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MC3374FTB bảng dữ liệu(PDF) 6 Page - Motorola, Inc |
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MC3374FTB bảng dữ liệu(HTML) 6 Page - Motorola, Inc |
6 / 8 page MC3374 6 MOTOROLA ANALOG IC DEVICE DATA CIRCUIT DESCRIPTION The MC3374 is an FM narrowband receiver capable of operation to 75 MHz. The low voltage design yields low power drain and excellent sensitivity in narrowband voice and data link applications. In the typical application the mixer amplifies the incoming RF or IF signal and converts this frequency to 455 kHz. The signal is then filtered by a 455 kHz ceramic filter and applied to the first intermediate frequency (IF) amplifier input, before passing through a second ceramic filter. The modulated IF signal is then applied to the limiting IF amplifier and detector circuitry. Modulation is recovered by a conventional quadrature detector. The typical modulation bandwidth available is 3.0 to 5.0 kHz. Features available include buffers for audio/data amplification and active filtering, on board voltage regulator, low battery detection circuitry with programmable level, and receiver disable circuitry. The MC3374 is an FM utility receiver to be used for voice and/or narrowband data reception. It is especially suitable where extremely low power consumption and high design flexibility are required. APPLICATION The MC3374 can be used as a high performance FM IF for the use in low power dual conversion receivers. Because of the MC3374’s extremely good sensitivity (0.6 µV for 20 dB (S+N/N, see Figure 3)), it can also be used as a stand alone single conversion narrowband receiver to 75 MHz for applications not sensitive to image frequency interference. An RF preamplifier will likely be needed to overcome preselector losses. The oscillator is a Colpitts type which must be run under crystal control. For fundamental mode crystals choose resonators, parallel resonant, for a 32 pF load. For higher frequencies, use a 3rd overtone series mode type. The coil L2 and RD resistor are needed to ensure proper operation. The best adjacent channel and sensitivity response occur when two 455 kHz ceramic filters are used, as shown in Figure 1. Either can be replaced by a 0.1 µF coupling capacitor to reduce cost, but some degradation in sensitivity and/or stability is suspected. The detector is a quadrature type, with the connection from the limiter output to the detector input provided internally. A 455 kHz LC tank circuit must be provided externally. One of the tank pins (Pin 8) must be decoupled using a 0.1 µF capacitor. The 56 kΩ damping resistor (see Figure 1), determines the peak separation of the detector (and thus its bandwidth). Smaller values will increase the separation and bandwidth but decrease recovered audio and sensitivity. The data buffer is a noninverting amplifier with a nominal voltage gain of 2.7 V/V. This buffer needs its dc bias (approximately 250 mV) provided externally or else debiasing will occur. A 2nd order Sallen–Key low pass filter, as shown in Figure 1, connecting the recovered audio output to the data buffer input provides the necessary dc bias and some post detection filtering. The buffer can also be used as an active filter. The audio buffer is a noninverting amplifier with a nominal voltage gain of 4.0 V/V. This buffer is self–biasing so its input should be ac coupled. The two buffers, when applied as active filters, can be used together to allow simultaneous audio and very low speed data reception. Another possible configuration is to receive audio only and include a noise–triggered squelch. The comparator is a noninverting type with an open collector output. Typically, the pull–up resistor used between Pin 14 and VCC is 100 kΩ. With RL = 100 kΩ the comparator is capable of operation up to 25 kHz. The circuit is self–biasing, so its input should be ac coupled. The regulator is a 1.07 V reference capable of sourcing 3.0 mA. This pin (Pin 17) needs to be decoupled using a 1.0 – 10 µF capacitor to maintain stability of the MC3374. All three VCCs on the MC3374 (VCC, VCC2, VCC3) run on the same supply voltage. VCC is typically decoupled using capacitors only. VCC2 and VCC3 should be bypassed using the RC bypasses shown in Figure 1. Eliminating the resistors on the VCC2 and VCC3 bypasses may be possible in some applications, but a reduction in sensitivity and quieting will likely occur. The low battery detection circuit gives an NPN open collector output at Pin 20 which drops low when the MC3374 supply voltage drops below 1.2 V. Typically it would be pulled up via a 100 k Ω resistor to supply. The 1.2 V Select pin, when connected to the MC3374 supply, programs the low battery detector to trip at VCC < 1.1 V. Leaving this pin open raises the trip voltage on the low battery detector. Pin 15 is a receiver enable which is connected to VCC for normal operation. Connecting this pin to ground shuts off receiver and reduces current drain to ICC < 0.5 µA. APPENDIX Design of 2nd Order Sallen–Key Low Pass Filters Input R1 R2 C2 C1 Bias Avo = K Low Pass Output 0 to fo Hz + – The audio and data buffers can easily be configured as active low pass filters using the circuit configuration shown above. The circuit has a center frequency (fo) and quality factor (Q) given by the following: fo + 1 2 p R1R2C1C2 Q + 1 R2C2 R1C1 ) R1C2 R2C1 ) (1–K) R1C1 R2C2 If possible, let R1 = R2 or C1 = C2 to simplify the above equations. Be sure to avoid a negative Q value to prevent instability. Setting Q + 1 2 + 0.707 yields a maximally flat filter response. Freescale Semiconductor, Inc. For More Information On This Product, Go to: www.freescale.com |
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