[time-nuts] 50 ohm drivers
Bruce Griffiths
bruce.griffiths at xtra.co.nz
Sat Mar 3 21:47:33 EST 2018
These devices are only suitable for driving source terminated 50 ohm transmission lines not a 50 ohm load to ground (or even 1/2Vcc) to produce CMOS levels at the load.
If you are driving a low pass filter or similar intending to produce a sinewave output then its somewhat easier.
Even paralleling CMOS outputs won't produce quite a full CMOS swing across a 50 ohm load.
The classical solution was to either double the swing and use both source and load termination or use the Thevenin equivalent using a switched current source.
If AC coupling were allowed push pull drive of a 1:1 RF transformer from a pair of complementary 25 ohm Zout CMOS drivers would produce a full amplitude swing across a 50 ohm load, however some dc biasing would be required at the load to achieve CMOS levels.
Bruce
>
> On 04 March 2018 at 15:17 "David C. Partridge" <david.partridge at perdrix.co.uk> wrote:
>
> Brice said:
>
> > >
> > . Some fast CMOS devices (esp clock drivers) have an output R close to 50
> > ohms as they are intended to drive 50 ohm source terminated transmission
> > lines.
> >
> > >
> Any in particular that you'd recommend? I need to drive a 50ohm line and a
> single gate inverter doesn't have the grunt to do so ...
>
> Thanks
> David
>
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