DC block for spectrum analyzer

[large_ghostman]

I have a spectrum analyzer (HP one), i only really use the LF input on it, that is up until now. The main input uses a large RF connector (totally forgot what these are called), it states on the input that it cant tolerate DC and DC should not exceed 0V. Origanaly i came with a DC block that screwed into the input, then what ever was connected screwed into that, since moving we no longer have the DC block, I tried ebay and got a shock at what they cost!!
So I wondered if it was possible to build one? I thought a capacitor would do it, but because of the cost of these blocks i assume there must be more to it than a simple capacitor.
Anyone got any information please, I dont want to damage the machine is was expensive and is a great bit of kit I want to look after. Especially now as my interest in amateur radio grows

 

[JimB]

Yes the DC block is just a capacitor.

So why does it cost an arm and a leg?
Because it works from very low frequencies up to several hundred Mhz.
The hard part is making it with a minimum of stray reactance to upset the (near) perfect 50Ohm termination which the analyser designer spent so much money in acheiving.

JimB

 

[large_ghostman]

So does this mean that making one isnt practical? maybe I should just make sure What ever output from my circuit connects to it I have a cap in the circuit to block the DC, Then all I need to do is make sure the circuit works with the cap there??? I would have saved up for one but I also need to buy a 75ohm to 50 Ohm converter for it and they also cost alot, for now I will continue to use the other input

 

[kubeek]

That depends on what performance do you expect from it, but still it should not be too hard to make one that would work satisfactory up to say 100Mhz.

 

[crutschow]

What is the maximum frequency you want the DC block to work?

 

[large_ghostman]

Hmm I am not sure, The analyzer is a HP 8568B, its range is 100Hz - 1.5 GHz, I dont know what frequency the block we had did, my main interest in in radio in general, like old fashioned radio's and amateur radio, so i guess as many of those frequencies as I can.
I am not even sure I would have any DC problems with that kind of equipment, iits more of making sure I dont ruin something that even today would cost all my body parts to replace! and it works fantastic! it agrees 100% with my in calibration HP signal generator.

 

[crutschow]

For frequencies up to a few hundred MHz you could likely use a box like one of **broken link removed**these with a 0.1uF ceramic (or other low inductance) capacitor in series between the two connectors to provide a DC block. That box has Type-N connectors which may be what the analyzer uses. Pomona Electronics makes that box with various combinations of BNC and Type-N connectors.

At higher frequencies the box will not look like 50 ohms and this mismatch will result in some signal distortion and incorrect readings. You can determine that point by looking at a sweep signal from your signal generator and noticing any differences in the analyzer reading with and without the DC block.

Note that 0.1uF will give a low frequency cutoff of about 32kHz with a 50 ohm input. If you want a lower frequency you will need to use a larger capacitor.

 

[KeepItSimpleStupid]

Here are a few 50 ohm DC blocks: **broken link removed**

The connector would help. FWIW, the 8568B uses BNC from the spec sheet.