* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download + v if
Standing wave ratio wikipedia , lookup
Transistor–transistor logic wikipedia , lookup
Analog television wikipedia , lookup
Analog-to-digital converter wikipedia , lookup
Schmitt trigger wikipedia , lookup
Superheterodyne receiver wikipedia , lookup
Cellular repeater wikipedia , lookup
Index of electronics articles wikipedia , lookup
Wien bridge oscillator wikipedia , lookup
Phase-locked loop wikipedia , lookup
Operational amplifier wikipedia , lookup
Audio power wikipedia , lookup
Dynamic range compression wikipedia , lookup
Crystal radio wikipedia , lookup
Power electronics wikipedia , lookup
Continuous-wave radar wikipedia , lookup
Valve audio amplifier technical specification wikipedia , lookup
Radio receiver wikipedia , lookup
Resistive opto-isolator wikipedia , lookup
Switched-mode power supply wikipedia , lookup
Valve RF amplifier wikipedia , lookup
Single-sideband modulation wikipedia , lookup
Radio transmitter design wikipedia , lookup
Opto-isolator wikipedia , lookup
Detector Input Impedance Consider an idealized, lossless detector, driven by an unmodulated carrier: 1:a + + avif (0-pk) vif (0-pk) _ RIN RL _ RL 2a 2 RDET IF (0 P ) + PL avif (DC) 2 2 v IF RMS 2 a RL _ RL 2 av 2 RL 2 vIF RMS R L 2a 2 PIF Minimum Power to the Detector For a modulated signal, the the minimum peak AC voltage into the detector must exceed the diode turn-on voltage. If the carrier voltage coming out of the final IF amplifier is EC(0-pk), then aEC(1-m) > VD ; or EC VD a 1 m) The minimum carrier power required is 2 VD 2 a 1 m VD2 1 EC (min) PC (min) 2 RIN RL 1 m 2 RL 2 2 2a The minimum total power into the detector must be: 2 V PDET (min) PC (min) 1 m2 2 D RL 1 m 2 2 2 1 m Note that the minimum total power required for undistorted output is not affected by the transformer turns ratio! Minimum Receiver Gain In terms of sideband power, PIF (min) VD2 1 m2 2 PSB , DET (min) 2 2 2 2 m 1 RL 2 m 1 1 m VD2 RL m2 2 V 2 m 2 V 2 1 2 D D 2 2 R 1 m 2 R 1 m 1 1 m L L The receiver should have sufficient total gain (GT) such that the signal out of the final IF amp will be sufficient to drive the detector without distortion whenever the signal level entering the antenna has sufficient power to make the detector output exceed the minimum required output SNR (12 dB for voice communication). This occurs when the sideband power entering the antenna is: PSB , ANT (dBW ) kT0 B(dBW ) NF (dB) SNRMIN (dB) Minimum Receiver Gain (cont) Expressing PSB,DET in dB, we have: VD 2 PSB, DET dBW 10log 20log 1 m 1 2 RL Note that the first term on the RHS only reflects detector parameters, and the second term reflects only modulation index. It is convenient to refer to the first term as “Reference Detector Power”, PDR(dBW), as it captures the two important detector parameters in one number. The minimum required total gain for the receiver is: GT (dB) PSB , DET (dBW ) PSB , ANT (dBW ) Receiver Sensitivity Receiver Sensitivity is a key specification which defines the minimum signal level required at the receiver input to produce a specified SNR at the detector (demodulator) output. It is usually expressed in dBf (femto-watts) or microvolts (RMS, 50 ohm source) . We have already determined the minimum sideband power required at the receiver input to produce a specified SNR , so total power required would be: PSEN W PSB , ANT W 2 m 2 1 PSEN dBf PSB , ANT dBW 10 log 2 m 2 1 150dB VSEN VRMS PSEN W 50 VSEN VRMS VSEN VRMS 106 Example Example: An AM (m = 0.6) receiver has the following characteristics: Detector RL = 5000 Detector VD = 0.2 V B = 20 kHz NF = 15 dB 10 log10 2 m 2 1 8dB 20 log10 1 m 1 3.5dB Threshold SNR = 12 dB RF Front End kT0 B 161dBW IF Strip SNR = 12 dB @ PANT = PSEN Detector PSB,ANT VD2 0.2 PDR 4W 54dBW 2 RL 2 5K 2 PSENS dBW 161dBW 15dB 12dB 8dB 126dBW 24dBf 0.25 pW PSB , DET dBW PDR (dBW ) 3.5dB 50.5dBW VSENS 0.25 pW 50 3.5V GT dB PSB, DET PSB, ANT 50.5dBW 143dBW 92.5dB Dynamic Range From an AC Load Line Analysis, we can determine the maximum 0-Pk amplitude of vIF < VIF(max). If we are dealing with a modulated AM signal, the maximum 0-Pk amplitude of vIF is EC(1+m) < VIF(max). We have previously determined that to avoid negative peak clipping, the minimum 0-Pk AC voltage into the detector must exceed the diode turn-on voltage: aEC(1-m) > VD . VIF max VD EC a 1 m 1 m Dynamic Range The ratio of maximum to minimum signal range for undistorted output is called “Dynamic Range”. Expressed in dB it is: VIF (max) 1 m EC (max) 20 log10 20 log10 a E (min) V 1 m D C Example: The final amplifier in the IF strip of our previous receiver will accommodate a maximum AC output of 6 volts 0-pk, and has an output impedance of 100 ohms. Since RDET = RL/2 = 2500 , the transformer should have a = 5 to match the IF amp to the detector. For modulation index m = 0.6, the dynamic range is: 6 1 0.6 20log10 5 15.7dB 0.2 1 0.6 Automatic Gain Control The previous example uses reasonable numbers, and the result, 15.7 dB, is not a respectable value for dynamic range. If the radio’s maximum range is 50 miles, then the IF will clip at ranges less than 1.5 miles, and this would be unacceptable. The solution is to provide a means for reducing the gain of the receiver when the output signal begins to approach the clipping range. Since the detector output contains a DC level proportional to the received carrier level, this DC level can be extracted from the detector output using a low-pass filter, and fed back as a control voltage to reduce receiver gain.