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

[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]

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.   

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.

[robert1111]

Thank you for the replies.  Can anyone share their tips on using an 0V DC input rated SA?  What precautions did you take against static electricity and inadvertent DC voltages?  A use case is connecting a small indoor antenna to the input?  Can the 0V DC input rated SA be safely used without a DC block?  Thank you.

[joeqsmith]

For low risk use, I don't do anything beyond the basics for ESD.   Rare I have a need for a DC block as it would be unusual for me to attach to a DC source.  Keep in mind that a block may not protect the device from DC.  We are talking about electronics 101 so it shouldn't be a surprise.  The block is a capacitor.  The capacitor has a low impedance at high frequencies.  As you connect to a DC source, you create a step function which can have very high frequency content.   

I am in the process of putting together some software for a low cost VNA to measure power distribution networks which are DC.  In this case DC blocks are used to attach to the device.  I put a warning it the manual for the beginners about the risks.   

>> Can the 0V DC input rated SA be safely used without a DC block?
In the right hands, sure.  In the wrong hands, I doubt you could add enough protection and still have a useful instrument.   

 
https://desco.blog/2020/12/16/esd-basics-training/

[TheUnnamedNewbie]

Thank you for the replies.  Can anyone share their tips on using an 0V DC input rated SA?  What precautions did you take against static electricity and inadvertent DC voltages?  A use case is connecting a small indoor antenna to the input?  Can the 0V DC input rated SA be safely used without a DC block?  Thank you.

The 0 volts has little to do with static electricity. ESD events are pulses, that might actually manage to get past the DC block.

If it is a general, low-frequency (<10 GHz or so?) SA, it will probably be decently ESD robust, so you don't have to take any special precautions on top of what you should already be doing in the lab (so wear straps, use ESD safe tools, etc).

If it is a higher frequency device, you want to short out the center pin to the shield first. You can do this by (carefully) touching a grounded strap to the center pin and ground shield of the connector at the same time, before connecting it to the SA.

[rfclown]

I don't put a DC block on my "0V max" analyzers unless there is known DC, like when measuring an RF amplifier without a DC clock on the output. A typical scenario is an LNA that gets its DC from the RF output line and therefore needs an external bias tee to measure it (which will serve as the DC block to the analyzer). I don't worry about putting antennas on the input an my analyzers while indoors (or outdoors for that matter).

[Joel_Dunsmore]

For ESD protection, and power protection, you can't beat a good limiter.  And the best limiter (in my opinion, and not just because I designed it) is the 11930 A/B  https://www.keysight.com/us/en/assets/7018-06792/technical-overviews/5966-2006.pdf

of course there are lots of other limiters you might use. These not only protect against over power, but do a very good job of ESD limiting. For DC, you want to use a DC block such as: https://www.keysight.com/us/en/product/11742A/blocking-capacitor-0-045-26-5-ghz.html

of course there are lots of other DC blocks.

The problem with DC is SA's that are measuring down to DC will have an input mixer with diodes, and DC will turn on and blow out the diodes due to over current. so the SA might be spec'd to have 100 mV or something below the diode threshold.  On the other hand ESD is on the order of 4kV but very little energy so the limiters are basically a diode string to ground which will clip the ESD to prevent over-voltage damage to the front end of the SA. 

[djsb]

Mini-circuits do DC blocks

https://www.minicircuits.com/WebStore/dc_blocks.html

I use one on my Signal Hound SA44B.