[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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