{"id":10463,"date":"2022-04-29T00:06:16","date_gmt":"2022-04-29T00:06:16","guid":{"rendered":"https:\/\/liquidinstruments.com\/?p=10463"},"modified":"2025-12-11T22:22:41","modified_gmt":"2025-12-11T22:22:41","slug":"using-the-lock-in-amplifier-to-build-an-am-radio-receiver","status":"publish","type":"post","link":"https:\/\/liquidinstruments.com\/application-notes\/using-the-lock-in-amplifier-to-build-an-am-radio-receiver\/","title":{"rendered":"How to build an AM radio receiver with a Lock-in Amplifier","gt_translate_keys":[{"key":"rendered","format":"text"}]},"content":{"rendered":"

This lab tutorial discusses a typical undergraduate electronics lab exercise and how it can be effectively conducted using Moku:Go to teach the fundamentals of an AM Radio receiver and Lock-in Amplifier<\/a>.<\/p>\n

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Moku:Go<\/a><\/h2>\n

\"Moku:Go\"<\/a>Moku:Go combines 15+ lab instruments in one high performance device, with 2 analog inputs, 2 analog outputs, 16 digital I\/O pins and optional integrated power supplies.<\/p>\n

Introduction<\/h2>\n

The purpose of this lab is to introduce the basics of the AM radio receiver and demonstrate the basics of using a Lock-in Amplifier. You will be using the Lock-In Amplifier<\/a>, Digital Filter Box<\/a>, and Spectrum Analyzer<\/a> instruments and the integrated power supply to design and optimize an AM radio receiver.<\/p>\n

Amplitude modulated<\/a> (AM) radio, while largely succeeded by frequency modulated<\/a> (FM) radio, is still an incredibly useful method to transmit messages via radio waves. In this lab you will design and implement an AM radio receiver. You will learn to find the local AM radio frequency and implement a radio receiver using the Lock-In Amplifier. Figure 1 displays the AM radio signals picked up in Canberra, Australia using the Spectrum Analyzer instrument.<\/p>\n

 <\/p>\n

\"Figure<\/p>\n

Figure 1. Example of the Spectrum Analyzer in the Canberra region<\/em><\/p>\n

Background<\/h2>\n

AM Radio<\/h4>\n

In AM radio the amplitude of the signal is modulated; this is compared to FM radio where the frequency of the signal is modulated. This difference can be seen in Figure 2 where the amplitude of the wave is clearly varying in the AM modulated waveform, while in the FM modulated waveform the frequency of the sine wave varies with time. Both types of radio transmissions have advantages and disadvantages. Commercial AM radio stations operate in the 535 kHz – 1605 kHz range and thus typically have a longer range compared to FM in the 88 – 108 MHz range, however it is more susceptible to noise and is far more suited to talk radio compared to music-based radio shows.<\/p>\n

 <\/p>\n

\"Figure<\/p>\n

Figure 2. <\/em>Example of an amplitude modulated waveform and a frequency modulated waveform using the Waveform Generator on Moku:Go.<\/em><\/p>\n

 <\/p>\n

AM radio operates by using a sinusoidal carrier wave which is modulated with the message signal (audio signal); this audio is the information that is being sent. In this type of modulation, the amplitude of the carrier wave is altered by the message signal (hence the term AM).<\/p>\n

The modulated signal corresponding to a particular radio station can be clearly seen as a spike in the frequency domain (e.g. Figure 1), though it is usually hard to see in the time domain. The Moku:Go FIR Filter Builder helps by allowing us to set up a narrow bandpass filter around the radio station, removing almost all of the signal other than the station.<\/p>\n

An example can be seen in Figure 3, where the FIR Filter Builder<\/a> picks out an AM radio station at approximately 600 kHz. The AM carrier, modulated with the voice signal, can clearly be seen in the blue trace. The red trace (antenna input) shows that without a narrow bandpass, it\u2019s impossible to pick this or any other radio station; in fact, the signal is completely dominated by the ~25 kHz switching of the dimmable LED lighting in the office where the screenshot was taken.<\/p>\n

\"Figure<\/em><\/p>\n

Figure 3. FIR Filter Builder isolating an AM radio station (blue trace) from the background signal (red).<\/em><\/p>\n

 <\/p>\n

To receive and then listen to the message signal, a radio receiver is required to receive the specific AM radio frequency and demodulate it to separate out the carrier signal from the message signal. The block diagram for a simple AM radio receiver is shown in Figure 4.<\/p>\n

\"Figure<\/p>\n

Figure 4. Block diagram of the AM radio receiver <\/em><\/p>\n

 <\/p>\n

A receiver operates by using a radio antenna to detect radio waves; however, this signal is usually relatively weak, therefore an RF amplifier is required to boost the signal so it can be further processed. As the antenna will capture all possible frequencies, a tuner is required to find the specific frequency needed.<\/p>\n

\"Figure<\/p>\n

Figure 5. Example schematic of the LC circuit  <\/em><\/p>\n

 <\/p>\n

Analog Demodulation<\/strong><\/p>\n

The analog demodulation tuner typically consists of an LC (Inductor-Capacitor) circuit, which can be seen in Figure 5. Depending on the inductor and capacitor used, the circuit will resonate at a specific frequency. All other frequencies above and below this resonant frequency will be blocked. The message signal can be rectified to only give a DC signal and demodulated from the carrier wave via a diode and bypass capacitor. The message single can then be amplified and sent to speakers, headphones, etc.<\/p>\n

 <\/p>\n

Lock-in Amplifier<\/strong><\/p>\n

A lock-in amplifier is a powerful device that can separate out the modulated signal from a noisy background, in our case, a specific AM signal from an array of signals. This means that the lock-in amplifier can function as a radio receiver since it contains several key components from a radio receiver.<\/p>\n

The Moku:Go Lock-in Amplifier is able to demodulate a modulated signal, for example a radio wave, by using a phase-sensitive detector (PSD). It uses a sinusoidal reference signal with the same frequency as the carrier signal. It can track any variances from the reference signal and hence able to track frequency drift.<\/p>\n

The PSD multiplies or \u201cmixes\u201d the two signals together, generating sum and difference terms of the two signals. The required frequency and reference signal consist of the same frequency and hence the difference between the frequencies is zero. Therefore, the required radio wave signal is set to DC.<\/p>\n

The mixed signal is then sent through a lowpass filter which removes the AC components of the modulated signal. This leaves only the DC signal that is proportional to the signal amplitude, here the signal can then be amplified using a DC amplifier.<\/p>\n

The output amplitude can be found from the signals sent through the mixer and lowpass filter. These can be found in either rectangular or polar coordinates. The amplitude R can be found from converting between the coordinates. For AM signals, only the amplitude or R is required (in polar coordinates); the phase of the signal can be ignored.<\/p>\n

 <\/p>\n

Pre-lab Exercises<\/h1>\n