[time-nuts] How to measure Allan Deviation?

[Dr Bruce Griffiths]

Didier Juges wrote:
> Dr Bruce Griffiths wrote:
>   
>> Didier Juges wrote:
>>   
>>     
>>> Dr Bruce Griffiths wrote:
>>>   
>>>     
>>>       
>>>> Didier Juges wrote:
>>>>   
>>>>     
>>>>       
>>>>         
>>>>> Dr Bruce Griffiths wrote:
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>>>>>         
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>>>>>> In principle this measurement could be made with a time interval counter:
>>>>>> PPS -> START
>>>>>> delayed 10KHz -> STOP
>>>>>>
>>>>>> Vary the delay and watch the jitter jump when the leading edge of the 
>>>>>> PPS signal occurs during the 10KHz burst which was phase coherent with 
>>>>>> the previous PPS pulse.
>>>>>> The only problem is finding a suitable variable delay device with 
>>>>>> sufficiently low (<=1ns??) jitter.
>>>>>>
>>>>>>   
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>>>>>>             
>>>>> The delay device can be triggered by the 1 PPS, then will drive the ARM 
>>>>> input of the counter, so as long as the delay device's jitter  is less 
>>>>> than the 10 kHz period, if we adjust the delay to 0.99985 second 
>>>>> (between the last 2 periods of 10 kHz before the 1 PPS), then the TI 
>>>>> counter will START on the last 10 kHz pulse before the pps, and STOP on 
>>>>> the 1 PPS.
>>>>> Tek has some time delay generators in the TM-500 and 7000 plug-in 
>>>>> series. I knew one day I would need one of those, I now know why :-)
>>>>>
>>>>> Didier
>>>>>
>>>>>
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>>>>>   
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>>>> Since the HP5370 arm input is high impedance(1 Megohm) and its input 
>>>> range lies betwen -2V and +2V its desirable to drive it from a back 
>>>> terminated source with an amplitude of 2V or less to avoid overdriving 
>>>> (the comparators are better behaved if the input signal remains within 
>>>> the specified range) the input and ensure that the pulse at the ARM is 
>>>> relatively clean and reflection free. A suitable buffer can be built 
>>>> using 3 74AC04 inverters as depicted below: The 74AC04 inverters load 
>>>> currents are well within the specified limits even if the buffer output 
>>>> is shorted to ground. Thus the driver should have a long service life.
>>>> ARM driver
>>>>
>>>>
>>>> Bruce
>>>> ________________
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>>> The trigger levels on the 5370 are strange. The normal inputs have 
>>> trigger levels that can be adjusted between -1.5 and +0.6 V or something 
>>> like that.
>>> I am not sure where that is coming from.
>>> For the ARM input, if someone is going to drive it with long coax 
>>> cables, the best would be a 50 ohm termination right at the instrument's 
>>> input.
>>> The schematic did not make it, but I believe I understand what you mean: 
>>> 3 gates with common input, each output has a 150 ohm series resistor and 
>>> the outputs (far end of the resistors) are tied together, to provide a 
>>> good 50 ohm drive to the cable. A series capacitor would probably be 
>>> recommended to center the signal around ground (assuming it's 50% duty 
>>> cycle).
>>>
>>> Didier
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>>>   
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>> Didier
>>
>> The trigger level limits are derived from the input amplifier and 
>> trigger circuit characteristics for the START and STOP inputs.
>>
>> The ARM input feeds directly (via a 1X FET buffer) to an  ECL comparator 
>> ((MC1651).
>> This comparator has an NPN longtailed pair input stage the 
>> characteristics of which determines the usable input signal range.
>> Driving an amplifier or comparator input stage into saturation degrades 
>> its timing characteristics somewhat.
>>
>> Bruce
>>
>>   
>>     
> I understand that the levels are what they are due to the use of ECL 
> logic, I was wondering from a user's perspective, was the 5370 intended 
> to be used only with ECL logic circuits? I am surprised that they did 
> not design a trigger range that included at least the "old" normal TTL 
> levels 0 to 5V. TTL was very common when the 5370 was designed (it uses 
> a lot of it internally, not so much in the timing circuits though)
>
> Didier
>
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>   
The 5370 STOP and ARM inputs can cope with 5V TTL/CMOS levels if used 
with the 10X input attenuators.

The high input impedance HP5363 time interval probes allow a much wider 
-10V to +10V input range.

The ARM input has no built in attenuator, however you can use an 
external attenuator with it.


Neither CMOS nor TTL were intended to drive 50 ohm loads so a high 
impedance probe was usually required.
It is possible to use resistors designed into the circuit to create very 
wide bandwidth 10:1 or 20: probes when driving a 50 ohm instrument.

Bruce