Hello, John,
Your phase noise plot seems quite similar to that sent by Said some time
ago (4 Jan), that looks like a lot of spurs quite evenly separated. It
is likely from the price and p/n that your unit is one of the "newer"
ones with 60MHz xtal and the DDS inside the loop - and I would think
that the pn from the DDS would be removed by the loop. But since the
division from 60MHz is done by a CPLD, perhaps the signal is corrupted
by this CPLD, or perhaps it is corrupted by a switching regulator inside
the unit.
The 60MHz signal is available inside the unit, in a small coaxial
connector. Perhaps it would be interesting to also analyze it.
Regards,
Javier
El 04/02/2012 10:43, John Miles escribió:
From your comments, it sounds like you may be measuring one of the
earlier 5680A's that could be tuned over a large range with a DDS output.
These earlier ones had a 50.25 MHz internal osc which was locked after
multiplication to the rubidium frequency. Then there was a DDS on the
output that could be programmed over a wide frequency range.
That sounds plausible. I haven't taken the time to bring myself 100% up to
speed with the many different variations and options for these little boxes,
but I could swear I'm looking at a low-resolution, unfiltered, uncleaned-up
DDS.
The newer ones have a 60 MHz internal oscillator. This is multiplied up
to 6840 MHz and subtractively mixed with ~5.3125 MHz from a DDS to get
to the rubidium frequency. The feedback from the rubidium cell locks the
60 Hz which is divided by 6 for the 10 MHz output. No DDS on the output
generation. The DDS in the loop can digitally adjust the rubidium lock
frequency that tunes the 60 MHz, to fine tune the output only around 10
MHz.
Do you know which type of 5680A you are measuring?
This one is marked S/N 0339-65969, purchased from
http://www.ebay.com/itm/260930018124 . It seems to have the pinout
documented at http://vk2xv.djirra.com/tech_rubidium.htm for serial
0127-96634 (pin 1=V+, pin 2=GND, pin 3=+5V, pin 7=RF out.) However, VK2XV
claims that s/n 96634 was non-programmable. I haven't tried hooking up a
serial terminal to mine, but I did notice that there was some negative
voltage on one of the other pins (9?) that might correspond to an RS-232
signal level.
Can't complain too much at $40/each, anyway I believe the seller's claim
that these are new or nearly so, even though they look like they've been
removed from an installation. The internal foam insulation is usually
discolored from heat, while it looks great in the two units that I bought
from this seller.
-- john
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