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Auto Scan tuning of Pocket FM radio

Published on Jul 20 2010 // FM Transmitter Circuits


Auto Scan tuning of Pocket FM  radio

This article describes the Pocket PC FM radio selected developed and produced by Philips TDA7088T integrated block , double row 16- pin flat package , the working voltage of 3V, the circuit than includes FM radio receiver from antenna to output the audio signal frequency discrimination All features, also contains a search tuning circuit , the signal detection circuit , mute circuit and compression in the frequency offset of the frequency locked loop FLL circuit . TDA7088T Circuit IF frequency is designed to 70kHz, without external circuit IF transformer , in which the internal RC frequency selected by the IF filter circuit to complete . Machine like the digital tuner radio tuner , as with power button (RUN), the other is the reset button (RESET). Circuit switched on , click the Search button , the circuit automatically frequency of low-end to high-end search from the radio, once the search for radio signals , tuning automatically stops . If you then click the Search button , the circuit continues to search for high frequency radio . When tuned to the frequency of the most high-end FM receiver , just click the Reset button , the vibration frequency that is returned to the bottom, the search tuning begins again . Here the production machine and debugging methods . This machine uses the headset as the antenna , the headphone cable to the FM signal from the sensor manifold block TDA7088T { 11} feet into the mixer circuit , resulting from mixing with the local oscillator frequency 70kHz signal . Radio signal into the manifold block { 11} and { 12} feet , inductor L2, capacitors C10, C11, C12 constitute the input circuit , the circuit ‘s frequency by the L1, C5 and varactor diode D1 decision . The figure shows the capacitor C1 is mute , C6 for the intermediate frequency feedback capacitor , C7 for the low-pass capacitor , { 15 } pin input for search tuning , C14 for the filter capacitor , { 16 } feet of electrical tuning , AFC output . In the assembly in accordance with the road map , the components welded to the circuit board . Require special attention here is the 2 coil, it was only a small number of turns oscillation coil , installed in the manifold block between 4 and 5 feet , and the other turns over the input tuning circuit coil , installed in the { 11} and { 12} feet between the election as very low IF frequency , local oscillator signal is easy to input from the antenna circuit went to cause interference , so the two coils to be placed into a perpendicular position . Welding pin arrangement to pay attention to TDA7088T direction , do not mistake , welding using electric iron shell to ground , not power in the case of circuit board soldering on the circuit board components. After installation is complete , we must carefully check to avoid wrong welding, Weld , and short-circuit phenomena , the next step after confirmation debugging . General into production is to debug Radio Scanner , amateurs do not have equipment can be used to debug high-frequency signal generator , also available to local FM radio station for debugging . First received by a DC power supply { 16 } feet up the radio frequency coverage , will be transferred to Vcc-0.1V DC power supply voltage value on ( such as the power used by radio Vcc = 3V is transferred to 3-0.1 = 2.9V), Slide the coil pitch oscillation to adjust its inductance so that the radio signal received 87 5MHz , then transferred to Vcc-1.6V DC power supply voltage value in the vicinity , as long as received 108MHz signal on it. Frequency coverage adjustment is completed, remove the DC power supply , that is, complete assembly debugging . Machine to receive frequency 88 ~ 108MHz range (due to SC1088 Manifold with TDA7088T and the pin block and the same performance can substitute each other directly ) . Circuit see above figure. Special specifications can be transferred to the machine to receive frequency 76 ~ 106MHz, machine size is only 78mm × 44mm × 14mm.