Why DC be not given input to a spectrum analyser?

Hi,

Can somebody tell me why exactly dc cant be given to a spectrum analyzer?

And where to find the information about how much dc would be tolerated for a specific analyser?

Normallly for spectrum analysis should avoid DC input, you can see there are a yellow tag under the input port said "+30dBm, 0VDC", that means no DC should input.

Some instruments (say NE5242A network analysis )allows DC input, the yellow tag of the input port said "+30dBm, 40VDC", then 40V is the tolerated voltage.

But what exactly happens when a dc input is given?..how does the damage being done to the analyzer?
My spectrum analyzer has an input terminal marked as

Can you tell me what exactly this specification refers to?

When DC input, the SA should be damaged. If lucky, only the RF variable attenuator will be damaged, the result is if you input 0dBm signal, the SA will display about -30dBm. Because SA has several attenuators, each for different freq band, sometimes only one attenuator is damaged, others are good. So SA can't measure amplitude at some freq band, but at others freq bands, SA is good. The worse case is the mixer of the SA is damaged, so the noise floor is very high.

You tag means: the AC and RF signal Maximum input signal should be 30dBm.
I guess your SA has some functions like oscilloscope, so you can select AC coupled or DC coupled. When you selected DC coupled, should not input DC signal.
When you select AC coupled, the maximum DC voltage is 50V.
Because I never used the DC or AC coupled SA, I am not sure about that, the last two points meaning on your tag.
Hope someone else can help you. Good luck.

The input attenuators should be able to handle DC up to the specified power level of 30 dBm = 7 V. The input mixer can be most likely damaged by lower levels, if the attenuation is disabled. As a special problem, DC may be undetectable for the automatic level switching.
Usually, your instrument will have a manual, that can be expected to answer more exactly, what you want to hear in the forum, without even telling a manufacturer and type.

I think, the quoted input terminal marking is pretty self-explanatory and can guide you in most cases.

P.S.: Spectrum analyzers generally can't display DC signals. There are however reasons, why they don't always offer AC coupling:
-For a SA with a frequency range downto e.g. 9 kHz it would be a bulky component
-Any additional component in the input path increases the VSWR by some amount

Thank you tony & Fvm ,, i think its much clearer now Btw im using R&S model FSQ 1155 5001.08 SA.

Get youself one of these:

Narda Microwave 562 - Noise Suppressor Inside / Outside DC Block (04243) - TestParts

Bolt it onto the front, and NEVER take it off. It will save you a PILE of cash someday!

That DC block has a frequency range of 10MHz - 12 GHz. The FSQ signal analyzer can measure down to 20 Hz.

There are several reasons to limit either RF or DC at the input of a spectrum analyzer (or for that matter a network analyzer too), the exact one depends on the design of the input stage but generally:

1] The very first thing that the input signal enters is usually a dc blocking capacitor, this is often a special part that is resonance free across the entire working range of the instrument such parts are made by Dielectric Labs , Presidio and others and these can have quite a low dc voltage rating before they are damaged.

2] Either immediately after the blocking capacitor or in place of it you may find an attenuator pad (fixed or variable depending on design) and, once again, this may be damaged by high levels of either dc or rf.

3] Sometimes, the first input stage may be either a schottky diode mixer or sampler and again these are damaged by high levels of rf or dc.

I hope this helps...

OldRFguy

One point to note about all devices that are DC coupled the max DC input voltage is not root(input-impeadence x max-power) but often a quater of that voltage...

Part of this is due to the difference between loaded and unloaded voltage (that is the input may well not be the AC input impeadance of 50 ohm but open CCT thus the 50 ohm source impedence is not forming a potential divider...)

Many state-of-the-art SAs (especially the ones which can display very low input frequencies such as 20Hz) have 0VDC tolerance. This is because of the fact that the RF input of such SAs is DC coupled. Hence any applied DC input will reach the first mixer in the RF path. That mixer is usually realized as a passive mixer and consists of several diodes. What happens if you apply a DC voltage to a diode? If it is forward-biased, the current will flow exponentially. That might damage the mixer. In my lab, we have a R&S FSU. We always keep a DC block at the input of the analyzer to prevent such damage. But then we have to keep in mind the insertion loss of the DC block all the time.

Thank you all ..thank you for the valuable inputs