[time-nuts] HP 8568B

[Alexander Pummer]

On 6/28/2014 6:48 PM, Hui Zhang 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 fou
>   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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it could happen that thes ystem reports the external reference but the 
time base reference oscillator doe not lock to it.  To check that use 
the same source fo the reference and also for he input and set the span 
small few kHz  the resolution  1/10 of the span and if you still see the 
freq error than the spectrum analyzer internal time base 10MHz need to 
readjusted since it cannot lock to the external reference, that is 
normal aging that instrument may be 25 years old
73
KJ6UHN
Alex