[time-nuts] HP 8568B

[Tommy phone]

Certainly the point of an external reference is to replace the internal reference with one more accurate, so when external is selected the internal is out of the picture. However it is my understanding that not all of the oscillators in the signal path are locked to either reference in the '66.

From Tom Holmes


> On Jun 28, 2014, at 10:34 PM, "Hui Zhang" <ba6it at 163.com> wrote:
> 
> Hi Tommy:
>     I used each span to do my test, they are also have frequency diffrence, the 42Hz is result of 1kHz span.  I also use marker counter function, sometime I use 'peak serach' function, it will gave me a peak value, very helpful. I am thinking about the reason you and Alex talked about 'internal LO's are locked to the 10 MHz reference', I thought the internal OCXO will not join work if a EXT REF in, am I wrong?
> Thanks you!
> 
> 
> Hui Zhang
> 
> 
> 
> 
> 
> 
> 
> 
> At 2014-06-29 10:24:22, "Tommy phone" <tholmes at woh.rr.com> wrote:
>> Hi Hui...
>> The first thought is that you are tied to the resolution of the span divided by the number of displayed points -1. Try a narrower span. If you look at the absolute accuracy spec that error may be within the '66's capability.
>> 
>> Also, I believe there is a marker counter function that will give you a more accurate reading.
>> Another consideration is that not all of the internal LO's are locked to the 10 MHz reference.
>> Don't panic yet :)
>> 
>> 
>> From Tom Holmes
>> 
>> 
>>> On Jun 28, 2014, at 9:48 PM, "Hui Zhang" <ba6it at 163.com> wrote:
>>> 
>>> Hello all:
>>>   Rencently I bought a second-hand HP8568B Spectrum Analyzer, it kind of vintage but works well. Only problem is its time base lost accurate. When I use it measured my HP Z3801A output (locked to GPS), the center frequency read is 9.999912MHz, about 88Hz diffrence. So I decide to use  my Z3801A for its external reference. When I connected the cable and set the reference switch in rear paneal to EXT, the CRT displayed "EXT REF", I believed it worked. But when I use it measure a nother GPSDO (Trimble ThunderBolt locked to GPS), it still have 42Hz diffrence. I am very sure both of my two GPSDO is good, if I use my HP53132A counter to compare them they will has less 1E-10 (1mHz) diffrence. And then I tie a BNC three-way connect from my Z3801A, one way to 8568B's EXT stand input, one way to the signal input in front panel, the  diffrence is still 42Hz - it's 10.000042MHz. I was fully confused, do I need a calibration after I used external 10Mhz stand? I read the manual again but not f
> o
>> u
>>> nd any infomation about how to do it. What do I do now? Anyone give me a suggestion? Any infomation will be appreciate.
>>> 
>>> 
>>> Hui Zhang
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