I have seen many spectrum analyzers with the warning on the RF input port: “!!! Avoid electrostatic discharge and DC voltage !!!”.
Out of precaution I always connected my signal source with a DC block but now I find the DC block filters my signal too much.
The signal comes from a line driver, there should generally be no DC voltage but I can’t completely guarantee that.
Since I have seen this multiple times I assume that this is something generic.
I understand that excessive ESD might be bad … but DC voltage?
Does anyone know the reason for it and how seriously to take the DC voltage?
I mean, some amount of DC voltage (uVs) may always present.
Am I too over cautious? Is it safe to have a low DC voltage, say +/-1V on the signal?
Best Answer
in your example, the input power level is 10dBm max. A DC signal could be considered to be an RF signal at 0Hz. 10dBm into 50 Ohms is 0.707V RMS, so 1 VDC is above the +10dBm maximum. 1 VDC is +13dBm into 50 Ohms.
This huge signal at 0Hz can cause measurement accuracy problems with signals at other frequencies, for example compression due to front-end overload. To have DC and still have measurement accuracy, 125mV is probably the absolute maximum that I would ever put on the front end, and I would stay under 70.7mV to make a good measurement. But you should just use a DC block unless there is a good reason not to. The rating is 0 VDC for a good reason, so don't put DC on it. This is not generic advice.
I don't speak for Keysight Technologies.