FAQs from Rohde & Schwarz

HMF2525 / HMF2550 AM modulation informations


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?


AM modulation has some particularities in terms of power in combination with the modulation index.

In comparison to CW we have the following rules:

HMF2525 / HMF2550 AM modulation informations
(m is the modulation index)

This means that you get the following gain with different modulation indexes when talking about peak power:

HMF2525 / HMF2550 AM modulation informations

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:

HMF2525 / HMF2550 AM modulation informations

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:

HMF2525 / HMF2550 AM modulation informations

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:

HMF2525 / HMF2550 AM modulation informations