DC block (inner/outer/both?) - Page 1
Forum section: electronics / rf-microwave

author: jmw
author_rank: Frequent Contributor
timestamp: July 20, 2022, 03:57:46 pm
content: When connecting a DUT to SA or scope, what the reasons for picking one style of DC block over another?

author: ejeffrey
author_rank: Super Contributor
timestamp: July 20, 2022, 04:27:58 pm
content: 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.

author: coppercone2
author_rank: Super Contributor
timestamp: July 20, 2022, 08:49:14 pm
content: 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?

author: ejeffrey
author_rank: Super Contributor
timestamp: July 20, 2022, 09:02:21 pm
content: 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.aspxI 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.

author: coppercone2
author_rank: Super Contributor
timestamp: July 20, 2022, 10:35:39 pm
content: 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?

author: joeqsmith
author_rank: Super Contributor
timestamp: July 21, 2022, 12:38:12 am
content: I keep a large film cap on hand with banana jacks handy for a low frequency high voltage block.https://www.eevblog.com/forum/testgear/brymen-bm789/msg3691099/#msg3691099Recent posts about their use with an SAhttps://www.eevblog.com/forum/testgear/dc-block-for-spectrum-analyzer-minimum-safe-low-cutoff-frequency/

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: July 22, 2022, 01:27:27 pm
content: The 'outer' I've seen as 'ground isolator' and the 'both' as 'galvanic isolator', for when you want to connect things that are at different ground potential or want to block 'low'frequency ground loops from forming.

author: TheMG
author_rank: Frequent Contributor
timestamp: August 05, 2022, 01:35:14 pm
content: Quote from: coppercone2 on July 20, 2022, 10:35:39 pmAges ago I used one to stop a 60Hz ground loop that was causing hum on a sound system, due to stray currents on CATV coax.

author: coppercone2
author_rank: Super Contributor
timestamp: August 05, 2022, 02:09:00 pm
content: I figured sound system would typically use isolation transformers (they seem popular) but I guess both work. I thought pro audio had a standard ground breaker 1:1 transformer with XLR connectors on both sides or whatever

author: TheMG
author_rank: Frequent Contributor
timestamp: August 06, 2022, 11:34:08 pm
content: This was a home audio setup, so unbalanced RCA connections between the various components including the TV (before optical S/PDIF).