Electronics > RF, Microwave, Ham Radio

DC block needed for 0V DC input rated Spectrum Analyzer?

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robert1111:
Hello.  Is a DC block needed for 0V DC rated Spectrum Analyzer to protect against static electricity?  Or will an attenuator suffice for protection?  Are SA 0V DC rated inputs not protected from small, inadvertent DC voltages or static electricity?  Thank you for your help.   

bob91343:
I can't believe 0 vdc is a requirement.  Perhaps a few miliivolts or maybe a couple of volts.

Ana attenuator won't eliminate dc.  It will reduce it.  A capacitor does eliminate dc.  That is also known as a dc block.

Ringmodulator:
Hi,

a DC block will not protect from static electricity.

It is used, to keep DC away from the SA input, hence the name.

The SA has a input impedance of 50 Ohms (normally). Any voltage will be shorted to ground via 50ohms.

If you want to measure rf on top of a DC voltage, for example to check the purity of a DC power supply, you will damage the input of the analyzer, because it can not dissipate the power from the DC shunted via 50 Ohms to ground

Chris

srb1954:

--- Quote from: robert1111 on June 16, 2021, 11:12:04 pm ---Hello.  Is a DC block needed for 0V DC rated Spectrum Analyzer to protect against static electricity?  Or will an attenuator suffice for protection?  Are SA 0V DC rated inputs not protected from small, inadvertent DC voltages or static electricity?  Thank you for your help.   

--- End quote ---
A DC block or capacitor won't protect against static discharge. These discharges typically have very fast edge rates and will be coupled through the capacitor with minimal attenuation.

What you need is a limiter which will clamp any transients or application of excessive RF power sufficiently to protect the spec analyser input. Check out the HP 11867A for an example.

The limiter will also provide some protection against inadvertent application of DC voltages but if you want to measure signals riding on top of DC voltages you will also need the DC block in front of the limiter.

ejeffrey:
It's definitely a good idea to put a DC block on a spectrum analyzer, especially one that doesn't have an input DC block enabled by default.  It won't really protect against ESD but it functions as a connector saver and it protects against accidental connection to DC power.  Even with an SA that has a switchable DC block I prefer to have one, only removing it if I am doing low frequency measurements.

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