•−•  ••−•    −•−•  •−  ••−•  •
RF Cafe Morse Code >Hear It<

Job Board

About RF Cafe™

Sitemap

Spectrum analyzers and DC blocks - RF Cafe Forums

Because of the high maintenance needed to monitor and filter spammers from the RF Cafe Forums, I decided that it would be best to just archive the pages to make all the good information posted in the past available for review. It is unfortunate that the scumbags of the world ruin an otherwise useful venue for people wanting to exchanged useful ideas and views. It seems that the more formal social media like Facebook pretty much dominate this kind of venue anymore anyway, so if you would like to post something on RF Cafe's Facebook page, please do.

Below are all of the forum threads, including all the responses to the original posts.


 Post subject: Spectrum analyzers and DC blocks
Posted: Sat Apr 01, 2006 11:45 pm 
 
Lieutenant

Joined: Sat Apr 01, 2006 11:16 pm
Posts: 3
Hi-- I'm a newbie to this forum, so please forgive unfamiliarity with posting a new topic question on: Spectrum analyzers and dc blocks.

I often need to look at some RF signal coming down a coax feedline from an antenna with a built-in LNA. This means there is low-voltage on the coax powering the antenna mounted LNA. Since spectrum analyzers (a Tek 495P in my case) cannot have DC on their RF input, a DC block must be used.

I have been cautioned that many DC blocks have large enough values of series capacitance to be a risk to the mixer diode in spectrum analyzers.
Many DC blocks, in order to achieve a low-frequency response below a few hundred KHz, use series capacitors as large as 1.0 uF or even 2.2 uF.

I am told that a discharged 1 uF capacitor will take more than enough power to charge-up to kill the mixer diodes (12 microjoules at 5V = 120 ergs. The burnout spec of typical Schottky mixer diodes is
hard to find, but seems to be around 1 to 10 ergs for at least some parts.

I have been warned that a DC block with a 1uF or 2uF capacitor is very risky to use. It has been suggested that, unless I specifically need very low frequency response, that 0.1 uF is about the most capacitance to risk using in a DC block and even then, I should avoid exposing it to more than around 5 VDC.

Can anyone on this forum confirm the risk to the mixer diode from inserting a series 1 uF or 2 uF? Since I so often have a need to look at RF coming from amplified antennas, I had considered installing a DC block INSIDE the spectrum analyser (with semi-rigid line and SMA connectors). Any thougths or feedback on this will be much appreciated!

Thanks!!

MPB
Micanopy, FL

_________________
My 911 has a "1" in front of it


 
   
 
 Post subject:
Posted: Sun Apr 02, 2006 12:25 pm 
 
General
User avatar

Joined: Wed Aug 20, 2003 11:47 am
Posts: 85
Location: Santa Barbara, CA
Most Spectrum Analyzers (at least the ones I have used) do not run straight into the first Mixer but have a 10 dB pad in front specifically to protect against this sort of problem. Should be ok. If your analyzer does not have a pad, you can add your own.

Alternatively, why not make your own DC block putting whatever capacitance value you want. I can't imagine a DC block with 2.2uF having very good high frequency response anyway.


 
   
 
 Post subject: DC Block for spectrum analyzer
Posted: Fri Apr 14, 2006 5:54 am 
 
Lieutenant

Joined: Sun Mar 05, 2006 10:38 am
Posts: 4
Hi,
DC block for spectrum analyzer may affect the reading on a spectrum analyzer. It is always preferrable to buy a DC block from any reputed vendors like Agilent (HP), RF labs, or others. For professional quality results, a calibrated DC block is recommended. Don't rely upon ann input attenuator (that is in-built) of a spectrum analyzer for taking care of incoming DC signals. It often happens that both attenuator and input mixer go bad due to wrong input. When buying a DC block, take care of selecting appropriate end-connectors. If un-calibrated readings are sufficient, a simple home-made dc block (using very low cap, like 0.1uf) may be used. But, this is not recommended solution, and difficult to get repeatable readings. Hope this helps.




Posted  11/12/2012
Custom Search
More than 10,000 searchable pages indexed.

Your RF Cafe
Progenitor & Webmaster

Click here to read about RF CafeKirt Blattenberger... single-handedly redefining what an engineering website should be.

View the YouTube RF Cafe Intro Video Carpe Diem!
(Seize the Day!)

5th MOB: My USAF radar shop

Airplanes and Rockets: My personal hobby website

Equine Kingdom: My daughter Sally's horse riding website