Why my generator output power is higher when the signal is modulated ?

Thread Starter

LJO

Joined Dec 30, 2019
4
Hello everyone,

During a RF measurement I noticed that my output power when the signal is modulated is higher than when it is not.
My setup for this test is as following :
My RF generator (RIGOL DSG815) is connected to my spectrum analyzer (R&S FPC1000). My modulating signal is coming from a function generator (plugged on the "EXT MOD IN" input of my HF generator).

Setting for my RF generator :
-Center frequency = 433,42MHz
-Level : -50dBm
-Modulation : AM
-Depth : 100%
-Coupling (AC)
-Input impedance of the external modulating input (100kOhm)

Setting for my RF analyzer :
-Center frequency =433,42MHz
-Span = 10MHz
-RBW = 500kHz
-I did my power measure by setting the analyzer into "Max hold" mode

Setting for my function generator :
-Square wave
-1Khz
-Duty 50%
-1Vpp (+-500mV) -> (I read on the RF generator manual that it is the recommended amplitude for this signal)

What I see during my measurements :
-When my output is not modulated, I measure my -50dBm as expected).
-With the modulating signal is activated, I measure an output power at -44dBm (My expectation is to have a power of -50dBm too).

Are my expectations wrong ? Or is it my setup that is not correct ?
I am annoyed for my measurements because the finality is to measure the RF sensitivity of my product, so I am afraid to perform a bad measure !

Thanks for your help guys !
 

DickCappels

Joined Aug 21, 2008
8,686
Hello everyone,
During a RF measurement I noticed that my output power when the signal is modulated is higher than when it is not.
-I did my power measure by setting the analyzer into "Max hold" mode

What I see during my measurements :
-When my output is not modulated, I measure my -50dBm as expected).
-With the modulating signal is activated, I measure an output power at -44dBm (My expectation is to have a power of -50dBm too).

Are my expectations wrong ? Or is it my setup that is not correct ?
I am annoyed for my measurements because the finality is to measure the RF sensitivity of my product, so I am afraid to perform a bad measure !
!
(A lot of text removed for clarity, emphasis added)
1642667255162.png
The maximum level is higher with a modulated carrier than with an unmodulated carrier, per the illustration above.
 

Ramussons

Joined May 3, 2013
1,228
In a modulated AM signal, the 1/3 of the power is from the "modulating" signal, 2/3 from the Carrier being modulated.

In a 100 kW AM broadcast station, the Modulator will be rated for 35 kW.

Putting it another way, Transmit power = Carrier Power + Modulator Power. The Modulator Power <= 50 % Carrier Power to avoid Over Modulation.
 

Thread Starter

LJO

Joined Dec 30, 2019
4
Thanks to both of you for this explanations.

Another dummy question for my measurements... For example when I found that the limit of sensitivity of my product under test is when I set -50dBm on my HF generator (Conducted measurement and by not taking into account the losses in the cable/connections for illustration). What value should I consider as my product sensitivity ?

Don't know if it make sense for you, sorry for the dummy questions

Thanks for your help
 

Thread Starter

LJO

Joined Dec 30, 2019
4
View attachment 258221
The maximum level is higher with a modulated carrier than with an unmodulated carrier, per the illustration above.
Hello Rick,
Thanks for that. My product is in On Off Keying, so if I understand well, my RF output is either at max power (For binary 1) either 0 (For binary 0). The thing I do not understand is when I set my RF generator to -50dBm, I measure -43dBm on my analyzer. I expect to have -50dBm as max power here. If not, what does the output power setting at -50dBm means in that case ?
 

Thread Starter

LJO

Joined Dec 30, 2019
4
In a modulated AM signal, the 1/3 of the power is from the "modulating" signal, 2/3 from the Carrier being modulated.

In a 100 kW AM broadcast station, the Modulator will be rated for 35 kW.

Putting it another way, Transmit power = Carrier Power + Modulator Power. The Modulator Power <= 50 % Carrier Power to avoid Over Modulation.
Hello Ramussons,

Thanks for your reply.
In my case, I plug a remote controller (with a modification to have no RF output power) on the external modulating input of my generator to give it the frame as my modulating signal (I just solder 2 wires to get my serial 3V3 data as my modulating signal). So I expect to only have the power from my generator and my remote only give data to be modulated and no power.

Do you mean the extra power I measure can come from my remote (even if it is just a GPIO of a microcontroller ?)
edit : I can also tell you that I tried with a function generator (to replace remote frames) set as in my first post.
Can you help me to have a better understanding of this?
 

DickCappels

Joined Aug 21, 2008
8,686
Hello Rick,
Thanks for that. My product is in On Off Keying, so if I understand well, my RF output is either at max power (For binary 1) either 0 (For binary 0). The thing I do not understand is when I set my RF generator to -50dBm, I measure -43dBm on my analyzer. I expect to have -50dBm as max power here. If not, what does the output power setting at -50dBm means in that case ?
It means that your generator, your analyzer or both are out of calibration or that you have a faulty cable or connector.

My best effort at remote diagnosis.
 
Top