Lab Four: AM Detectors
AM detectors are an obvious essential part to any AM radio. AM waves must be demodulated in order to separate the carrier wave from the modulated wave. The madulated wave is what must be extractd in order to get a clear audible tone out of the amplifier.
Constructing AM Detectors
The simple detector circuit shown in Figure 1 is created on the breadboard. This AM detector will have significant clipping of the signal due to the turn-on voltage of the diode. The diode will not activate until the signal is high enough and will not capure the lower amplitude pieces of the signal. This clipping is shown in Figure 2. When fed a very small signal such as 100mV, there is no output read on the oscilloscope. This is once again due to the higher turn-on voltage of the diode.
When the resistor is swapped for a 10k\(\Omega\) resistor, the time constant of the RC circuit increases greatly. The output of this is shown in Figure 3. This increases the time constant so much so, that the AM detector does not correctly distinguish the modulated frequency. This effect is further exaggerated when the resistor is increased to of 100k\(\Omega\) and becomes more like a DC voltage.
Biased Diode Detector
Now the diode is biased so that the it can detect smaller voltage inputs and to reduce clipping. The biased diode detector circuit is shown in Figure 4 and constructed on the breadboard. With this circuit, there is little to no clipping. With increasing the amplitude of the AM signal, it it found that the output becomes clearer.
Adding the AM Detector to the Amplifier
The AM detector is connected to the LM Amplifier. For this particular ampliefer, the output is shown in Figure 6. The sound quality of this combination sounds very good, and has a clear output signal. It is unclear why the input is being shown as it is on the oscilloscope, but it is confirmed that the wave in actuality is the correct modulated waveform.
Through this lab, a deeper understanding of how information is encoded in waves and how to decode that information. The time constant for the detector must be properly outfitted so that the signal is properly detected and not clipped in any way. Clipping a wave will result in a loss of sound quality and distortion to the audio. The combination of amplifiers is also essential to the overall design, and this was also emphasized in this lab.