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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.


Minimum Frequency



Maximum Frequency






Minimum Capacity



Maximum Capacity



Capacity Ratio



Ideal curve of capacitance versus frequency. Unit is pF and MHz.

Performance • Test

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.

Measurement 1

This is what you see directly after power on. Resistor R1 not assembled. Amplifier works fine, looks slightly overdriven ...

A universal VCO board with the MC100EL1648DG

Measurement 2

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.

A universal VCO board with the MC100EL1648DG

Measurement 3

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.

A universal VCO board with the MC100EL1648DG

Frequency and Tuning Sensitivity vs. Tuning Voltage

Measurement 4 : Frequency and Tuning Sensitivity vs. Tuning Voltage

Frequency and Tuning Sensitivity vs. Tuning Voltage

Measurement 5 : Now with DIY inductor. N=3, D=4, L=10



Type Package Config. U max [V] C min [pF] C max [pF] R series [Ω]
BB833 SOD323 Single 30 0.75 9.3 1.8 (typ)
BB131 SOD323 Single 30 0.7 ~ 1.055 8 ~ 17 3.0 (max)
BB135 SOD323 Single 30 1.7 ~ 2.1 17.5 ~ 21 0.75 (max)
BBY31 SOT23 Single 30 1.6 ~ 2 16.5 (typ) 1.2 (max)
BBY39 SOT23 Double 30 1.6 ~ 2 16.5 (typ) 1.2 (max)
BBY40 SOT23 Single 30 4.3 ~ 6 26 ~ 32 0.7 (max)
BB149A SOD323 Single 30 1.95 ~ 2.22 18.22 ~ 21.26 0.75 (max)

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t1 = 3876 d

t2 = 381 ms

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