[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
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>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>
>>>
>>>
>>
>>
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>>
>>
>>
>
>
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