producing rippled voltage supply

[taleebe]

I'd like my DC voltage supply to have an AC ripple on it so that i may test PSRR of some DUT.I have this elaborate circuit built and it indeed produces such a supply upto 20MHz.first i built the circuit with a 100pf ceramic capacitor in place to block the signal generator from DC and was expecting some attenuation due to the resulting RC filter but as ive discovered the circuit produces a rippled supply even without a capacitor,However the AC ripple produced is attenuated and the attenuation factor increases with increasing frequency.Im comfortable with attenuation as i can just adjust amplitue using my signal generator but i am worried about the attenuation increasing with frequency. If i have a 200mv ripple i'd like it to stay at 200mv even when i change the frequency!Someone has suggested i buy a better scope but im not sure if thats it.
As a side note my DUT sinks up to 1A of current and i;m doing PSRR tests upto 20MHZ so conventional ways of producing a rippled supply(op amp,transformer in series with source,power FET) are out the door[/img]

 

[IanP]

Try to connect RF choke (inductor) in series with 3R3 resitor and 100nF ceramic cap between PS (+) and (-) terminals ..
Start with 1µH, if it is not enough, increase the choke's inductance to, say, 10µH or whatever value you can get your hands on ..

Regards,
IanP

 

[taleebe]

really,now im excited...could you explain the thoery behind your theory please!

 

[IanP]

Keep in mind that Zl=ωL ..
Higher frequency means higher impedance ..
DC (0Hz) means 0Ω ..

Inductor just stops RF signal to be attenuated by 3R3 resistor ..
Also, I added 100nF cap between the source's output terminals to provide RF path from (+) to (-) ..

Regards,
IanP

 

[taleebe]

appreciate it, ok so i forget about the other cap and just slide in the cap at the source, but remeber im looking for the ripple ampltude to stay at the same level with changing frequency, so i was trying to avoid caps and inductors beacuse their impedance changes with frequency unlike resistors...........one question;i have a 50 nF ceramic cap but how will the RF path created by the capacitor help?
-thanks