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Design >> RF Design >> Power delivered at the load if the load is 50ohm//100Mohm
https://designers-guide.org/forum/YaBB.pl?num=1398352725

Message started by rf_man on Apr 24^th, 2014, 8:18am

Title: Power delivered at the load if the load is 50ohm//100Mohm
Post by rf_man on Apr 24^th, 2014, 8:18am

Hi Gurus,

I am confused at the power delivered to load. When the load is 50ohm//100Mohm, equivalently the load is 50ohm, then in my opinion with Psource = 20dBm (6.3Vpp), voltage swing at the load should be 6.3Vpp as well. But in Spectre simulation, the voltage swing at the load is 12.6Vpp, that is exactly the power level at the load when the load is an open(100M). But the load is actually a 50ohm//100Mohm? Why the load can't be seen as 50ohm?

Kind regards,

RF design beginner

Title: Re: Power delivered at the load if the load is 50ohm//100Mohm
Post by aaron_do on Apr 24^th, 2014, 6:16pm

Hi,

if you set up the simulation properly, there's no reason 50||100M ohm will not look like 50 ohm...

regards,
Aaron

Title: Re: Power delivered at the load if the load is 50ohm//100Mohm
Post by RFICDUDE on Apr 24^th, 2014, 7:38pm

The reason this happens is because the source doesn't know that the load it is driving is 100M ohms.

The "power" parameter for the power source is the "available power" that the source will deliver to a load of the same resistance as the source. A very simple model of this is a voltage source in series with a resistor of 50 ohms where the voltage of the source is exactly twice the voltage that you would measure across a 50 ohm load with specified source power delivered to the load. If the load resistance is exactly the source resistance then half of the source voltage is dropped across the source resistance and half across the load and the power delivered to the load will be equal to the power setting on the source.

In your case the load is 100Meg ohm so you see all the source voltage across the load and the power delivered to 100Meg ohm load is much less than the power the source is set to.

Hope this helps.

Title: Re: Power delivered at the load if the load is 50ohm//100Mohm
Post by aaron_do on Apr 24^th, 2014, 10:06pm

Hi RFICDUDE,

he actually said the load was 50||100M ohm so I don't think the 100Mohm resistor would matter...

Aaron