[time-nuts] Rubidium Performance: DDS noise effect on 10 MHz

Javier Herrero jherrero at hvsistemas.es
Sat Feb 4 10:13:49 UTC 2012

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.



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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