[time-nuts] Simple GPSDO Multiple Outputs - buffered line driver options?

[Bob kb8tq]

Hi

Cross talk between the reference in and the measured signal on the 53131 series can be an
issue . The counter drops resolution in the vicinity of 10 MHz as a result. There is also an  issue 
with  the sampling process rather than direct feedthrough. 

If you are measuring things like phase noise, having a lot of 10 MHz running around the lab will put a 
spur in the phase noise plot. It may be close enough in that you don’t notice it every time. The same 
sort of spur will play nasties with things like ADEV measurements. Getting a 10V RMS signal down 
120 db is tough …

Finally if you happen to be playing with radios, WWV is at 10 MHz. It does not take a lot of reference 
signal to get back into the typical receiving antenna.

======

If you are daisy chaining counters, there are several ways to do it:

1) Drive the “ext ref in” and daisy chain off of “ext ref out”. This way the signal is buffered at each device.
It may add a bit of noise, but you can go a long way doing this.

2) Put a coax Tee connector at each instrument. If the device is high Z in, this can do ok. If it is a 50 ohm
termination all the time … not so much. Even with the high Z input it’s better for short runs than long ones.

3) Mix the two approaches. If you have a variety of gear, use the ones with ref in / ref out as buffers. String
the other gear in-between those boxes. 

Bob

> On Jan 21, 2020, at 10:27 AM, Taka Kamiya via time-nuts <time-nuts at lists.febo.com> wrote:
> 
> I've tried daisy chaining 4 HP5335A.  By the time signal got to the 4th box, it was too weak to reliably drive the 1 pps.  I had a terminator at the last tee with short length of RG58s between boxes.  I guess some box puts relatively heavy load on the signal?
> 
> I'd like to know why 10V is a bad idea.  (besides too close to the upper limit)  Cross talk between what/where?  
> 
> To OP:I have video amplifiers Extron MDA-3V successfully used for this.  1 port in, 3 ports out ones by Extron are very inexpensive at 10 dollars+/-.  I never did formal testing but reading on all counters matched exactly.  They are 75 ohms but it didn't matter in my use case.  You can change/adjust internal resisters if you are concerned.  
> 
> --------------------------------------- 
> (Mr.) Taka Kamiya
> KB4EMF / ex JF2DKG
> 
> 
>    On Tuesday, January 21, 2020, 7:45:22 AM EST, Bob kb8tq <kb8tq at n1k.org> wrote:  
> 
> Hi
> 
> What are you driving? 
> 
> Most “normal” gear is pretty happy with a fairly wide range of input levels. Obviously things
> like termination and long lengths of coax can get into the act. For 4 outputs, a passive splitter
> with 6 db of loss should do just fine. You have only taken the output voltage down by 2:1 ….
> 
> Just for reference:
> 
> https://www.avionteq.com/Document/53131A-specification-sheet.pdf <https://www.avionteq.com/Document/53131A-specification-sheet.pdf>
> 
> Calls out a 200 mv to 10V RMS input level as acceptable For a variety of reasons, 10V RMS
> is a really bad idea (cross talk ….). Lower is better in this case.
> 
> Bob
> 
>> On Jan 21, 2020, at 4:19 AM, skipp isaham via time-nuts <time-nuts at lists.febo.com> wrote:
>> 
>> Hello once again to the Group,
>> 
>> May I ask what the current relatively simple options are for
>> expanding a Thunderbolt or equivalent... output for distribution
>> to multiple devices?
>> 
>> Although I expect only two or three isolated / buffered outputs
>> will be required in my example. I'm worried about signal level
>> if a passive system (Mini Circuits divider or equivalent type)
>> is used.
>> 
>> Would appreciate a few quick opinions on what is practical and
>> seems to work well.
>> 
>> thank you in advance
>> 
>> regards,
>> 
>> skipp
>> 
>> skipp025 at yahoo dot com
>> 
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