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High Impedance Buffer and Broadband Amplifier for Digital FreqHigh Impedance Buffer and BroadbandAmplifier for Digital Freq. Meters(First published by) in the Wireless Institute of Australiamagazine, October 1980, last update 14 June 2003.)AN ARTICLE FOR THE WELL EQUIPPED AMATEURWith the introduction of synthesized transceivers employingthe hetrodyning of several mixer crystals with the VCO output ofa PLL system, there has grown the need to measure frequencies atlow levels.
Buffer Amplifiers Solve VCO Problems May 11, 2003 Abstract: Poor isolation of an oscillator circuit's output can cause adverse effect. This article describes key application issues surrounding VCO circuits such as load pulling, supply pushing, phase noise.
In the majority of cases, because we are dealing withsolid state devices, we have levels that are around the order of10 dBm or less (0 dBm = 1 mW).The Impedances around such circuits are not very appropriatefor measurement with devices of relatively low impedances,particularly when the circuit impedances can range anywherebetween tow hundred and several thousand ohms. Consequently ahigh gain and high impedance device is required if we are toobtain any measurements and accurate measurements respectively.
Iam sure that we are all familiar with the operating principle ofthe GDO, in the same way, loading of any oscillator will cause aresultant shift if frequency.These two devices, the RF buffer and the broadband amplifier,were primarily designed for the input to the front end of afrequency meter and its prescaler, in particular the EA DigitalFrequency Counter. The application was the measurement of a YaesuFT-901D transceiver because some problems were experienced on the10m bands.Those familiar with this transceiver know that the crystals andthe VCO cover an approximate frequency range from 15 MHz - 43MHz.
![Buffer Buffer](http://img.disqueenfrance.com/nimg/9b/1b/bd391d1f90375cdb02b444f34739-200x200-1/wideband_mmic_vco_w_buffer_amplifier_4_0_8_0_ghz_mc58625_on_sop_integrated_circuit_chip.jpg)
The probe and amplifier were used to obtain measurementsover this range with no noticeable shift in the final frequencyof the transceiver.THE HIGH IMPEDANCE BUFFERThree requirements should be met by the probe:. High input impedance - should be greater than 1 M ohm. Low input capacitance - typically less than 10 pF. Wide bandwidth - useful over several octaves.A JFET was chosen as the active device on the input of thebuffer. The JFET was followed by a PNP bipolar tansistor - whichis used for impedance transformation. The circuit configurationbootstraps the source resistor to minimize input capacitance.The FET is a process 50 type with a typical gain of 12 dB at400 MHz and a noise figure of 4 dB.