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| DC Block for spectrum analyzer: minimum safe low cutoff frequency |
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| matthuszagh:
Most DC blocks for RF signals seem to have a low cutoff frequency of 10 MHz (corresponding to a series capacitance of 318 pF for 50 ohm Z0). Presumably, this is generally considered safe for an SA input to protect against a transient from a reasonable DC voltage. I have 2 questions: (1) how do we quantify what an acceptable level of input power or energy to the SA input is? and (2) how low can we set this cutoff frequency before the transients start to get dangerous. 1: for instance, do we set thresholds for max power and average power through a 50 ohm resistor? What would reasonable values for these thresholds be? 2: it's possible to find DC blocks with much lower cutoff frequencies (for example this one (https://www.centricrf.com/content/pdf/C0927.pdf), which goes down to 9 kHz). Obviously, if it's possible to answer (1), that will also answer (2). |
| bob91343:
The governing issue here is the thermal time constant of the input circuitry. I would imagine something on the order of 50 or 100 milliseconds, corresponding perhaps to 10 or 20 Hz. And down at least 10 dB at this frequency. |
| matthuszagh:
--- Quote from: bob91343 on May 31, 2022, 05:42:32 pm ---The governing issue here is the thermal time constant of the input circuitry. I would imagine something on the order of 50 or 100 milliseconds, corresponding perhaps to 10 or 20 Hz. And down at least 10 dB at this frequency. --- End quote --- This is helpful, thanks. Out of curiosity, how did you arrive at that thermal time constant? Is there somewhere I can read more about this? |
| bob91343:
It was just an educated guess. I figured one second was much too long - the parts are tiny. |
| joeqsmith:
You can get them with a much lower cutoff like the P2130A. I think you are confused about input protection. Fast transients like ESD will blow right through... I would consider the min / max freq, voltage, how tight of tolerance you need (effects of the voltage)... Maybe invest in a humidifier and not poke the equipment with things you shouldn't? Maybe look at finding some sort of limiter? Maybe make a limiter? |
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