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

[John Miles]

>  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