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VCO-1648.php 14106 Bytes 10-02-2015 16:10:07
Micro-Projects Tools : A universal VCO Board
With the MC100EL1648DG and a PGA-103+ as buffer.
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
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.
This is what you see directly after power on. Resistor R1 not assembled. Amplifier works fine, looks slightly overdriven ...
Now R1 is assembled and optimised. About 4.2 kΩ to produce such a picture. Current increased a lot. Now almost 120 mA - and the level
dropped to 5 dBm - Huuuu ?!?
You'd rather use a lowpass filter.
This is a close-up of the spectrum. The vco was free-running. Not bad at all, if you consider the parts used. We think this
circuit is a good choice to learn about oscillators, pll's and similiar subjects. Because it has one beau- tiful property : it does oscillate.
Measurement 4 : Frequency and Tuning Sensitivity vs. Tuning Voltage
Measurement 5 : Now with DIY inductor. N=3, D=4, L=10