Electronics > RF, Microwave, Ham Radio

DC block (inner/outer/both?)

(1/2) > >>

jmw:
When connecting a DUT to SA or scope, what the reasons for picking one style of DC block over another?

ejeffrey:
An inner DC block is that standard and what most people mean when they say "DC block".  It's almost always a good idea to use one on a spectrum analyzer as it keeps DC/low frequency signals from entering the SA front end.  This is especially helpful when using amplifiers that are biased via the input and/or output.  Most spectrum analyzers have a DC block built in, but an external one often has a higher voltage rating, cannot be bypassed electronically, and functions as a connector saver.  I don't usually use a DC block with an oscilloscope, but just use the built-in AC coupling mode if needed.

Double DC blocks are used to break ground loops or allow operation when  the signal source is operating at a different ground potential from the instrument.  They are of some use but they can also cause their own problems.  For one thing, they have terrible common mode rejection.  The entire cable acts as an antenna and the inner/outer DC block can allow common-to-differential conversion.  Put another way, in coax the outer ground is your EMI shield.  A DC block is a break in the shield.  If you want to do floating measurements a balun may be a better option.  Still, if you are measuring long coax runs (such as CCTV runs) that may have a large ground potential difference, a double DC block is an easy way to make the measurement without a large ground current flowing through your instrument if you don't care about these issues.

coppercone2:
for RF devices its always gonna be 1 from what I have seen. The other two would break safety ground as well. I imagine if you have things like HV RF tube amps, that becomes a concern.

Usually people will use the first picture or a transformer. I have never even seen the other two types. I assume that they are somewhat rare?

ejeffrey:
Inner DC blocks are certainly the most common but I have seen and used inner/outer DC blocks.  Here is an example: https://www.pasternack.com/sma-dc-blocks-inner-outer-category.aspx

I have heard of outer DC blocks, but I can't really figure out what they would ever be used for.  I'd be interested if anyone knows what they might be good for.

coppercone2:
What is a real world use example for using those dual or outer blocks?

I am wondering how useful it is to have on in the connectors box. Is it something that could come up with HAM?

Navigation

[0] Message Index

[#] Next page