about dbm/200khz problem..

[super]

why it is 200khz bandwidth

Dear all :
The doc spec write as below ,
Frequency Emission Level
2Mhz to 50Mhz -30dbm/200khz

1. what's it mean ?
2. How should I do when I measure spurious use Spectrum Analyzer ?
(ex: set RBW=200khz and measured spurious below -30dbm is pass ?
or others )


Thanks.

 

[vfone]

if bandwidth measurement 200 khz

-30dBm is an absolute level.
The spurious in band 2MHz – 50MHz shall not pass this level when the Spectrum Analyzer RBW is set on 200 kHz.

 

[biff44]

Yep.

Think of the spectrum analyzer as a power meter with a sweeping downconverter and an IF bandpass filter in front of it. Changing the IF filter bandwidth will change what the power meter can see.

If you have a spurious single tone, it gets thru to the power meter pretty much whatever IF bandpass setting (RBW) you use.

If you have multiple close together tones, or a broadband noise floor, then a wide IF bandwidth will let in much more noise power, while a narrow IF bandwidth will let in less noise power.

So, if the customer did not specify the RBW setting, you could "cheat" and just measure everything with a 3 Hz RBW setting, and your sideband noise will look great!

Rich

 

[super]

Dear sir :
Thanks your reply.
To confirm again ,
1. It means when I measure use Spectrum Analyser I need to set RBW=200Khz start=2Mhz stop=50Mhz,
if measured power level >-30dBm then it's fail , <-30dBm it's pass.
Right ?

2. But my boss tell me it's need below -70dbm not -30dbm , I don't know why.
If I transfer it to dbm/hz is -83dmb/hz ,it become more severe than -70dbm and unit also different.
I don't know which part is wrong ?

Thanks.

 

[biff44]

I would vote for #1. Unfortunately, your boss has a bigger vote!

Maybe he is thinking this:

-30 dBm in a 200 KHz bandwidth is like saying -33 dBm in a 100 KHz bandwidth is like saying (-33 dBm -50 dB) = -83 dBm in a 1 Hz bandwidth.

If your spectrum analyzer can measure in a 1 Hz resolution bandwidth AND you are measuring pure white noise, you can set it for that and do the measurement that way.

However, you are making life very hard on yourself if the signal contains any discrete spurious tones in addition to white noise. You might fail the test using the "#2 method" then, while you might legally pass it using the "#1" method".

Example: you are looking 20 MHz away from the carrier, and you have the spectrum analyzer set at 200 KHz RBW. Lets say that at this offset you have white noise at -50 dBm/200KHz, and you also have a discrete tone at -40 dBm. The spectrum analyzer will simply integrate all the noise it can see in the 200 KHz RBW, so it will show a roundy bump with a top of -39.5861 dBm.

This is because -50 dBm is 0.00001 mw (from 10^-50/10), and -40 dBm is 0.0001 mw, so the total integrated power is 0.00011 mw, or 10 Log(0.00011) = -39.5861 dBm. Since you are 9 db within the "-30 dBm in 200 KHz" spec, you ship the unit!

If instead you set the spectrum anlayzer to 1 Hz bandwidth, as it sweep exactly to 20 MHz, it sees a -40 dBm discrete tone, and shows it at -40 dBm. This is 43 dB worse than "-83 dBm per Hz", so now with that interpretation you can not ship the unit.

I would read thru your specification, line by line, and try to prove to your boss that the spec is designed to measure both white noise and discrete spurs, and that your's is the proper interpretation. A valid way to do this is to make the measurement, show him that there is a discrete spur 20 dB out-of-spec, and the only way to pass the test is your way. Bosses like this sort of argument, as they want to ship the product!

A lot of systems, communications systems for instance, only care about the total integrated noise power, and do not care what percentage comes from white noise or discrete noise sources--just that it is below a total integrated level.

Rich

 

[super]

Dear biff44 :
Thanks your kindly explain.
Because previous project is follow this spec (2Mhz to 50Mhz -70dbm) , but no one know reason and I consult another person he give me this spec (2Mhz to 50Mhz -30dbm/200khz ) but he don't know which one is right.

I will check it again.

Thanks again. Many Many thanks.

 

[FvM]

If you are refering to a particular EMC or communication equipment regulation, it should be possible to find out, what's the correct interpretation. It may be in doubt, however, which regulation applies for your application.