Just a small board. With a buffer amplifier. And space for your own L-C tank circuit. And a small repetition of this thomson oscillator formula.

The circuit is straightforward. (as usual). The MC100EL1648 is a voltage controlled oscillator amplifier that requires an external parallel tank circuit consisting of the inductor (L) and capacitor (C). It does the main job. For isolation reasons we added a buffer, consisting of a PGA-103+ from Mini Circuits. (Almost any other will also work). In order to get a "pure" sinewave, resistor R1 is foreseen. It must be optimised empirically. Two voltage regulators are used to ensure stable operation.

In order to measure the spectral purity - and see if the calculations above do make any sense, we want to design a vco from 70 - 90 MHz. To be on the safe side - and to take into account any tolerance spreads, the boundaries are moved 10% up and down. We therefore entered 63 MHz and 99 MHz into the javascript form above. With some tuning, we come across an inductance of approx. 390 nH. Now all we need is to find one (or two) suiteable varicaps. We see, that a BB135 has a maximum capacitance of 16 pF at approx. 1V, whilst the minimum capacitance of 6.6 pF is obtained at approx. 6.5 V. The series resistance is (max) 0.75 Ω which corresponds to a Q of 270 (midband). This is acceptable. The tank circuit needs High-Q components, as this has a direct impact on the resulting phase noise of the oscillator.

As the DIY-coil was slightly higher than the box, we used a 470 nH (Fastron, 1206, ±5%, SRF min. 550 MHz, Q>52 from Reichelt) to get it running. Supplied by 15 V, current consumption was 80 mA, 20 mA for the oscillator and 60 mA for the amplifier.

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