[time-nuts] IC for 1 PPS Output

[Brooke Clarke]

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
>>>
>>>
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>>>
>>>
>>>
>>>
>>>      
>>>
>>    
>>
>
>
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