Lesson 18

Modulation and Demodulation

In radio communications, there are two primary components of the transmitted signal. They are baseband signal and the modulated signal. 

Let’s look at an example of an AM signal that might go out over a local broadcast station. We’ll call it “AM 1000 – WHRP.” 

  • Engineer Maria would set up the transmitter. An oscillator would generate a carrier signal. That’s the constant frequency of the broadcast signal, in our case 1,000 kilohertz.
  • Disc Jockey Renee would generate the input signal. It might be talk, news, oldies records. It’s the audio signal we want to broadcast. This is called the baseband signal. It refers to the frequency range occupied by a message signal prior to modulation.
  • Combining the two is called modulation. When that happens WHRP is on the air. The modulated result goes out over the air to their listeners.

Diagram of amplitude modulation

We showed an example of Amplitude Modulation or AM. In AM minor variations in the strength of the carrier transport the signal. AM signals have two sidebands, but ham radio commonly uses a single sideband. You can generate a single-sideband phone signal from an AM signal. Do that by using a balanced modulator followed by a filter. 

What about FM? FM doesn’t vary the strength of the signal. Instead, it makes minor modulations to the frequency to carry the data.  

When developing this course we looked at the results of thousands of practice tests. From that we determined which exam questions are most difficult for students. This next topic tends to trip people up, so let’s spend some extra time on it.  

The question: Which of the following can be used to generate FM phone signals? The correct answer is reactance modulation of a local oscillator.

Why are the others wrong? Well, two of them talk about modulating the amplifier. In a radio circuit, the modulation is usually complete by the time the signal hits the amplifier. More on amps in an upcoming lesson.

The remaining choices are balanced modulation or reactance modulation of the local oscillator. Balanced modulation is used in double-sideband suppressed-carrier signals. So, we can throw that out. Stay focused on reactance modulation because the question asked about FM signals. 

The function of a reactance modulator is to produce PM or FM signals by a varying capacitance. PM is short for Phase Modulation.  

A function called de-emphasis is commonly used in FM communications receivers. De-emphasis provides compatibility with transmitters using phase modulation. Don’t confuse that with a pre-emphasis network circuit. That is added to an FM transmitter to boost the higher audio frequencies.

De-emphasis circuit

Pre-emphasis circuit

So, everything for a transmit signal is in place. Now we need a circuit to receive it. These are demodulation circuits. With your demodulation circuit you are “decoding” the AM or FM signal. Choose the right demodulation circuit for your application to get the best output.  

Two common tools in demodulation circuits are detectors and discriminators. A diode envelope detector functions by rectification and filtering of RF signals.  

Let’s break it down to AM and FM again. In the AM environment, your SSB receiver may use a product detector. That’s a common type of detector circuit for SSB signals.

Schematic of diode envelope detector

Now on to an FM receiver. Here, a frequency discriminator stage is a circuit for detecting FM signals. So what’s a stage? It’s a mixer circuit that does the work of modulating or demodulating signals. A mixer takes an input signal and adds a local oscillator frequency to create a new output signal. 

Schematic of a mixer circuit

You don’t just get a single frequency output from a mixer circuit. Four principle frequencies appear at the output of a mixer circuit. They are the two input frequencies along with their sum and difference frequencies.

It’s important to manage the amount of signal energy that hits the mixer circuit. When input signal levels are too high for a mixer circuit, spurious mixer products are generated.

In our amateur radio we want both transmission and reception in one device. Knowing how a signal is generated and received we can do that. Combine oscillator and mixer stages into a function called a phase-locked loop circuit. 

A phase-locked loop circuit can handle frequency synthesis and FM demodulation. What are the detailed elements of a phase-locked loop circuit? It’s an electronic servo loop consisting of a phase detector, a low-pass filter, a voltage-controlled oscillator, and a stable reference oscillator.

Schematic of a phase-locked loop circuit

A digital radio uses different ways to create, or synthesize, a modulated signal. A direct digital synthesizer is a type of frequency synthesizer circuit. It uses a phase accumulator, lookup table, digital-to-analog converter, and a low-pass anti-alias filter.

The Direct Digital Synthesizer or DDS generates waveforms digitally. A DDS has a lookup table which contains amplitude values that represent the desired waveform. While DDS are high performing, they are not immune from interference. A term called “spectral impurity components” translates to “interference” for DDS circuits. The major spectral impurity components are spurious signals at discrete frequencies.

Many modern amateur radios use a combination of analog and digital circuitry. This is a way to optimize price and performance.  Many have a digital signal processor or DSP. Let’s step through how a DSP works.

  • Inside the DSP is an analog-to-digital converter. It feeds the DSP with its information.  
  • This analog-to-digital converter has a sampling rate. The sample rate is an aspect of receiver analog-to-digital conversion. Sample rates are the number of samples per second output from the converter.  They are usually expressed in kilohertz.
  • Converter output sample rate is important for accuracy. The sample rate needs to be at least twice the rate of the highest frequency component of the signal. 
  • The volume of data in a sample is expressed in bit depth or bits. Focus on 10 bits as a minimum required. This is for an analog-to-digital converter to sample a signal with a range of 1 volt at a resolution of 1 millivolt. 

When comparing radios to buy, pay attention to sample rate as a metric. Sample rate determines the maximum receive bandwidth of a direct-sampling software defined SDR.  

Software defined radio diagram

SDR’s use direct digital conversion, also called direct sampling. Direct sampling in an SDR is when incoming RF is digitized by an analog-to-digital converter without being mixed with a local oscillator signal.

Tools used to help digital signal processing include Fast Fourier Transform. FFT is used for converting digital signals from the time domain to the frequency domain. 

Also, decimation is a function reducing the effective sample rate by removing samples. A decimator requires an anti-aliasing digital filter. It removes high-frequency signal components that would otherwise be reproduced as lower frequency components.  

The minimum detectable signal level for an SDR is determined by two factors. For digital signals that’s reference voltage level and sample width in bits. Those two set the minimum detectable signal level for a direct-sampling SDR receiver. Assuming no atmospheric or thermal noise.

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