[time-nuts] IC for 1 PPS Output

John Ackermann N8UR jra at febo.com
Fri Apr 7 13:05:40 EDT 2006


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
>>
>>
>> 
>>
> 
> 





More information about the time-nuts mailing list