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Measuring AM Modulation with a Z90 or Z91

 
 
AM Modulation Review

Amplitude Modulation, or AM, varies the strength, or amplitude, of the radio frequency carrier signal with the modulating frequency.

If we look at the composite signal in the time domain (plotting voltage versus time) we see the traditional waveform as shown below.
 
30% AM modulated signal, time domain view. (Horizontal axis time, vertical axis amplitude)

If we look at the composite signal in the frequency domain (plotting voltage versus frequency) we see a different view of the same signal.
 
30% AM modulated signal, frequency domain view. (Horizontal axis frequency, vertical axis amplitude)

 


We may measure the percentage modulation using data from either the time domain or frequency domain.
 
In the time domain:
 

 
Emax and Emin are in the same units, volts or millivolts most commonly.

 
In the frequency domain:
 

 

Esideband and Ecarrier are in decibels above or below a reference level in this formula, most commonly dBm, i.e., decibels above or below 1 milliwatt. However, since the formula uses only the difference between two dB referenced values, we may use any reference level and hence an instrument such as the Z90 which is not normally calibrated in terms of absolute levels.

Usually, if the modulation percentage is relatively high, say 30% or greater, the time domain approach will give the best accuracy. For low modulation percentages, 30% and less, the frequency domain approach has greater potential accuracy.

Let's work through a couple of examples.
 
Time Domain Example

First, let's consider a signal modulated 30% with a 1 KHz sine wave, using the signal generator's modulation percentage meter to set the level. In this case, the signal source is a Boonton 102D signal generator.

In the time domain, using an oscilloscope we see a signal similar to that shown below.
 

Using the Tektronix TDS430 oscilloscope's built-in measurement facilities, we determine Emax and Emin as:
     Emax = 240 mV
     Emin = 135 mV

We then calculate the modulation percentage:

The measured modulation is quite close to the signal generator's 30% setting.

 

Frequency Domain Example

Let's try a frequency domain measurement. Here's the same 30% modulated signal, viewed in the frequency domain with a Z90.
 


The difference between the carrier and the two modulation sidebands seems to be about 20 dB. We can then calculate the modulation percentage based on this difference:

This is not nearly as accurate as our envelope (time domain) measurement. Why? Because an error of only one or two dB in measuring the difference between the carrier and side band levels causes a relatively large error. 

Low Percentage Modulation

Let's look at a more difficult case, 5% modulation. As you can see, accurately measuring the peak and trough of the modulated waveform will be difficult, particularly with an analog oscilloscope without on-screen measurement cursors.
 

Using the TDS430A's cursor measurement functions, I determined Emax = 197 mV and Emin = 179 mV, and computed the modulation percentage as 4.8%.

Looking at the same signal in the time domain with the Z90 shows the modulating sidebands are considerably lower than the carrier.

 


I measured the difference between the carrier and modulating sidebands as 52 dB. Using this value, we can determine the modulation percentage as 5.01%.

 
Detecting Incidental FM

One final observation that we may make much easier in the frequency domain is to determine if the signal generator has simultaneous AM and FM modulation. Almost all signal generators before the era of phase lock and DDS techniques have some degree of incidental frequency modulation that occurs simultaneously with the desired amplitude modulation. Simultaneous AM and low level incidental FM will reveal itself in different amplitude of the upper and lower sidebands.

To illustrate this, I intentionally simultaneously modulated my Boonton 102D signal generator with 1 KHz, setting the AM level at 30% and FM level corresponding to a deviation of  100 Hz. This is a low amount of FM, but it shows up as a quite noticeable difference between the upper and lower sidebands. You will note a very small difference between the two sidebands in the 5% AM modulation as well, perhaps 1 dB or so. This is the result of a very small unwanted simultaneous AM and FM modulation.