Question
I use the HMF2550 as a Sinus generator in combination with AM modulation.
The power level of my modulated signal is too low. If I switch off modulation (pure CW) the signal level is fine again.
What's the background of this behavior?
HMF2525 / HMF2550 AM modulation informations
Answer
AM modulation has some particularities in terms of power in combination with the modulation index.
In comparison to CW we have the following rules:
(m is the modulation index)
This means that you get the following gain with different modulation indexes when talking about peak power:
Let's perform the following steps after loading the default setting on the generator:
- Z is set to 50 Ohms
- f is set to 50 MHz (or 25 MHz with the HMF2525)
- Amplitude is set to 5 V pp
Important notice: When connecting the HMF25xx series on your measurement device, always use a DC blocker to avoid destruction due to accidental DC offset
This setting leads us to the following CW power level:
Upp/2*0,7= 2,5V * 0,7 = 1,75 Veff
P = U²/R = 61,25 mW
10 * log 61,25 mW = 17,87 dBm
When measuring the power level using a CMA180 we get the following results:
This is what we would expect when factoring in cable loss.
When now enabling AM modulation with an index of 0 % the measurement values change to:
What happened?
The amplitude value setting of the HMF25xx complies to the output power for 100 % modulation index.
As we would have about 6 dB gain in comparison to 0% modulation index, these 6 dB now have to be subtracted.
17.46 dBm - 6.02dB = 11,44 dBm
When now changing the index to 100% we will get the maximum power according to the amplitude level set before:
