Chapter 3: Amplitude Modulation Reception EET-223: RF Communication - - PowerPoint PPT Presentation

Chapter 3: Amplitude Modulation Reception

EET-223: RF Communication Circuits Walter Lara

Tuned Radio Frequency (TRF) Receivers

• Simplest/Oldest AM receiver (see Fig 3-1)
• Consists of:

– RF Amplifier:

• Amplifies weak signal from antenna
• Low noise characteristics
• Tuned to carrier and sideband frequencies

– Detector: extracts the intelligence from the AM signal – Audio Amplifier: provides sufficient power to drive loudspeaker

Figure 3-1 Simple radio receiver block diagram.

Receiver Characteristics

• Sensitivity: minimum input signal (voltage) required to

produce a specified output signal

– The lower, the better – Must be greater than noise floor (input noise) – Range from mV (cheap ) to µV (expensive)

• Selectivity: extend to which a receiver can differentiate

between desired signal and other undesired signals or noise

– Optimum value equals bandwidth needed for carrier and sidebands (e.g., for AM, 30-KHz) – TRF receivers suffer from variable-selectivity problem (because

• f tuned circuits)

AM Detection (Demodulation)

• Non-linear device (e.g., BJT or Op Amp) used for

detection which results on components at:

– Carrier frequency (fc) – Lower-side frequency (fc - fi) – Upper-side frequency (fc + fi) – DC Component – Intelligence frequency (fi)

• LPF used to suppress RF components leaving only

intelligence and DC components

Figure 3-2 Nonlinear device used as a detector.

Diode Detector

• Simple and effective
• Nearly perfect nonlinear resistance characteristic
• Advantages:

– Can handle high power – Acceptable distortion levels – Highly efficient (~90% achievable) – Support Automatic Gain Control (AGC) circuits

• Disadvantages:

– Tuned circuit power absorbed by diode (reduces selectivity) – Doesn’t provide amplification

Figure 3-3 Diode detector.

Superheterodyne Receivers

• Developed in the early 1930, still dominant
• Advantages:

– Constant selectivity over wide range of received frequencies (unlike TRF’s) – Better sensitivity – Lower distortion (better linearity) – Provide amplification

• Disadvantages:

– More complex, costly – Image frequency problem (more later)

• Block Diagram shown at Fig 3-6 (see next side)

Figure 3-6 Superheterodyne receiver block diagram.

Superheterodyne Receiver Components

• Main components are:

– RF Amplifier: pre-amplifies RF signal (if required) – Local Oscillator (LO): provides steady sine wave – Mixer (aka first detector): mixes RF signal with LO sine wave to produce an RF signal at fixed/known frequency – Intermediate Frequency (IF) Amplifier: provides bulk of RF amplification at fixed frequency (constant BW, avoiding variable-selectivity problem) – Detector: extracts intelligence from RF signal – Audio/Power Amplifier: amplify as need by speaker

Superhereodyne Receiver Frequency Conversion

• The Mixer, being a nonlinear device, produces the

following components:

– Frequencies at all original inputs: fLO, fc , fc + fi , fc - fi , – Sum and difference components of all original inputs: fLO ± fc , fLO ± (fc + fi ), fLO ± (fc – fi ) – Harmonics of all above frequencies – A DC component

• The IF Amplifier is tuned to only accept components

around 455 KHz: fLO – fc , fLO – (fc + fi) fLO – (fc - fi)

• The IF Amplifier output is a replica of original AM

signal, except that carrier frequency is now 455 KHz

Figure 3-7 Frequency conversion process.

Figure 3-8 Frequency conversion.

Superhereodyne Tuning

• Center frequency of tuned circuit at front end of IF

Amplifier is always constant (455 KHz)

• Center frequency of tuned circuit at front end of

Mixer is adjusted to select incoming radio station

• LO frequency tracks tuned frequency to keep a

constant difference of 455 KHz

• Front-end circuits are made track together by using

variable ganged capacitors (see Fig 3-9)

• Alternatively, varactor diodes can be used. These

have small capacitance that varies as function of their reverse bias voltage (see Figs 3-11 & 3-12)

Figure 3-9 Variable ganged capacitor.

Figure 3-11 Varactor diode symbols and C/V characteristic.

Figure 3-12 Broadcast-band AM receiver front end with electronic tuning.

Superhereodyne Image Frequency Problem

• Frequency conversion performed by mixer-oscillator

sometimes allows undesired station to be fed into IF Amplifier

– See Fig 3-13 for problem illustration

• Designing receivers with high image frequency

rejection is an important design consideration

• Not a major problem on standard broadcast since

stations properly spaced to allow good selectivity

– See Fig 3-14 for illustration

• If needed, double conversion technique can be used

to solve problem (details on Chapter 7)

Figure 3-13 Image frequency illustration.

Figure 3-14 Image frequency not a problem.

Automatic Gain Control (AGC)

• Lowers amplifier gain when strong signal amplitude

is present to keep transducer output constant

• Avoids having to adjust the volume control for weak

vs strong signals

• Needed because signal strength can vary due to

many factors such as:

– Channel-to-channel variance on signal strength – Changes on weather and ionosphere conditions – Changes on receiver location (e.g., AM car radio)

• Recall Diode Detector has built-in support for AGC

(see how at Figs 3-18 & 3-19)

Figure 3-18 Development of AGC voltage.

Figure 3-19 AGC circuit illustration.

AM Receiver Analysis

• Typically, power gain or attenuation of receiver

stages is specified in dBm or dBW. Recall:

– dBm = 10 log P / 1 mW – dBW = 10 log P / 1 W

• Dynamic Range is a measure of how well a receiver

can handle large and small signals at the same time

– Computed as dB difference between largest tolerable input level and its sensitivity level (minimum level) – State-of-the-art receivers perform at ~100 dB

Figure 3-26 Receiver block diagram.