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SL5067 Datasheet(PDF) 8 Page - General Electric Company |
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SL5067 Datasheet(HTML) 8 Page - General Electric Company |
8 / 13 page SL5067 8 APPLICATION NOTES Overview The key to good modulator performance is to ensure good and compact circuit layout with adequate grounding of all supplies. Earth loops must be avoided or kept as small as possible since RF coupling either through the air, or through the ground plane itself is the single most important factor in degrading modulator performance. Double sided board with a groundplane should be used, and all sensitive pins must be properly decoupled as close to the device as practicable. Oscillator design and layout The oscillator should be kept as small as possible to minimise parasitics. It is recommended that the circuit diagram shown in these application notes is used if the entire UHF band is to be covered. For lower frequencies or for applications requiring less tuning range, component values can be adjusted. Surface mount components should be used throughout the circuit and particular care must be taken with placement as the two coils should be as close to the oscillator pins as possible.( See Figs. 16 and 17) For applications at low VHF frequencies, it is suggested that the values of the coupling capacitors on pins 9, 10, 11 and 12 are increased, 2.2pF capacitors (or greater) may be used for frequencies up to 500MHz but it must be remembered that the larger the coupling capacitor used, the smaller the tuning range will be, as the varactor diode capacitance will form a lower percentage of the total tuning capacitance of the loop. For fixed frequencies (or small tuning ranges) up to 100MHz, 15pF or 18pF capacitors may be used. Varactor tuning of the SL5067 should not be attempted unless the application either uses a synthesiser, or a temperature compensating network is used. The capacitance of most varactor diodes changes greatly with temperature, and this must be compensated for if the modulator is to remain on tune to the correct channel. For applications requiring tuning over only a few channels, an air variable capacitor plus appropriate temperature compensation may be used. Modulated outputs Care must be taken with the routing of the modulated outputs and also with the mod index pin, pin3. It is suggested that pin 1 is used, and that the unused modulated output on pin 2 is terminated in a way which looks as physically and electrically similar to the used output on pin 1. Experiments have shown that a RF coupling problem can exist between pins 2 and 3. This manifests itself at frequencies over 600MHz in applications where pin 3 is not taken directly to ground. Good decoupling of pin 3 (with 10pF and 10nF) will help to reduce these effects. The modulated outputs must be routed away from the oscillator tank as there is danger of the local oscillator signal coupling directly into the modulated outputs. This will produce distortions in the modulated signal giving bad performance in such characteristics as differential phase and gain. For VHF and other applications below 500MHz RF coupling is not such a problem, however similar care should still be taken with layout in order to maximise device performance. Use of a balun It is possible to further improve device performance with the use of a balun to remove the effects of common mode coupling. Although using a balun will add to component cost, it may be the only way to achieve acceptable performance at higher frequencies where common mode noise has made it impossible to achieve a low enough minimum power signal to give the necessary dynamic range in the output signal. A low cost balun wound on a ferrite bead former should be sufficient to provide adequate performance in the majority of applications. Sound tank circuit Care must also be taken with the layout of the sound tank, in order to minimise harmonics,and reduce coupling between the audio and video parts of the circuit. The sound tank must be situated as close to the device pins as possible. If this is not done, RF may couple into the sound tank, via the tracks connecting the sound oscillator to the inductor and capacitor. In practice, it is easiest to mount the sound tank capacitor close to, or directly on pin 15 and 16, with the inductor slightly further away. This appears to give the best linearity. In some cases where some coupling and/or distortion problems are occurring, the addition of small 2p2 capacitors from either side of the tank circuit to ground may improve both FM deviation and linearity. For optimum performance (in the FM case) the sound tank should be selected to give a Q of around 10. The circuits shown in the datasheet give a value of approximately 9, and are the suggested normalised values to be used. Lower values of Q will give greater FM deviation per volt input (kHz/Volt), but also increase the level of the 3rd harmonic of the sound subcarrier. This is shown in Fig. 10. The Q of the inductor chosen should be at least 2.5 times the Q of the tank circuit itself. It is not recommended that a Q of over 16 is used, as the amplitude of the sound subcarrier fundamental will start to decrease once a Q of approx 12 has been reached. Thus if a Q of 20 were used in order to give good harmonic performance, there would be an unacceptable trade off in terms of picture carrier to sound subcarrier ratio, which would be approx 20dB. MISCELLANEOUS POINTS Board layout and decoupling Good decoupling techniques must be used throughout with the use of surface mount components wherever possible. For best performance, all supplies and sensitive pins should be decoupled as close to the device as possible, with a combination of capacitors, say 100pF and 10nF to ground. The use of double sided board with a groundplane is strongly advised. This should be of particular help in the reduction of oscillator coupling. Mod index pin As already stated, great care must be taken with the mod index pin, pin 3. This should be decoupled with chip components as close to the pin as possible. Ideally the mod index should be defined with a DC voltage, thus requiring the use of two external resistors, see Figs 4 and 5. It is also possible to define mod index through the use of a single resistor connected to ground or VCC depending on whether negative or positive modulation is required. Synthesiser drive It is suggested that any synthesiser (if used) is driven differentially. This is done by taking both of the prescaler outputs (pin 7 and 8) to the synthesiser via 1nF or 10nF capacitors. FM/AM select The voltage on the FM/AM select pin should be defined by two external resistors between vcc and ground, see Figs 6 and 7. The application diagram Fig. 14 shows a potentiometer, RV2 which is used to define the voltage on this pin in the demo board in practice it is suggested that in low total resistance value (5V or less) is used between VCC and GND since this will ensure a constant voltage on pin 18 irrespective of any small internal resistance variations between devices, thus ensuring a constant PC/SC ratio. It should be noted that the sound subcarrier level is referenced to the AGC sidebands rather |
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