[SOLVED] Which type of DC block for my Spectrum Analyzer?

[Nezs]

Good day,

I wish to connect my spectrum analyzer with a TWT and I want to prevent any damage from DC input.

The TWT output is a waveguide output. I am using a waveguide to SMA adapter.
Hence, theoretically, the RF output will be referenced to the ground level of the TWT.
However, I am worried if there is a difference between the ground level of the TWT and the SA, hence a DC offset.

In that case, what type of DC block should I use?

Thank you for your help in advance.

 

[FvM]

DC blocks with specified properties are available form measurement equipment vendors and should be used, where appropriate. In the present case, I don't see why.

If you manage to get more than a view mV ground potential difference, something seems to be terribly wrong with the setup, I fear. Isn't it, that you'll use external attenuators anyway? So the SA input should be protected against some DC, too.

 

[jiripolivka]

Good day,

I wish to connect my spectrum analyzer with a TWT and I want to prevent any damage from DC input.

The TWT output is a waveguide output. I am using a waveguide to SMA adapter.
Hence, theoretically, the RF output will be referenced to the ground level of the TWT.
However, I am worried if there is a difference between the ground level of the TWT and the SA, hence a DC offset.

In that case, what type of DC block should I use?

Thank you for your help in advance.

As your TWT output is a waveguide, then no DC can reach the SA input and no DC block is needed. I would be concerned, however, by a high-RF power output from your TWT. TWTs are typically used as high-power wideband amplifiers with an output power exceeding several watts. I would advise you to use a good waveguide attenuator or a directional coupler allowing less than a specified maximum input to the SA. Take care that those devices can dissipate a high RF power from a TWT.

For most applications, without the reference to a TWT, a DC block should be used on any SA input, to prevent burning the internal 50-Ohm device by DC current. Find a good DC block specified to pass a low frequency and higher, of the band of your interest. To use SA from 10 MHz to RF, the DC block should pass 10 MHz and more. If you intend to test any devices with DC in them, take care that the DC block can carry the maximum voltage, be it 12 V or 100 V.

 

[Nezs]

Thank you very much for your advice!

Yes, i will be using 70 dB attenuators. Even if there is any offset, it will be attenuated as well!

Thanks again!

 

[biff44]

to answer your original question, you are worried about the ground of the TWT being at different potential from the S/A. You would use an "inside/outside" DC block to isolate the two grounds.

If it were not high power, you could simply connect your waveguide to coax transition to the TWT output by putting a layer of kapton insulation between the 2 flanges and using nylon screws. But doing so will cause some RF leakage out of the flange.

I would also make sure that the S/A is connected to earth ground on the AC power plug, as well the metal chassis of the TWT. That way if there are significant leakage currents, like from a power transformer, the earth grounds will keep the potential differences to only a few volts. You do NOT want your spectrum analyzer chassis to be the earth ground return for the TWT chassis!

**broken link removed**