>There is a footnote in the 53131/53132 manual that explains this:
>In your case, your oscillator is quite poor. Your oscillator varies by hundreds of microHertz each measurement. The ADEV is about 1e-10. In this case, it is not a counter problem; it is instability in your oscillator (or reference). The good news is that for this type of oscillator, your 53132 counter performs well and you do not need a more expensive time/frequency comparison instrument.
>If you need more information, let me know.
>----- Original Message -----
>From: "Hui Zhang" <ba6it at 163.com>
>To: "Discussion of precise time and frequency measurement" <time-nuts at febo.com>
>Sent: Saturday, August 31, 2013 8:06 AM
>Subject: [time-nuts] HP53132A's LSD question
>> 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
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