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 of 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.