Transcript
Non Sinusoidal Signals Noise Amplifiers Detectors
IRTS Region 4 Module 7
Recap of Sinusoidal Signals
• To specify a wave its Amplitude, i.e., Vpeak [V max] and the time to complete one complete cycle (period) is needed • The frequency in Hertz (Hz) is the number of complete cycles in one second IRTS Region 4 Module 7
Phase
• If two wave have exactly the same frequency and cross the zero line at different times they have a phase difference • The red wave crosses the zero line ninety degrees before the black. It leads the black by 90 or ¼ cycle. The black lags the red by 90 IRTS Region 4 Module 7
Harmonics
• A second wave whose frequency is an exact multiple of another is called a harmonic. The other (lower frequency) is the fundamental • The example shown is the second harmonic (in red) of the fundamental. It is a twice the fundamental frequency IRTS Region 4 Module 7
D.C. Component
• The wave shown is not symmetrical around 0 volts but has an offset or d.c. component added to it
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Non-Sinusoidal Signals
• An audio speech signal is the sum of sinusoids of a range of frequencies (20Hz-20kHz) and differing amplitudes • For speech, frequencies in the range from 300 Hz to around 2.7kHz make a significant contribution to intelligibility and the signal is tailored to this range for amateur use to conserve bandwidth IRTS Region 4 Module 7
Non-Sinusoidal Signals
• The square wave comprises an infinite series of odd-harmonics (3rd, 5th, 7th) of decreasing amplitude IRTS Region 4 Module 7
Noise
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Noise • Random motion of free electrons in conductors and semiconductors increases with temperature - known as thermal noise • PN = kBTB , kB is Boltzmann’s constant, T the temperature above absolute zero, B bandwidth (Hz) • Thermal noise power, does not depend on frequency, but on temperature and bandwidth. Noise power per Hertz bandwidth, i.e., noise power density is therefore equal throughout the frequency spectrum, depending on T. It is white noise IRTS Region 4 Module 7
Noise • Thermal noise generated in the early stages of a receiver (receiver thermal noise) is of significance at VHF and UHF • At lower frequencies, atmospheric noise (band noise) is up to 40dB greater than thermal • Noise power is directly proportional to receiver bandwidth
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Amplifiers
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RF (Radio Frequency) Amplifier
• • • •
Amplifies weak input signal Provides some selectivity Should be low noise Has a manual or automatic gain control (AGC) or switched attenuator to prevent overload by strong signals IRTS Region 4 Module 7
IF Amplifier
• IF – Intermediate Frequency • Provides gain and Selectivity at IF • Fixed selective tuned circuits at IF • Usually preceeded by a crystal or ceramic filter IRTS Region 4 Module 7
Audio Amplifier
• Amplifies the audio signal • Has manual gain control
• Provision for loudspeaker, headphone or line output IRTS Region 4 Module 7
AF Small Signal Amplifier
+V
Output Input
Low Zin ~1K
Medium Zout ~ 5K
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Power Amplifier
• Class C amplifier (non-linear, but most efficient) may be used for CW • Tuned network to match output impedance to 50 ohm of feeder/antenna • Provides harmonic filtering • May have Pi tank Circuit with tune/load controls IRTS Region 4 Module 7
Linear Amplifier
• SSB signal passes through buffer/driver stage to Linear Power Amplifier • Must be linear (Class A or, more efficiently, AB1, AB2) • Must not be over driven as non-linearity and thus splatter will occur
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Power Amplifier
• Does not need to be linear for FM • Has matching and filtering network
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Amplifier Class & Bias • Class-A, B, AB and C are defined by the bias and operating region of the transistor IC
IC Distorted Output
Output
VBE Input signal normal bias voltage
VBE Input signal low bias voltage
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Amplifier Classes • Class-A Biased well on for high fidelity but also results in low efficiency and high heat dissipation on poweramps • Class-B Gives only only half the waveform, so usually used in Push-Pull configurations. Fairly efficient, but can get crossover distortion
• Class-AB A variation of Class-B with but biased on each transistor to conduct for slightly more than half cycle for better fidelity
• Class-C Nonlinear but efficient - high distortion needs filtering Used for FM and in GSM mobile phones • Other Classes exist also
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Class B (Push Pull)
•Incorrect biasing will result in Crossover Distortion
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Transmitter Amplifiers • Modulation schemes which carry information in their amplitude require good linearity in all stages, or else distortion will occur • AM & SSB require linear amplification eg Class-A • An FM-only transmitter does not need to be linear, so a Class-C amplifier can be used which is more efficient • CW is only on or off, so Class-C is also fine for this.
• Data Modulation: Frequency or Phase-shift keyed schemes are like FM and could use Class-C. If Amplitude changes then a linear amplifier is needed • Non-linear amps need filtering to avoid harmonics or bandwidth spread IRTS Region 4 Module 7
Poweramp Circuits • Modulation schemes which carry information in their amplitude require good linearity in all stages, or else distortion will occur
• AM & amps need filtering to avoid harmonics or bandwidth spread
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Automatic Level Control • Splatter, distortion and damage can occur if a Poweramp is overdriven • Heat dissipation and output power varies with nature of drive and modulation. • Power in SSB Transmissions is given is PEP (Peak Envelope Power) so may need to back-off transmissions on SSB unless speech processors are used to average out voice peaks
• Automatic Level Control, ALC, can display the need to reduce the drive level, or do so automatically. • Excess SWR (Standing Wave Ratio) detection is often also built in as a protection measure IRTS Region 4 Module 7
Detectors
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Detector (Demodulator)
• Recovers modulation from the signal • Amplitude modulation (AM) detector is often a simple diode rectifier IRTS Region 4 Module 7
Product Detector
• For CW and SSB a product detector (mixer) is used • SSB – Oscillator on IF frequency is mixed with IF signal to give audio as difference (Carrier Insertion Oscillator, CIO) • CW – Oscillator offset from IF frequency by say, 800Hz, is mixed giving an audible beat note (Beat Frequency Oscillator, BFO) IRTS Region 4 Module 7
FM Detector
• In the case of FM (frequency modulation) the modulation causes the carrier frequency to vary • Amplitude variation due to propagation or noise is first removed by a LIMITER if a Foster-Seeley Discriminator is used as a demodulator • Ratio detector does not require a Limiter IRTS Region 4 Module 7
