[time-nuts] IC for 1 PPS Output

[Brooke Clarke]

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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