[time-nuts] Switching 1 pps signal

Sun Jul 7 16:07:07 UTC 2019

Hi

As far as the coax goes, termination is a good idea. As far as the signal goes … maybe not so much ….

Using 5V logic as an example:

If you build up a driver that has a 50 ohm source impedance, you have a (so far) do-able project. The 
output delivered into a light load could be the same as the driving voltage. Simply put, a 5V driver will
deliver a 5V signal. 

If the driver is built just to drive a 50 ohm load, you can still get 5V into the load off of a 5V driver. The
output impedance will very much *not* be 50 ohms in this case. 

The textbook case is to have both ends of the cable properly terminated. You now have a 50 ohm driver
and a 50 ohm load. Your 5V signal at the driver end is a 2.5V signal at the load. If you are after 5V 
at the load, you need a 10V driver. 

Logic gets picky about levels. If it’s 5V logic, it may or may not be very happy with a 2.5V “logic high” 
signal. The edge rate will (probably) max out around 1.25V which again may or may not be what you
are after. 

The next issue is that with a 10V drive, an un-terminated device gets 10V. There is a lot of gear out 
there that is in the “damage” range at this sort of voltage. Indeed, you can get into these issues on 
gear that was new back in the 1970’s.  Indeed there is gear that 5V will nuke ….

So what to do …..

The most practical approach is to source terminate only. As mentioned in other posts, this may not
be ideal for isolation. 

Another practical solution is to simply treat these as logic signals and use gates to switch them. If
your sources are things like GPS modules, Cs standards, or GPSDO’s, that is usually an acceptable 
approach. If you are running a Hydrogen Maser …. maybe not. 

The “why” depends a bit on how much the “couple of picoseconds” wobble in the high speed gate 
bothers you. A lot of devices are noisy enough that their noise will be way more than that . Indeed 
logic families vary in terms of jitter so you need to be a bit careful about what you use for what. 

Bob

> On Jul 7, 2019, at 10:59 AM, Taka Kamiya via time-nuts <time-nuts at lists.febo.com> wrote:
> 
> Being a Ham and experienced 10base-2, I'm rather careful with termination but thanks for the reminder.  I've decided to use coax relay 18GHz that I have a lot of, and purchased cheaply.  That way, there is no question.  
> 
> --------------------------------------- 
> (Mr.) Taka Kamiya
> KB4EMF / ex JF2DKG
> 
> 
>    On Sunday, July 7, 2019, 4:01:32 AM EDT, Glen English VK1XX <glenlist at pacificmedia.com.au> wrote:  
> 
> In that case, my suggestion is think of them all as transmission lines 
> in coax. terminate as needed.
> 
> Others here will also have points I have missed I am sure, or 
> alternative equally valid suggestions.
> 
> beign critical (maybe I am being over critical) :
> 
> watch out also for reflections from unterminated (open ) relay contacts, 
> the 2x pulse you might get back at the driver might cause harm or 
> trouble on the supply rail.
> 
> IE so source terminate ideally, even if it costs you a little swing or 
> at least a little series R like 10 ohms  source R to mop up reflections 
> back on the source... if its swinging 2V with source term = 10 ohms into 
> 51 ohms, that wont hurt too much.
> 
> rather than just totem pole strong CMOS driver , consider LVPECL style 
> driving, also.
> 
> IE it depends what drivers and receivers you choose.... simple would be 
> all 1.8V CMOS I guess, most stronger buffers will drive 50 ohms and 1.8V
> 
> if you plan on multiple drop, then you'll need to consider where you 
> place the termination...  which is why differential rocks over single 
> ended because the swing can drop before the trouble develops with double 
> termination.  of course if you AC couple into single ended setups that 
> can work but messy.
> 
> 
> glen
> 
> 
> On 7/07/2019 2:07 PM, Taka Kamiya via time-nuts wrote:
>> I should have mentioned this clearly earlier, but all converting and switching, then measuring happens in ONE 2U rack case.  Length of each cable is minimal.  I thought mention of TICC would make it sort of clear but it didn't.
>> I have LOTS of coax relays.  I'll use them.  It's a gross overkill but I've seen 80s HP equipment have overly generous parts selections, too.  It's hard to explain EXACTLY as the contraption isn't built yet.  That made it necessary to use generic terms.  I apologize for causing confusion.
>> 
>> ---------------------------------------
>> (Mr.) Taka Kamiya
>> KB4EMF / ex JF2DKG
>>   
>> 
>>       On Sunday, July 7, 2019, 12:00:52 AM EDT, Glen English VK1XX <glenlist at pacificmedia.com.au> wrote:
>>   
>>   Hal, Good point.
>> 
>> and I have never seen a spec for phase stability for Cat-7 cable !
>> 
>> for RG58, OR OTHER polyethylene, might be up to 150ppm /deg C. maybe as
>> good as 10 ppm/deg C for some LMR.
>> 
>> With 1000 feet of cable might be an issue for fine stuff. a few nano
>> seconds each way over temperature might not matter for his project...
>> 
>> I forget it is not a microwave  RF  VNA application. Easy to resolve
>> anyway with a difference balance term somewhere.
>> 
>> g
>> 
> 
> 
> 
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