FM Generation &

FM Detection Prepared by

Sam Kollannore U. Asst. Professor in Electronics M.E.S. College, Marampally

FM Generation • Prime Requirement • Subsidiary Requirement Methods • Direct method

– Capacitance / inductance of an LC oscillator tank is varied in proportion to the voltage supplied by the modulation circuits

– Reactance FET – Bipolar transistor – Vacuum tube – Varactor diode

• Indirect method – FM generation through Phase modulation (Armstrong method)

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Basic FET reactance modulator

• Impedance z seen at A-A is reactive – made inductive or capacitive by a single component change

• Value of this reactance is proportional to the transconductance of the device, which can be made to depend on the gate bias and its variation

• Connected across the tank circuit of the oscillator to be frequency modulated

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Theory

• To determine ‘z’, a voltage ‘v’ is applied to the terminals A-A between which the impedance is to be measured and the resulting current ‘i’ is calculated.

• Impedance = applied voltage/resulting current • To make the impedance to be pure capacitive

1. Bias network current ib must be negligible compared to the drain current. (ib << i)

2. Gate-to-Drain impedance (Xc) must be greater than the Gate-to-Source impedance (R) by more than five fold. (Xc>> R)

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observations

1. Equivalent capacitance Ceq depends on the device transconductance and can be varied by the bias voltage.

2. This capacitance can be adjusted to any value by varying the components R and C.

3. The expression gmRC has the correct dimensions of capacitance.

4. Gate-to-drain impedance Xc must be larger than the Gate-to-source impedance R.(ie. Xc >> R)

5. Resistive component for this particular FET reactance modulator will be 1/gm – vary with the applied modulating voltage. ie. acts as variable resistance and appear across the tank circuit of the master oscillator varying its Q and therefore its output voltage. ie. a certain amount of AM will be created. Therefore oscillator being modulated must be followed by an amplitude limiter.

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Types of Reactance Modulators

Name ZGD ZGS Condition Reactance Formula RC capacitive C R Xc >> R Ceq = gmRC

RC inductive R C R >> Xc Leq = RC/gm

RL inductive L R XL >> R Leq = L/gmR RL capacitive R L R >> XL Ceq = gmL/R

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Varactor Diode Modulator

Diode is reverse biased to provide the junction capacitance effect and since this bias is varied by the modulating voltage which is in series with it, the junction capacitance will also vary, causing the oscillator frequency to change accordingly.

Limited applications Used together with a reactance modulator to provide

automatic frequency control. Used for remote tuning

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Stabilized Reactance Modulator - AFC

Reactance modulator cannot operate on crystal controlled oscillator.

But it must have the stability of a crystal oscillator if it is to be the part of a commercial transmitter

Achieved by Stabilized Reactance modulator - corrects any drift in the average frequency of the master oscillator.

Reactance modulator operates on the tank circuit of an LC oscillator

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• Isolated by a buffer • Amplitude limiters for noise elimination • Amplified by Class C power amplifiers • Fraction of output from the limiter is fed to a mixer • Difference signal from the mixer is amplified and fed to Phase

Discriminator • Discriminator produces a DC voltage corresponding to its

input signal frequency – Dc correcting voltage • Output of discriminator is connected to the reactance

modulator so that it corrects any drift in the average frequency of the master oscillator. www.vidhyaguru.com

• Crystal oscillators cannot be successfully frequency modulated

• LC oscillators are not stable enough for communication/broadcast purpose

• Stabilized reactance modulators are complex in nature. • So indirect method is used – FM generation through

Phase modulation. • Based on Crystal oscillators – shows high frequency

stability – often used.

Indirect Method of FM Generation Armstrong Method

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• System terminates at the output of combining network

• Remaining blocks (mixers and multipliers) are used to convert NBFM into WBFM.

Indirect Method of FM Generation Armstrong Method

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Principle of operation – using vector diagrams

AM signal (amplitude varies – no

phase/frequency variation)

AM voltage added to an unmodulated voltage keeping 900 apart – to produce some phase modulation (complex

and nonlinear)

Solution: carrier of AM removed – Two

sidebands are added to the unmodulated voltage

As the modulation increases, phase deviation also increases – thus phase modulation is obtained

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• Our requirement is FM – so PM may be changed into FM by prior bass boosting of the modulation – so the modulating voltage is equalized before it enters the balanced modulator – using a simple RL equalizer

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Effect of frequency changing on an FM signal

• Effect of frequency doubler – both centre frequency and deviation are increased by the same factor.

• fc±δ → 2fc+2δ and 2fc-2δ ie. Frequency deviation is doubled to ±2δ. Hence modulation index is also doubled.

• Effect of mixer – centre frequency changed without changing its maximum deviation.

• fc±δ mixed with fo → fc-fo+δ and fc-fo-δ (considering the difference signal)

• It is possible to raise the modulation index 9 fold without affecting the centre frequency by multiplying both by 9 and mixing the result with a signal having frequency 8 times the original frequency

• Further considerations in Armstrong System – refer Kennedy….

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Basic FM Demodulators Conditions 1. Conversion should be done efficiently 2. Conversion should be done linearly 3. Detection circuit should be insensitive to amplitude

changes 4. Should be simple in operation with very simple

adjustments only Basic method: Converting the Frequency modulated IF voltage of

constant amplitude into a voltage which is both frequency and amplitude modulated

This voltage is then applied to a diode detector which reacts only to amplitude changes and ignores the frequency variations

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Slope Detection

• Frequency modulated signal fed to a tuned circuit whose resonant frequency is tuned to one side of the centre frequency of the FM signal.

• Output will have an amplitude proportional to the frequency deviation of the input carrier.

• This voltage is applied to a diode detector with an RC load of suitable time constant

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Drawbacks of slope detector

• Ineffecient • Linear only along a limited frequency range • Reacts to all amplitude changes • Difficult to adjust (two different tunings)

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Balanced Slope Detector

• Travis detector/Triple tuned discriminator/Amplitude discriminator • Uses two slope detectors connected back to back to the opposite

ends of a centre tapped transformer • Hence fed 1800 out of phase

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• Primary tuned to fc (which is the IF) • Top secondary tuned above the IF by an amount δf ie fc + δf • Bottom secondary tuned below the IF by an amount δf ie fc – δf • Diode detector with RC load • Output taken across the series combination of the two loads • Working principle – explanation

– Input freq = fc; Voltage across T1 and T2 are same ; V0 = 0 – Input freq = fc + δf; Output of D1 > output of D2; V0 is positive and max. – Input freq = fc – δf; Output of D2 will be large negative ; Output of D1 is small

positive; V0 is negative and max.

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S-shaped Frequency modulation characteristics

• More efficient than slope detector • Difficult to align (Three different tuning) • Better linearity – but not good enough • No amplitude limiting

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Phase Discriminator

• Centre tuned discriminator/Foster –Seeley discriminator • Advantages

– Primary and secondary tuned to the centre frequency of the incoming signal ie. simple to align

– Better linearity than slope detector

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1. Voltage applied to each diode = Primary voltage + Corresponding half of secondary voltage

2. Primary voltage and secondary voltages are i) exactly 900 out of phase when fin = fc

ii) less than 900 out of phase when fin > fc

iii) more than 900 out of phase when fin < fc 3. Since the phase difference between primary and secondary

windings differ, then the corresponding vector sums will also differ.

i.e. individual diode output voltages will be equal only at fc. At all other frequencies, the output of one diode will be greater than that of the other.

Thus the magnitude of the output will depend on the deviation of the input frequency from fc.

Phase Discriminator – Principle

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Phase Discriminator - Phasor Diagrams • L3 is an RF choke having large reactance compared to C & C4

• Hence voltage across L3; VL≈V12

• Voltage applied to D1; Vao = Vac + VL = ½ Vab + V12

• Voltage applied to D2; Vbo = Vbc + VL = - Vac + VL = -½ Vab + V12

• Now Va’b’ = Va’o – Vb’o

α Vao – Vbo

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Frequency response of the phase discriminator

Drawback • Does not provide any amplitude limiting

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Ratio Detector

• Modified discriminator circuit to provide amplitude limiting

• Sum voltage Vao + Vbo remains constant and the difference voltage varies

• Any variations in the magnitude of this sum voltage is considered as spurious – suppressed in Ratio Detector – thus amplitude limiting is achieved.

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Basic changes • One of the diode is reversed • Large capacitor C5 • Output is taken from a different point

Operation • With Diode D2 reverse biased, O is now positive w.r.t. b’

→ Va’b’ is now a sum voltage rather than difference as in the phase discriminator.

• C5 will keep this sum voltage constant. www.vidhyaguru.com

• Output is now taken between O and O’ with O as ground point.

• R5 = R6

• V0 = Vb’o’ – Vb’o = Va’b’/2 – Vb’o

= (Va’o + Vb’o)/2 – Vb’o

= (Va’o – Vb’o)/2

Behaves identical to that of discriminator Output characteris cs → S - shape

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Amplitude Limiting • If V12 is a constant DC – no current flowing to charge/discharge

the capacitor C5 • If V12 tries to increase, extra diode current flows, charging C5. • Voltage Va’b’ remains constant at first – because voltage across C5

does not respond suddenly. • Current in the diode’s load has risen, but the voltage across the

load has not changed. i.e the load impedance has decreased • Secondary of the ratio detector transformer is more heavily

damped – Q falls – gain of the amplifier driving the ratio detector falls - thus counteracting the initial rise in input voltage.

• Reverse operation occurs when the input voltage fall – i.e. gain of the driving amplifier rises.

• Diode Variable damping - varying the gain of an amplifier by changing the damping of its tuned circuit – maintaining a constant output voltage.

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