[time-nuts] IC for 1 PPS Output

John Ackermann N8UR jra at febo.com
Fri Apr 7 15:41:15 EDT 2006


Just got home and can upload the scope shots.  Here they are.  These
were taken with a Tek 2465B (400MHz) scope, with an HP 4 foot test
cable, using the scope's internal 50 ohm termination.

John
----


Brooke Clarke said the following on 04/07/2006 03:21 PM:
> Hi John:
> 
> When I use 3 x 47 Ohm resistors and 3 gates the rise time seems about 
> the same, but the voltage into a 50 Ohm load increases from 2.5 V p-p to 
> 3.6 v p-p, so I tend to agree that's the way to go.
> 
> Have Fun,
> 
> Brooke
> 
> 
> John Ackermann N8UR wrote:
> 
> 
>>Hi Brooke --
>>
>>I meant to say that there was a separate 47 ohm resistor on each of the 
>>inverter outputs, with the far end of those resistors tied together.
>>
>>Last time this came up, there was an argument about why you wouldn't use 
>>resistor values that in parallel would equal 50 ohms (e.g., 150 ohm if 
>>three gates were used), but Tom Clark jumped in to explain the basis for 
>>using the lower values -- I think it was mainly to ensure that you still 
>> have a valid TTL level into 50 ohms, coupled with looking at the 
>>internal resistance of the gate, and minimizing the ringing of the edge.
>>
>>In any event the scheme I described has been used with good success in 
>>the TAC-2 for a  bunch of years.  When I get home tonight, I'll post 
>>photos of the risetime that I'm seeing.
>>
>>John
>>
>>Brooke Clarke wrote:
>> 
>>
>>
>>>Hi John:
>>>
>>>I think you mean by "-- paralleling several sections of a 74AC04 hex 
>>>inverter through 47 ohm resistors" directly connecting the outputs then 
>>>using a series 47 Ohm resistor.  But the post by Normand makes the good 
>>>point that by using a seperate resistor on each gate the loading on the 
>>>gate is reduced.  So I could use a single 74AC04 chip with 150 Ohm 
>>>resistors on each gate.  I'll try that when I'm finished reading the mail.
>>>
>>>Have Fun,
>>>
>>>Brooke
>>>
>>>John Ackermann N8UR wrote:
>>>
>>>
>>>   
>>>
>>>
>>>>Brooke Clarke wrote:
>>>>
>>>>
>>>>
>>>>     
>>>>
>>>>
>>>>>Hi:
>>>>>
>>>>>I've got the TVB 1 PPS PIC circuit working and would like to add an 
>>>>>output IC to drive 50 a couple of Ohm cables.
>>>>>Need two outputs, the normal 1 PPS and also the 10 kHz output since I 
>>>>>think it would work with the SRS app note for making 1,000 TI 
>>>>>measurements in one second to better see small offsets.
>>>>>
>>>>>I seem to remember that there may be an internal resistance already in 
>>>>>some chips so putting a 47 or 50 Ohm resistor in series may result in an 
>>>>>impedance above 50 Ohms.  Any thoughts?
>>>>>
>>>>>Thanks,
>>>>>
>>>>>Brooke Clarke
>>>>>
>>>>> 
>>>>>
>>>>>       
>>>>>
>>>>
>>>>Hi Brooke --
>>>>
>>>>The circuit I use is stolen from Tom Clark's TAC -- paralleling several 
>>>>sections of a 74AC04 hex inverter through 47 ohm resistors.  For the 
>>>>current TADD projects, I'm using three sections that way, so each chip 
>>>>gives me two outputs, and I use a 47 ohm SIP resistor pack to keep the 
>>>>board space down.
>>>>
>>>>Tom came up with the 47 ohm value (though IIRC he used four sections in 
>>>>parallel) based on compromising for maximum voltage into a 50 ohm 
>>>>termination, and best rising edge.
>>>>
>>>>The 74AC chips don't have internal source resistors, but I think some of 
>>>>the op amp distribution amp chips might.  For square waves at these 
>>>>frequencies, I'd use 74AC parts over an analog circuit.
>>>>
>>>>John
>>>>
>>>>
>>>>_______________________________________________
>>>>time-nuts mailing list
>>>>time-nuts at febo.com
>>>>https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
>>>>
>>>>
>>>>
>>>>
>>>>     
>>>>
>>>
>>>   
>>>
>>
>>
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>>
>> 
>>
> 
> 

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