East Kent Radio Society EKRS

1

Intermediate Course (4) Receivers Karl Davies

Receivers

2 Summary  Block diagrams of receivers  The Superhet receiver  Detection of AM, FM, and SSB  How a Diode AM Detector works  Use of BFO and CIO  AGC

Crystal Receiver

3  Block diagram  Single tuned circuit; poor selectivity   No gain; poor sensitivity Requires large aerial and earth; only receives strong signals

Tuned circuit Diode detector Earphone

Crystal Receiver Circuit

4    L1, C1 - Tuned circuit – selects signal D1 – Detector diode – demodulates C3, R1 – Low-pass filter for audio Antenna C1 100pF L1 Coil C2 500pF D1 AA119 C3 100pF R1 100k X1 Crystal Earphone RF Earth

TRF Receiver (Tuned Radio Frequency)

5  Block diagram     RF amplifier gain increases sensitivity One or more tuned circuits All the gain is at one frequency – feedback is a problem AF amplifier provides more power for loudspeakers

RF amplifier Demodulator AF amplifier BFO

Superhet Receiver

6  Block diagram    Mixer changes variable RF frequency to fixed IF frequency IF amplifier provides selectivity with several tuned circuits Fixed IF can use non-tunable crystal or ceramic filters

Mixer Local oscillator IF amplifier Demodulator AF amplifier BFO

Mixer as a Converter

7      Mixer may be used as a frequency converter Changes the selected RF frequency to the IF frequency using a tunable LO signal.

Mixers have spurious responses – image frequency, half the RF… LO can be above or below the RF IF can be above or below the RF ~

RF 145MHz Mixer

~

LO 123.6MHz

145MHz –123.6MHz=21.4MHz

IF frequency

Image frequency is 123.6MHz-21.4MHz=102.2MHz

AM Envelope Detection

8   Demodulation of a modulated audio signal Detector output follows envelope of RF  Otherwise known as an “envelope detector”

Envelope

Modulated RF Demodulated Audio

Diode Detector Circuit

9    Diode D1 rectifies AC into DC C1/R1 is a lowpass filter - filters out the RF D1 conducts on positive half-cycle only Output

D1

Time

RF Input C3 100pF R1 100k Demodulated AF Output

Input

Diode Detector Operation

10  Volts and Current in the diode detector (SPICE Simulation) 2 D1 cathode output 1 0 -1 -2 800 600 400 200 0 0 0.5

1 1.5

Diode conducts on peaks only 2 2.5

3 3.5

D1 anode input

Anode

D1 current 4 Time/µSecs

Cathode

CW Demodulation - BFO

11  BFO = Beat Frequency Oscillator  Slightly offset BFO added to carrier to generate the “beat note”

+ Carrier Wave Envelope Detector BFO

…the origins of the “BFO” !

Beat Note

SSB Demodulation

12     SSB filter selects only the wanted sideband Product detector mixes to baseband CIO is at the frequency where carrier would have been Product detector is a balanced mixer

IF Sideband Filter IF Amp Product Detector AF Amp CIO = Carrier Insertion Oscillator

~

CIO

USB Demodulation

13  SSB demodulation is essentially mixing to baseband Lower Sideband Upper Sideband Frequency MHz

IF Filter

• Unwanted sideband may contain noise and other signals • Mixing with the carrier frequency is product detection Upper Sideband Frequency MHz

Mixer

Upper Sideband Frequency

LSB Demodulation

14  SSB demodulation is essentially mixing to baseband Lower Sideband Upper Sideband Frequency MHz

IF Filter

• Typically, IF filter is not moved; the local oscillators are offset.

• When mixed down, the LSB spectrum becomes inverted.

Lower Sideband Frequency MHz

Mixer

Lower Sideband Frequency

Demodulation Summary

15   CIO = Carrier Insertion Oscillator    A fixed local oscillator used to demodulate SSB It reinserts the carrier that was removed in the transmitter Wanted sideband is directly translated directly to Audio BFO = Beat Frequency Oscillator   A slightly variable local oscillator used to demodulate CW CW demodulated by envelope detector  BFO/Detectors can resolve SSB. A product detector can resolve CW  Two Issues   Input Signal Levels can vary greatly Detection methods described so far are for Amplitude based modulations, not FM

AGC

16    AGC = Automatic Gain Control AGC adjusts the gain to keep carrier level constant Signal level varies widely, but audio volume stays constant

RF Amp Mixer LO IF Amp AGC Circuit Demod AF Amp

FM Receiver

17    Uses FM Discriminator to detect small frequency deviations FM receivers use IF limiter amplifiers; no AGC needed Squelch detects high-frequency noise and gates audio

Mixer Local Oscillator IF amplifier Discriminator AF amplifier Squelch circuit