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Nov 23, 2011
VCO is definitely useful in so many RF projects, wide band Ghz VCO get more concern than VHF/UHF VCO. here is a Manhattan Island Pad construction for VHF VCO, low to 88Mhz and up to around 300Mhz.
Here is the schematic, a Seiler oscillator , Voltage controlled via a BB910, from 0V to 15V get 88Mhz to 180Mhz covered. My linearity ramp genarator output sawtooth ramp from about 1.5V to 13.5V , then cover 100Mhz to 170Mhz range.
All critical component in above pictures is given a range to try, and my construction use: C5:472, C4:22p C7:30p Air coil: 0.8mm coper wire round diameter 5mm, 4 turn. it's worthy to point out : Island pad with a ground plane lead make up a small capacitor, i use 0.8mm Pad for easy soldering each such pad had about 3pF, include this cap for design your PCB, or use single side copper clad board.
the image is not output from the oscillator, it's come from a frequency meter, divide input frequency by 32. I had 100M Oscilloscope, for frequency around 100M and above it's just use to view the Oscillation envelope
New experiment on this circuit
New experiment on this circuit
(2011/11/24)
All above test use transistor 2sc9018, which GBW (Ft) is 1Ghz. This circuit use common Collector Amplifier, a little poor performance than common base. so if you try let it work above 180Mhz, suggestion use transistor that GBW is above 3Ghz (worth to try). and i put a 2SC3355, which GBW 6.5Ghz, that work well up to 280Mhz. output power drop quick when work frequency is growth, this is not so wide band VCO, but from 88Mhz up to 300Mhz is also impressive me. depend your varactor, for common case 2:1 tune range is easy to achieve . and stability is about 10^-5. (10khz per second, no shield, no special consideration for stability. )
DC bias
DC bias
(updated 2011/11/26)
the transistor work at very high point(so much current flow to emitter ). and at such condition, the whole loop gain seems out of control: tune it from low frequency to high, you will found frequency abrupt jump to a very high frequency and drop sharply again. Origin R1 is 51k, you must low the DC supply to about 1.5V to eliminate the tune broken.
so i change the R1 to 3k and everything became very well. if you continue make emitter current lower, i.e use R1=2.2K, you need to 7V DC supply to stable the oscillator. anyway , keep Ie as low as possible. and frequncy stablity is improved.
more adjustment is C4, C7, at low DC bias C4=C7=22pF work more stable. and suggest star with C4=C7 to begin the fine tune.
VCO linearity
VCO linearity
@2013 10/18
this char is the linearity chart, use 2 back to back BB910, 1V to 16V. and another testing is not shown here, use only one BB910, which linarity is good form 0-6V, and from 6V, frequncy grow is begin slow, but not like this 12V to 16V broken.
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