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A3845SLW Datasheet(PDF) 6 Page - Allegro MicroSystems |
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A3845SLW Datasheet(HTML) 6 Page - Allegro MicroSystems |
6 / 8 page 3845 AM NOISE BLANKER 115 Northeast Cutoff, Box 15036 Worcester, Massachusetts 01615-0036 (508) 853-5000 TYPICAL APPLICATION A typical application uses the A3845xLW in a C-QUAM® AM stereo car-radio tuner with its input from between the RF tuned circuits and the mixer input. Although there is a 1.5 µs delay from the beginning of the noise pulse to the start of blanking, this is small compared with the impulse re- sponse time of the receiver. It takes almost 10 µs for the RF noise burst to reach 70% amplitude at the mixer input. The blanker RF input could be connected to the collector of the discrete RF amplifier, but the bandwidth is much wider there and false triggering from strong adjacent channel signals could occur. The A3845xLW noise blanker can also be used in dual-conversion AM tuners. The blanker RF input would then be connected at the first IF amplifier input and the blanker RF gate connected at the second mixer output. Because the first IF band-width is usually relatively wide, the noise pulses are nar- rower, and the RF blanking time will be correspondingly less. In this case, it may be necessary to reduce the value of capacitor C13 so that the noise separator does not extend the RF blanking time. COIL INFORMATION FOR HIGH-PERFORMANCE ETR AM STEREO RECEIVER WITH NOISE BLANKING Symbol Q N1:N2 N1:N3 Toko Part Number Antenna T1 1:1.6 7HN-60064CY RF T2, T3 120 10:1 RWOS-6A7894AO, L = 178 µH Local Osc. T4 120 5:1 7TRS-A5609AO Mixer T5 2:1 8.9:1 7LC-502112N4, CT = 180 pF Detector L2 100 A7BRS-T1041Z, CT = 1000 pF 0 -5 -10 -15 0.1 1.0 10 RF INPUT FREQUENCY IN MHz Dwg. GS-006 TYPICAL RF FREQUENCY RESPONSE The audio delay is determined by the value of R5: Audio Gate Delay ( µs) = 157 x 10-12 x R 5 where R5 should be greater than 33 k Ω. The amount of delay required will depend on the IF filtering characteristics of the particular receiver design. After the audio delay time, the audio one- shot is triggered. The audio switching MOSFETs (leads 9-10 and leads11-12) are controlled by the audio one-shot whose gate time is determined by the values of R6 and C8: Audio Gate Time ( µs) = 1.9 x R 6 x C8 The MOSFET audio gates also include charge-balancing circuits to eliminate switching transients. ® Registered trademark of MOTOROLA, INC. |
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