Transcript Receivers - TalkTalk
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