Tait TM9100 Service Manual

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Tait TM9100 Service Manual Manual pdf 71 page image

TM9100 Service Manual Circuit Descriptions 71© Tait Electronics Limited August 20053.3 Frequency Synthesizer CircuitryIntroduction For a block diagram of the frequency synthesizer circuitry, refer toFigure 3.4.The frequency synthesizer includes an active loop filter, one or two VCOsand buffer amplifiers, and a PLL IC. The last-named uses conventionalinteger-N frequency division and includes a built-in charge pump. Speed-up techniques ensure a transmit-receive settling time of less than 4.5mswhile retaining low noise characteristics in static operation.Power Supplies Several power supplies are used by the frequency synthesizer owing to acombination of performance requirements and the availability of suitablecomponents. The PLL IC includes analog and digital circuitry and usesseparate power supplies for each section. The digital section is run on 3V,while the analog section is run on approximately 5V. The VCOs and bufferamplifiers run off a supply of about 5.3V. The active loop filter requires asupply of 14 to 15V, and a reference voltage of approximately 2.5V.PerformanceRequirements Low noise and good regulation of the power supply are essential to theperformance of the synthesizer. A 6V regulator IC provides good lineregulation of the 9V supply and good load regulation. Good regulation ofthe power-supply line and load is essential for meeting the transient ACPrequirements. The regulator output voltage is electrically noisy, however,and filtering is essential. Filtering of the power supply is achieved with twocapacitance multipliers (Q508 and C585 for the VCO supply, and Q512 andC579 for the PLL and loop-filter supply). The VCO (or VCOs) use aseparate capacitance multiplier because these multipliers have poor loadregulation and the VCOs impart sufficient load transients to warrant aseparate supply.Effect of TuningRange For reasons of noise performance, the VCOs are designed to be tunedwithin a range of 2 to 12V. Active tuning circuitry is required. An activeloop filter incorporating an IC operational amplifier achieves this range witha suitable power supply voltage. Normal synthesizer switching behaviorinvolves overshoot, which dictates that the tuning voltage range must extendabove and below the range of 2 to 12V. The 14V limit is a result of limitson the working supply voltage of the IC operational amplifier.Switch-mode PowerSupply The power supply VCL SUPPLY for the active loop filter is provided by aSMPS, which is in turn powered by 9V. The SMPS consists of an oscillator(switching circuit) and a detector. The output voltage is monitored by afeedback circuit that controls the DC bias of the switching circuit tomaintain a constant output voltage.SynthesizerCircuitry The essential function of the PLL frequency synthesizer is to multiply a25kHz reference frequency to give any desired frequency that is an integermultiple of 25kHz. There are some constraints imposed by the capabilitiesof the synthesizer hardware, especially the tuning range of the VCOs.