[time-nuts] Z3805A/58503A time receiver : 2 questions

[Claude Fender]

Hello,

This is my first message here although I read this list for a few weeks.

 I bought a Z3805A/58503A frequency receiver  and I didn't notice on the pictures that the model number was not  written on the front panel. It's not important but is there a reason for that, if you know ?
The item : http://cgi.ebay.fr/ws/eBayISAPI.dll?ViewItem&item=251226027893 


I log some parameters of the receiver and today I notice a "gap" in the frequency.

Picture : http://uppix.net/e/2/4/cf39ac772e5a69fc616f5cf30f208.png

I have a Rubidium and I measure it's frequency with a 5334B counter locked to the GPS, and I can see the "gap" too :
http://uppix.net/f/b/a/d86c2737ad135264e4a2b78e503d5.png
measurements are with Gate Time of 10s, 60s and 100s, 


Do you know why this happens and how to prevent this behaviour ?

Thanks for yours advices 


Claude