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| Texas Instruments CD74HCT7046A Phase-Locked Loop IC |
Phase-Locked Loops. You probably wouldn't have heard of it unless you're studying electronics (like me) or you're extremely enthusiastic about such topics. It's just one of the many unsung heroes that quietly sat somewhere in your phone or computer, quietly keeping the components within to function properly without you knowing. In this post, I'd like to talk about this special circuit and how it revolutionised FM signal demodulation.
But first, let's have a look at what a Phase-Locked Loop is:
A Phase-Locked Loop (PLL) is a circuit that synchronises two signals (an input signal and a reference signal) so that the two signals will have the same frequency and phase. (The position of the wave, e.g. If two people are jumping side by side on a trampoline and they both go up and down at exactly the same time, their jumps are in phase (perfectly aligned); If one person is going up while the other is going down, their jumps are out of phase (not aligned).)
Here's how a basic PLL works:
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| Don't worry about the math formulas |
The phase detector will check if the two signals are in phase. If not, it measures the phase difference (how much are the two signals out of phase) and send a signal (usually a Voltage) across to the loop filter (usually a low-pass filter), which will smooth out the signal from the phase detector, which could be noisy or jumpy to create a steady control voltage signal for the synchronised oscillator (usually a Voltage-Controlled Oscillator (VCO)). The synchronised oscillator will then generate an output signal based on the control voltage signal from the loop filter. (e.g. If the loop filter sends a higher voltage, the VCO speeds up its output signal. If the voltage is lower, it slows down. This adjusts the output signal’s frequency and phase to match the reference signal.) The output will also be sent back to the phase detector so that the system can continuously comparing the input to the output, hence detecting and correcting any variations in phase or frequency. Once the output signal’s phase and frequency match the reference signal, the PLL is described as "Locked".
In short, A PLL keeps two signals in sync by adjusting one to match the other.
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| Signetics NE565 PLL IC, one of the earliest PLL implementation in an IC (Credit: Consolidated Electronics) |
Now that we know how a basic PLL works, let's look at where we might find them and why are they used. One great example would be your AirPods. As that they are connected to your iPhone through Bluetooth, the PLL is used to lock onto the exact frequency (usually around 2.4GHz) to communicate with your iPhone or iPad or whatever. This keeps the connection stable so you can listen to your Taylor Swift songs without cutting out, even if there’s interference from other devices. So, yes, PLLs work with other radio communication protocols as well, not just FM signals.
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| Sample implementation of a FM Demodulator using the LM565 PLL IC |
Now, to the bit where I would like to turn your attention to: FM Demodulation. The concept of using of FM (Frequency Modulation, i.e. encoding information in a signal by varying its frequency.) to broadcast data appeared in the 1933, invented by American engineer Edwin Armstrong. Earlier FM radio receivers include circuitry such as the Foster-Seeley discriminator and the Ratio detector, where both demodulates the FM signal by converting the change in frequency into corresponding voltage levels that represents the original audio with transformers, capacitors and diodes. (I'm not going into detail on the working principles of these circuits, but if you're interested, do look it up.)
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| Sample Circuit Diagram of the Foster-Seeley discriminator and the Ratio detector |
Both methods are fairly simple and doesn't require complex feedback systems found in PLLs. And most importantly, as electronics aren't as advanced back in the day, these circuits worked well enough for early FM radios at the time. But of course, they came with various drawbacks such as the circuit components (often bulky transformers and capacitors) requiring precise manual tuning and calibration to the centre frequency (the frequency of the unmodulated signal around which the signal’s frequency varies to carry data), which could drift over time or temperature, and it’s less effective at rejecting noise or interference.
Then by the 60s and 70s, the PLL technology emerged, and they replaced discriminators or earlier FM demodulation methods in most FM radios and communication systems. PLLs demodulate FM signals by comparing the phase of the incoming FM signal with the output from the synchronised oscillator. As the frequency of the FM signal changes, the phase difference between the two signals creates an error signal. As that error signal varies proportionally to the difference, it's essentially the demodulated audio signal. (e.g. if the FM signal’s frequency increases (representing a louder sound or higher pitch), the error voltage increases, mirroring the original audio signal.). In practice, that signal will have to be further smoothened using external filter circuits to resemble the original signal wave before modulation.
So, you see, PLLs are smaller (integrated in a tiny IC chip), cheaper (today, when it's so widely used), and more reliable in comparison. It automatically locks onto the FM signal’s frequency changes precisely and offer much better noise resistance, hence producing much cleaner audio and without the need of manual tuning or frequent recalibration compared to earlier methods. But still, it isn't perfect. It might have a slight time delay when locking, struggles to lock when the incoming FM signal is weak or noisy or if the FM signal shifts too rapidly beyond the PLL’s capture range. But nonetheless, PLLs offered a much more reliable, cheaper and versatile solution along with higher signal quality. it's also worth mentioning that PLLs not only transformed FM radio but also set foot in other use cases such as frequency synthesis and modern modulation schemes such as Quadrature Phase Shift Keying (QPSK) Modulation.
And that, ladies and gentlemen, is how PLLs revolutionised FM signal demodulation. What are your thoughts on the PLL technology? Do you think that the principles of PLLs influence future communication systems? Let me know in the comments below!
This post draws on the following resources:
- Phase-Lock Basics by William F. Egan (Wiley-Interscience, 1998)
- Modulation by F. R. Connor (Edward Arnold, 1973)
- University of Exeter Lecture Materials by Prof. Mustafa Aziz
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