Turn frequency back into voltage. Three ways.
Ten minutes of reading. Move the sliders, then the three detectors on the trainer will make sense.
What a frequency demodulator does
The amplitude of an FM wave carries no information, so a plain diode envelope detector (Lab 6) gives only a flat line. A frequency demodulator needs a part whose output changes with frequency: a tuned circuit on its slope, a phase shift that depends on frequency, or an oscillator that is forced to follow the carrier. The IT-4103 has one of each, and in this lab you use all three on the same varactor FM carrier.
Detuned resonant circuit: FM to AM on a slope
A parallel tuned circuit is deliberately tuned above the carrier, so the 455 kHz carrier sits half way up the left hand slope of its response (manual Figure 1). When the carrier frequency rises the operating point moves right, up the slope, and the output amplitude grows; when it falls, the amplitude shrinks. The FM wave comes out as an AM wave whose envelope follows the audio.
The slope is straight only over a short stretch. Push the carrier towards the peak, or swing it too far, and the envelope no longer follows the audio: the recovered wave flattens or even doubles in frequency. On the trainer the tuned circuit peaks near 480 kHz.
The diode, the low pass filter and the amplifier
The diode detector follows the envelope but leaves a ripple at the carrier frequency, and its output sits on a DC level. The Low Pass Filter removes the 455 kHz ripple; the Pre-Amplifier blocks the DC and amplifies the audio (gain 5 on the trainer). The same filter and amplifier follow the quadrature detector and the PLL. The filter corner is near 3 kHz, so a higher audio frequency is cut as well: that is the drop the manual asks you to notice in step 6.
Quadrature detector: frequency to phase
The input goes two ways: straight to a multiplier, and through a tuned phase shifting network to the other multiplier input. At the rest frequency the network shifts the signal by exactly 90°, so the two inputs are in quadrature and the average of their product is zero. Above the carrier the shift is less than 90° and the product has a positive average; below it, more than 90° and a negative average.
The multiplier output is the cosine of the phase difference. Near 90° the cosine changes almost in a straight line with frequency, which is why this detector has less distortion than the slope detector. A small capacitor in series with the signal and a parallel tuned circuit form the network on the trainer (tuned to 455 kHz).
Phase locked loop: an oscillator that follows
A phase detector compares the incoming carrier with the output of a voltage controlled oscillator. Its output, smoothed by the loop filter, is the control voltage of the VCO. If the VCO falls behind, the control voltage rises and speeds it up, until the VCO runs at exactly the input frequency, 90° out of phase. The loop is then locked.
Locked, the control voltage must follow the input frequency: v = (fin − f0) / Kv. For an FM input that control voltage is the audio. With no input the VCO runs at its free running frequency f0 (455 kHz on the trainer, Kv = 50 kHz per volt).
An unlocked loop only grabs a carrier that comes close to f0: that band is the capture range. Once locked it follows the carrier much further: the lock range. On the trainer the capture range is ±20 kHz and the lock range ±40 kHz.
Comparing the three detectors
| Detector | How it works | Strength | Weakness |
|---|---|---|---|
| Detuned resonant | slope of a tuned circuit turns FM into AM, then a diode detector | simplest circuit | slope is curved, so it distorts; any amplitude noise passes straight through |
| Quadrature | frequency dependent 90° phase shift, multiplier, filter | less distortion, better noise rejection, easy to put in one IC | needs a well tuned phase shift network |
| Phase locked loop | VCO forced to follow the carrier; its control voltage is the audio | very linear, almost immune to noise, no tuning, one IC | works only inside its lock range; must first capture the carrier |
Real FM receivers put an amplitude limiter before the detector, so noise riding on the amplitude never reaches a detector that would pass it on. The IT-4103 has one (Amplitude Limiter block).
FM broadcast receivers mix the station down to a 10.7 MHz intermediate frequency and use a quadrature or PLL detector inside the receiver chip.
The same loop as on the trainer is sold as one chip (the 565 or the 4046 family) and used in tone decoders, FM stereo decoders and clock recovery.
On the bench
Every experiment starts the same way as Lab 9: check the audio on CH1, tune the varactor carrier to 455 kHz on CH2 (1 µs/div, trigger on CH2), then patch the audio in. After the detector chain is in place, move CH2 to the Pre-Amplifier O/P, slow the time base to about 500 µs/div and trigger on CH1, so the original and the recovered audio sit on the screen together.
What you should see
| Setting (1 kHz audio) | Detuned resonant | Quadrature | PLL |
|---|---|---|---|
| Audio 0.5 V p-p (±10 kHz swing) | about 1.5 V p-p, slightly uneven | about 2 V p-p, clean | about 2 V p-p, clean |
| Audio 1 V p-p | about 2.8 V p-p, tops start to flatten | about 3.5 V p-p | about 4 V p-p, clean |
| Audio near MAX (1.8 V p-p) | clearly distorted, swing reaches the peak | rounded tops | about 7 V p-p, still clean |
| Audio raised to 5 kHz | all three drop to about one fifth: the Low Pass Filter cuts above about 3 kHz | ||
| Carrier moved to about 480 kHz | output at twice the audio frequency | smaller, one sided output (the DC shift is removed by the Pre-Amplifier) | still locked if you got there slowly (inside the lock range), clean audio |
Values are those of the simulator model of the IT-4103 with the varactor carrier tuned to 455 kHz. A real trainer differs a little from unit to unit; the way each control changes the picture is the same.
Check yourself
Q1Explain the working principle of detuned resonant circuits.
A parallel tuned circuit is tuned a little away from the carrier, so the carrier falls half way up one slope of its response. As the FM carrier frequency swings, the operating point moves up and down the slope and the output amplitude follows the frequency. The FM signal becomes an AM signal, and a diode detector with a low pass filter recovers the audio from its envelope.
Q2Briefly mention the shifting of the operating point as the incoming frequency changes (Figure 1).
With the carrier on the left hand slope, a higher incoming frequency moves the operating point to the right, towards the peak, so the output amplitude increases; a lower frequency moves it to the left and the amplitude decreases. The amplitude swing therefore follows the frequency swing, as long as the point stays on the straight part of the slope.
Q3Explain the purpose of the low pass filter before the output in these detectors.
Each detector leaves a ripple at the carrier frequency (and at twice it) on top of the audio. The low pass filter passes the audio and removes that ripple and other high frequency products, so the output is a clean copy of the message. On the trainer it also limits the audio band, so a 5 kHz tone comes out smaller than a 1 kHz tone.
Q4Suggest an electronic component which can be used as a phase shifting circuit (Figure 3).
A small series capacitor feeding a parallel LC tuned circuit (tuned to the carrier) gives 90° at the carrier and a phase that changes with frequency. An RC network, or a delay line or all pass network, can also be used as the phase shifter.
Q5Explain how the phase locked loop tracks the incoming signal and acquires the carrier.
The phase detector compares the input with the VCO and produces an error voltage, which the loop filter smooths and applies to the VCO. If the input is within the capture range, the error voltage pulls the VCO frequency to the input frequency (acquisition) and the loop locks. Once locked, any change of the input frequency changes the phase error, which changes the control voltage so that the VCO follows (tracking). For FM, that control voltage is the demodulated audio.
Q6Suggest a logic or electronic circuit that could perform the phase comparator function (Figure 4).
An analog multiplier (for example a Gilbert cell or a balanced mixer) gives an average output proportional to the cosine of the phase difference. For square waves an exclusive OR gate does the same job, and an edge triggered phase frequency detector (two flip flops with a reset) is used in many PLL ICs.
Q7What is the free running frequency of the voltage controlled oscillator in a phase locked loop?
It is the frequency at which the VCO oscillates when there is no input signal, so the control voltage is zero (the centre of its range). It is set by the timing resistor and capacitor of the VCO. On the trainer it is 455 kHz, the carrier frequency the loop is meant to receive.
Q8Differentiate between the lock range and the capture range of a phase locked loop.
The lock range (tracking range) is the band of input frequencies over which a loop that is already locked stays locked. The capture range (acquisition range) is the band over which an unlocked loop can acquire lock. The capture range is never larger than the lock range, because the loop filter limits how far the beat note can pull an unlocked VCO. On the trainer: lock ±40 kHz, capture ±20 kHz.
Q9Of the three detectors, which do you think is the most accurate and precise? Explain why.
The phase locked loop. Its output is the control voltage of the VCO, which is directly proportional to the input frequency over the whole lock range, so it has the lowest distortion. It ignores amplitude changes and most noise, needs no tuning, and tracks the carrier. The quadrature detector comes next; the detuned resonant circuit is the least accurate because its slope is curved and it responds to amplitude noise.