Analog demodulation (ADEM) is the process of recovering the message signal from an analog modulated signal.
Let c(t)=Acej(2πfct + θc) be the carrier that is modulated by the message signal m(t), with a maximum message frequency content of fm Hz.
The following table shows the types of analog modulation based on the carrier parameter being varied by the message signal.
|Modulation Type||Transmitted Signal||Bandwidth|
|Amplitude modulation double sideband with carrier (AM DSB)||
where, k is the modulation depth, usually expressed as a percentage of Ac
|Amplitude modulation double sideband with carrier suppressed (AM DSBSC)||2*fm|
|Frequency modulation (FM)||
where, fd is the frequency deviation, in Hz
|Phase modulation (PM)||
where, Θd is the phase deviation in degrees.
To measure the transmit quality, the analyzer needs to acquire a signal with bandwidth greater than the minimum bandwidth, as mentioned in the above table. Specify the acquisition bandwidth by configuring the RBW filter to be used. RFmx recommends that you use a Flat Top RBW filter. The analyzer signal processing chain is as shown in the following image.
The demodulator recovers the message signal based on the modulation type specified. For demodulating AM signals, specify whether the carrier is suppressed, in order to recover the message signal amplitude correctly. For demodulating FM and PM signals, specify whether the demodulator needs to correct the carrier frequency error. Carrier phase error correction is applicable only for PM signals.
Often, audio signals are pre-emphasized before frequency modulation. At the receiver, a de-emphasis filter is used on the demodulated signal to compensate for the pre-emphasis applied at the transmitter. This filter is a low pass filter with a 6 dB/octave roll-off having the following transfer function.
where, τ is the de-emphasis filter time constant, in seconds
This filter is bypassed when the de-emphasis is 0 seconds. Typical de-emphasis time constant values used for FM are 50 μs and 75 μs.
Enable audio filtering to filter the demodulated signal. The response of the supported audio filter types are shown in the following table.
|Audio Filter Type||Filter Response|
|Custom||A band pass filter is designed using the specified lower and upper cut off frequency.|
The following table shows the sample rate of the demodulated signal. The sample rate is adjusted based on the filter settings.