Hello All,
I'm building an RF attenuator for use in radio repair and restoration projects. It's a simple passive design using seven pi-attenuators in series that should give -1dB, -2dB, -4dB,-8dB, -16dB, -32dB and -64dB of attenuation from the output of my low budget MHS-5200A function generator. The seven switches will give a total 128 dB of attenuation in 1 dB steps, at least that is my intent. I calculated the resistors without difficulty, but now I need to add a series DC blocking capacitor which I assume will be in the neighborhood of 0.01 to 0.1 uF, but I don't know how to calculate it to be sure. Though I've studied it many times, I still have a tenuous grip on how impedance is applied from device to device. I assume, for example, that the input impedance of my attenuator will be 50 ohms as that is the output impedance of my function generator, and the output impedance will be "high" as it will generally be placed in the grids or input stages of whatever radio I'm working on. I really have no idea how "high", nor do I know if that particularly matters in this case. Desired frequencies for -3dB dropoff are maybe 500 Hz to 15 MHz, though those are just guesses. How does one calculate the capacitance, or is it simply a rule-of-thumb kind of thing?
Thanks in advance!
I'm building an RF attenuator for use in radio repair and restoration projects. It's a simple passive design using seven pi-attenuators in series that should give -1dB, -2dB, -4dB,-8dB, -16dB, -32dB and -64dB of attenuation from the output of my low budget MHS-5200A function generator. The seven switches will give a total 128 dB of attenuation in 1 dB steps, at least that is my intent. I calculated the resistors without difficulty, but now I need to add a series DC blocking capacitor which I assume will be in the neighborhood of 0.01 to 0.1 uF, but I don't know how to calculate it to be sure. Though I've studied it many times, I still have a tenuous grip on how impedance is applied from device to device. I assume, for example, that the input impedance of my attenuator will be 50 ohms as that is the output impedance of my function generator, and the output impedance will be "high" as it will generally be placed in the grids or input stages of whatever radio I'm working on. I really have no idea how "high", nor do I know if that particularly matters in this case. Desired frequencies for -3dB dropoff are maybe 500 Hz to 15 MHz, though those are just guesses. How does one calculate the capacitance, or is it simply a rule-of-thumb kind of thing?
Thanks in advance!