Calculation of Rin,out of audio filter

[Fovakis]

Ok, at high frequency the lowest impedance possibly seen at the filter input is trivially R1||R5||R2, so say somewhere about 12K or so at a guess, it rises towards DC as the reactance of the input coupling cap rises and the various filter capacitors become more significant reaching infinity at DC due to the input blocking cap.

Dear Dan you are very helpful. Are you academic- you have students?

Well, i understand the problem of mis-match at the output of mixers. He need to match with 50ohm, to work properly. In my case he don't. It is understood. I confused, because, at the audio system i know that the RLoad (in this case Zinput of the Active Filter) must be 10 times greater that Rsource (in this case the output port IF port of mixer ). We are at the "voltage" domain at KHz. So, the perfect matching i know it is not good for maximum voltage transfer. Οn the other hand there is not right termination of the IF port of the mixers. Which one is more critical?

Also, what is "AF"?

Regards,
George

 

[Dan Mills]

Not any kind of academic, I never got around to finishing my degree, never mind a masters or phd. I have however been around this stuff for 30 years which helps.

AF = Audio frequency.

RF uses matched impedance to opimise power transfer, which way back in the day was how audio was also done (They used to use 600 ohm rather then 50, but that is a detail), modern audio usually does voltage transfer which is where the load being > 10 times the source comes in.

Now you could terminate the mixer into a 50 ohm resistor to ground, then use a filter having an input impedance > 500 ohms wired in parallel, the effect will be that the mixer sees a ~50 ohm termination, but it will totally kill your noise performance because the filter will be seeing a tiny input voltage.
Better would be to design an audio filter having a 50 ohm input impedance and a stiff voltage output, hence my suggestion of a common base buffer stage (low input Z).

Note that the output from the mixer is NOT audio, it has components way up into the radio frequency spectrum, it becomes audio once you strip off all the ultrasonic and rf energy in the filter.

Seriously hit the books.

73 Dan.

 

[Fovakis]

Not any kind of academic, I never got around to finishing my degree, never mind a masters or phd. I have however been around this stuff for 30 years which helps.

AF = Audio frequency.

RF uses matched impedance to opimise power transfer, which way back in the day was how audio was also done (They used to use 600 ohm rather then 50, but that is a detail), modern audio usually does voltage transfer which is where the load being > 10 times the source comes in.

Now you could terminate the mixer into a 50 ohm resistor to ground, then use a filter having an input impedance > 500 ohms wired in parallel, the effect will be that the mixer sees a ~50 ohm termination, but it will totally kill your noise performance because the filter will be seeing a tiny input voltage.
Better would be to design an audio filter having a 50 ohm input impedance and a stiff voltage output, hence my suggestion of a common base buffer stage (low input Z).

Note that the output from the mixer is NOT audio, it has components way up into the radio frequency spectrum, it becomes audio once you strip off all the ultrasonic and rf energy in the filter.

Seriously hit the books.

73 Dan.

Okay i will do what you say! About the books... can i have the exact title of the books ?

ARRL HANDBOOK Experimental method in RF Design. this one?https://www.amazon.com/Experimental-Methods-Design-Wes-Hayward/dp/087259923X

i don't have a lot money.it worth for me ? i am a begginer...

Radcom handbook?

Terman?