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Spectrum analyzers and DC blocks - RF Cafe Forums

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mpb
 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


 
   
 
kpainter
 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.


 
   
 
asoft
 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.

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