[time-nuts] Motorola Oncore GPS Interface Board

Dr Bruce Griffiths bruce.griffiths at xtra.co.nz
Tue Nov 28 03:52:40 UTC 2006


Jason Rabel wrote:
>> Jason,
>>
>> The only problem with that part is that it's not a buffer! It's a 
>> switch. It has no drive capability at all.
>>
>> There are useful buffer chips for this task, but their names escape me 
>> just now. Perhaps the 74FCT245 or equivalent would work.
>>
>> Driving a full 5V TTL signal into a 50 ohm load is another matter - you 
>> need a lot of DC power (1/2 watt) to do that. I was just using a Liner 
>> Tech LT1010 for a similar job - it would get the job done, but folks 
>> here might complain about the phase noise.
>>     
>
> You're right... I guess I had a brain fart, I don't know where my mind was
> at that moment... ;)
>
> Anyhow, besides using a buffer chip (like the several mentioned in other
> posts), how would using a plain old voltage following op-amp compare? I
> haven't looked up any specs yet, I'm just throwing the idea out there.
>
>
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>   
Unless you use a wideband opamp the transition times will be inferior to 
those achieved with a relatively inexpensive logic gate such as a 
74AC540 or equivalent.
Depending on the opamp output stage you may also need a negative supply 
to ensure that the opamp output will actually reach zero volts.
To ensure stability it may be necessary to use a small resistor in 
series with the opamp output, in which case you may as well use this 
resistor to match the line impedance.

Bruce




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