> Peter,
>
> That depends. To use 1M Ohms input impedance, you need a 50 Ohms series
> impedance at the driver chip. Most sources such as the 58503A and Thunderbolt
> violate that requirement by having only a couple of Ohms output impedance,
> and are thus not suitable and do need the 50 Ohms termination at the scope
> least you get horrible ringing as shown in Tom's plots from yesterday.
>
> However that means you are pumping up to 100mA through your coax, and scope
> termination. That makes your coax ground jump many 10's of millivolts
> (depending on cable length and quality). This IR induced ground jump now also
> shows up on your 10MHz coax and messes with that signal, as the 1PPS return
> current partially goes through the 10MHz coax shield and generates a voltage
> rise on the shield. It's a cluster....
>
> You can take a multimeter and actually measure the voltage drop on your coax
> cable shield from one connector to the other. On units with longer 1PPS pulse
> you see the multimeter twitch once per second (Symmetricom XLI for example)
> even on a short 1m cable.
>
> But if you look at Tom's plots you see that there is some high frequency
> ringing on the 58503A 1PPS when terminated into 1M, I am not sure thats
> coming from cable reflections. For those high frequency rings a 1G scope may
> be better to see what's really going on in the driver.
>
> Think about it this way: why would you want to drive a 50 Ohms coax with a 5
> Ohms output impedance? That's an absolutely horrible impedance mismatch. But
> that is what the Trimble Thunderbolt does, and likely also the Resolution-T..
> Resulting in ringing up to 10V on your cable.
>
> Bye,
> Said
>
>
> Sent from my iPad
>
> On Sep 14, 2014, at 9:04, "Peter Reilley" <[email protected]> wrote:
>
>> I tried removing the termination and got a little better than 4 nS
>> risetime.
>>
>> Isn't the ringing frequency simply a function of the length of
>> the coax? Isn't it the price you pay for mismatched impedances?
>>
>> Pete.
>>
>>
>> -----Original Message-----
>> From: time-nuts [mailto:[email protected]] On Behalf Of Said
>> Jackson via time-nuts
>> Sent: Sunday, September 14, 2014 11:43 AM
>> To: Said Jackson; Discussion of precise time and frequency measurement
>> Subject: Re: [time-nuts] Correcting jitter on the 1 PPS
>> signalfromaGPSreceiver.
>>
>> Ok, did the math, a 4ns risetime should be ok on a 200MHz scope.
>>
>> You likely won't see the oscillations and reflections visible in Toms 58503A
>> plots for example, they are faster than the risetime.
>>
>> Bye,
>> Said
>>
>> Sent From iPhone
>>
>>> On Sep 14, 2014, at 8:35, Said Jackson via time-nuts <[email protected]>
>> wrote:
>>>
>>> Peter,
>>>
>>> You don't need nor do you want a 50 ohms end-termination on a
>> series-terminated 50 ohms coax cable.
>>>
>>> This has been discussed here extensively before, please check the
>> archives. Your last sentence is not correct.
>>>
>>> Also, you are running into your scope's BW limit if you are measuring a
>> 4ns risetime with a 200MHz scope..
>>>
>>> Bye,
>>> Said
>>>
>>> Sent From iPhone
>>>
>>>> On Sep 14, 2014, at 6:19, "Peter Reilley" <[email protected]> wrote:
>>>>
>>>> My rise time is about 4 nS. I am measuring that with my 200 MHz
>>>> scope. I am only using 50 Ohm termination, anything else is not
>>>> valid when using 50 Ohm coax.
>>>>
>>>> Pete.
>>>>
>>>>
>>>> -----Original Message-----
>>>> From: time-nuts [mailto:[email protected]] On Behalf Of Tom
>>>> Van Baak
>>>> Sent: Saturday, September 13, 2014 10:34 PM
>>>> To: Discussion of precise time and frequency measurement
>>>> Subject: Re: [time-nuts] Correcting jitter on the 1 PPS signal
>>>> fromaGPSreceiver.
>>>>
>>>>> The cables are not exactly the same lengths. Differences in length
>>>>> will result in a fixed offset. I am not concerned about such fixed
>>>>> errors, only jitter.
>>>>>
>>>>> I am comparing the rising edges which is what the spec defines as
>>>>> the reference edge.
>>>>>
>>>>> Pete.
>>>>
>>>> Pete,
>>>>
>>>> Correct, the survey position is determined only by the phase center
>>>> of the antenna, not by cable length. And cable length mismatches
>>>> should make no difference in your jitter measurements.
>>>>
>>>> But one thing to check is how sharp the 1PPS rising edge is -- right
>>>> at the input to your TI counter. I use a BNC tee with one leg open
>>>> allowing a 'scope check (set to 1M input). If your risetime is a
>>>> couple of ns like mine is, then all is well. Slow risetime can be a huge
>> source of timing jitter.
>>>> Check both 50R and 1M at the counter input. Use DC, not AC coupling.
>>>> Use fixed trigger, never auto-trigger. Pick a trigger level that
>>>> matches the maximum slope.
>>>>
>>>> Some examples of good/bad GPS 1PPS risetimes:
>>>> http://leapsecond.com/pages/gpsdo-rise/
>>>>
>>>> /tvb
>>>>
>>>>
>>>> _______________________________________________
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