[time-nuts] 50 ohm Driver
Graham / KE9H
ke9h.graham at gmail.com
Wed Mar 4 13:16:02 EST 2015
Building on top of John's comments, if you are using a logic gate, look at
maximum output (pull up) current per pin, set the series resistor so that
current is not exceeded into a short, then also see if there is a maximum
current draw for all gates combined, or some power input pin, and do not
You can also look at switching the termination resistor from a simple 50 Ohm
resistor to ground, to a "Thevenin" load, which is 100 Ohms from +V to the
and another 100 Ohm resistor from the load point to ground. This way you
still have a 50 Ohm termination, but only draw one half the DC current.
In the event of no input, the receiver voltage will go to half scale. Make
sure your system will not misbehave when this happens.
Alternate driver is to use a video line driver with sufficient bandwidth.
--- Graham / KE9H
On Wed, Mar 4, 2015 at 7:54 AM, John Ackermann N8UR <jra at febo.com> wrote:
> One comment on the parallel AC gate approach. It may not be directly
> applicable to Martyn's issue, but there is a common confusion about the
> value of the summing resistors.
> Per Tom Clark, who came up with the idea, they are *not* intended to
> provide a near-end line termination to 50 ohms, but are simply there to
> protect the paralleled devices if the gates have slightly different delays
> (in which case one gate could end up sinking the other two).
> So, the commonly used 47 ohm value isn't magic. You can use a lower
> value, and thus get more voltage at the far end. I haven't experimented to
> see how far you can take that idea before destroying gates.
> On 3/4/2015 5:26 AM, Martyn Smith wrote:
>> A quick question.
>> My output driver for a simple amplifier.
>> I use three gates (in parallel with resistors) from a 74AC14 to give me
>> about 0-3.2V into 50 ohm.
>> I want to have a driver that gives me a full 0-5V (at least 0-4.5V) swing
>> into 50 ohms.
>> Can anyone recommend an IC that can delivery this.
>> But it needs to have similar jitter performance to the 74AC14.
>> I use it up to 10 MHz.
>> Best Regards
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