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Szerokie VFO





NADESŁAŁ: SP6RGB

Artykuł pochodzi z czasopisma RadCom nr 3/2006. Ponieważ jest bardzo duże zainteresowanie tym tematem postanowiłem wrzucić "surowy" materiał, który otrzymałem od Roberta. Jest to skan oryginalnego artykułu przepuszczonego przez program OCR :-)

WIDE-RANGE TUNABLE OSCILLATOR WITH STABLE OUTPUT. Over the years, 'TV has included a number of oscillator circuits featuring the use of two active devices in order to permit the use of two-terminal tuned circuits with one side-grounded. For example, as long ago as TT' September 1970, the late LA8AK sent along details of a simple source-coupled FET oscillator that could function with an appropriately-tuned circuit from about 1 MHz right up to VHF: Fig 1.




FIG 1: SOURCE-COUPLED FET LC-TUNED OSCILLATOR USING TWO MPF120, 2N3819 ETC, AS DESCRIBED BY THE LATE LA8AK IN 1970. THE VALUE OF C1 SHOULD BE CHOSEN TO SUIT THE FREQUENCY, BUT OFTEN OF THE ORDER

OF 10PF. FOR USE FROM ABOUT 1 MHZ TO VHF,


A recent three-page item, JLC Oscillator Has Stable Amplitude', by Julius Foit of the Czech Technical University, in the 'Design Ideas' column of EDN (October 17, 2005), begins: "Many applications call for wide-range-tunable LC oscillators that can deliver a nearly-constant frequency, nearly-harmonic-free output, even when the circuit's output load changes. From a design viewpoint, eliminating both inductive or capacitive LC circuit taps and transformer couplings within the frequency-determining circuit simplifies fabrication and production, as does the option of grounding one side of the tuned LC circuit. These requirements suggest a circuit that can automatically and efficiently adjust loop gain internally, the basic requirement for oscillation. In addition, the circuit must provide sufficient gain to oscillate with low-impedance LC circuits and regulate the oscillation's amplitude to improve frequency stability and minimise THD (total harmonic distortion)." Julius Foit discusses in some detail the design processes leading him to come up with the complete oscillator circuit shown in Fig 2.



FIG 2. WIDE-RANGE LC-TUNED OSCILLATOR WITH STABLE AMPLITUDE AS DESCRIBED BY JULiUS FOIT IN EDN USES TWO N-CHANNEL BF245B JFETS. WITH SUITABLE LC VALUES IT IS CLAIMED TO WORK WELL FROM ABOUT 5kHz TO 50MHZ

In his application as a radar-marker generator, the oscillator operates at around 280kHz with an L of 147/UH and C of 2200pF. But it is claimed that the circuit will provide stable amplitude oscillation as low as 5kHz and up to 50MHz with no adjustment of component values apart from the L and C of the tuned circuit. Variations in load impedance will affect frequency stability and for VFO applications it would be advisable to use a buffer stage. At the higher frequencies, stability will be influenced by mechanical and constructional factors, as for any other oscillator. Foit claims that the best active devices for this oscillator are the selected N-channel, medium-grade BF245B devices with a drain current of 5mA at a gate-to-source voltage of 0V and a drain-to-source voltage of 15V.










Prawa autorskie © SP-QRP Wszystkie prawa zastrzeżone.

Opublikowane: 2006-02-24 (15290 odsłon)

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