[time-nuts] What's best an HP 58503A, Z3805A, or Z3801A upgraded to 58503A?

Bob Camp kb8tq at n1k.org
Sun Nov 30 16:13:55 EST 2014

> On Nov 30, 2014, at 12:14 PM, Dr. David Kirkby (Kirkby Microwave Ltd) <drkirkby at kirkbymicrowave.co.uk> wrote:
> On 30 Nov 2014 16:17, "Bob Camp" <kb8tq at n1k.org> wrote:
>>> 1) Z3805A http://www.ebay.com/itm/251527236609
>>> * 16 channels
>>> * double oven
>>> * dual outputs
>>> * $499 + free carriage
>>> 2) Z3805A http://www.ebay.com/itm/271324714268
>>> * 8-Channels GPS Receive
>>> * 6-Channel GPS Parallel tracking
>>> * double oven
>>> * dual outputs.
>>> * $399+ carriage, so probably the same cost as #1 above
>>> Why do #1 and #2 have the same part numbers, but a different number of
>>> channels??? One has 16, the other 6/8.
>> There are multiple versions of the 3805. There are two GPS modules in
> them and two OCXO’s. You mostly see them in pairs (260 OCXO with Furuno GPS
> / 10811 with Motorola GPS).
> So what GPS module is the best?  Furuno or Motorola? Does the number of
> channels indicate the module?

Both work fine. The sawtooth correction on the Motorola is slightly coarse. It’s the module the software was optimized for. The two issues pretty much cancel each other out. Neither module is as sensitive as a modern GPS. Neither one has WAAS correction. Neither one does anything but GPS. Neither one has > 50 correlators like a modern chip set. None of them (modern or the older ones) work as well with a poor antenna location as with a good location. 

Assuming the listing is accurate (many are not) the channels should indicate which module the unit has in it. Often the listings are cut and paste so they reflect only the model not the guts. 

> You have given me half the information I was
> looking for,  but not really supplied enough for me to make a decision.

To be very clear about this:

By far the biggest and most important issue in buying any of them is the actual condition of the particular unit you get. Without careful inspection (like under magnification) of the unit, it’s tough to know how much it’s been abused. I have examples of each that appear to have spent some time in a fish pond. I have some that quite obviously were dropped on the connectors. I have some that appear to be brand new. All of them came from sellers with very good reputations. 

The next biggest issue is the actual performance of the specific OCXO in the unit you get, after burn in for a few months. In terms of ADEV they vary unit to unit far more than any difference between the designs you are looking at. None of them I’ve seen are awful, but some are indeed very good. HP published several papers on the variation in the 10811, there are a number of sites on the web with plots documenting the variation. The 260 is going to be the same sort of thing. 

None of these GPSDO’s  were designed as a frequency standard, so what you are most likely looking at them for was a second order consideration in the design. The variation from unit to unit comes in part from that. The designers targeted a specific CDMA cell phone spec. The main feature of that spec (holdover performance) is of little or no interest to TimeNuts / Hams / Lab rats. The fact that these units are useful at all is as much by accident as by design.  

> I will try to get them to give me a photo of the actual item.

If you can get detailed interior and exterior photos of the one they will actually ship you, that might be of some use. It will at least rule out physical damage or major corrosion from flood…


> Dave
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