Series-LC-tank VCO breaks tuning-range records
A series tank circuit allows you a 4-to-1 frequency range.
Louis Vlemincq, Belgacom, Evere, Belgium; Edited by Paul Rako and Fran Granville -- EDN, October 20, 2011
This Design Idea applies a novel topology to an oscillator. It uses a series-connected LC (inductive-capacitive) tank circuit to give the circuit a higher tuning range than circuits that use a parallel-LC connection. The architecture of the oscillator permits wide frequency swings, well beyond the capabilities of the best hyperabrupt varactor. Engineers deem a VCO (voltage-controlled oscillator) capable of covering one octave as state of the art. This topology allows a 4-to-1 ratio in output frequency. The LC tank alone sets this frequency so that the parasitic capacitances of other components do not limit the output frequency. Unlike standard oscillators, this circuit works well at its frequency extremes.
At first glance, the central structure
of the oscillator resembles two transistors
that form a latching SCR (silicon-controlled-rectifier) structure (Figure
1). The structure is similar to that of
a thyristor, but you add degeneration
resistors that keep the circuit in a linear
mode of operation. The resistors make
the gain of this “SCR” smaller than one,
and it is dc-stable. The series-tuned
tank circuit increases the gain beyond
one at the resonant frequency, causing
the circuit to oscillate. No auxiliary components are necessary for oscillation,
and the node between the inductor
and the capacitor is free of other
connections, meaning that only the
varactor you use as the capacitor determines
the tuning range. The frequency
varies as the square root of the tuning
elements. To change the frequency by a
factor of two, you need a fourfold variation
of the tuning capacitance.Unlike a parallel-LC tank, the resonant current passes through the active element and is, therefore, limited. This limit in turn means that the ac voltage appearing across the tuning components is small—typically, less than 100 mV. The small signal reduces the effects of circuit nonlinearity and the impact of the self-biasing effects of the signal on the varactor. You can use control voltages as small as 0.3V across the varactor. If you use a 1-μH inductor, the circuit still oscillates with capacitor values of 4.7 pF to 4.7 μF—a ratio of 106-to-1.
For the detailed design, move the
LC tank to the emitter of PNP transistor
Q2 (Figure 2). The lower speed
of the PNP creates greater phase difference
and encourages oscillation.
Connect L2 and C2 at a common power
point on the power rail, emphasizing the criticality of the layout in this part
of the circuit. The oscillator “senses”
the tuned circuit through C2 and C4,
and anything inside that loop adds
uncontrolled parasitics to L2. These
parasitics would compromise the AGC
(automatic-gain-control) action and
degrade the performance and accuracy
of the oscillator.
Click to enlarge.
Q1 and associated components
implement the AGC. A parallel-LC
oscillator tolerates clipping of the signal,
but this series-LC circuit degenerates
into a multivibrator if you allow
the signal to grow so large that it clips.
The AGC servo action has the added
advantage of producing uniform output
amplitude. Use D5 to create a 0.6V dc
bias. R11 and R12 form a voltage ladder
that creates a dc-bias voltage close to
the forward-voltage drop of Schottky
diode D6. This bias allows D6 to work
as a more perfect rectifier of the small
output signal. C8 integrates the rectified
signal into a dc voltage proportional
to the amplitude of the circuit’s
output. Apply this dc signal to IC1,
the AGC amplifier, through a filter
comprising R15 and C8. 
The op amp
servo-controls the filtered dc signal
against the A-CTRL input-amplitude
signal you send to the circuit. This signal
allows you to set output amplitude
at 0 to 1V.In this example, the output amplitude is 0.9V. The frequency range extends from 35 to 140 MHz, a 1-to-4 ratio—twice that of conventional high-performance VCOs—and requires a fourfold increase in the capacitance ratio. The overall capacitance ratio is 1-to-16, exactly that of the varactor itself. At the lowest (Figure 3) and highest (Figure 4) frequencies of the output range, the quality of the sine wave remains excellent, thanks to AGC action.
Talkback
-
Just for the record, Edwards has been using a circuit
nearly identical to that of fig.1 in their film
thickness monitors since the late 70's
Jose A. Senna - 2011-2-11 09:47:23 PDT -
This is indeed quite a nice design, and the fact that it is amplitude-stable should indeed allow operation at a level much less likely to produce harmonics. Of course, the tuned-circuit "Q" will vary with frequency, and so there is a potential for a variation in distortion based on that. Adding a tuned buffer, controlled by the same tuning voltage, would probably provide the desired reduction in harmonics.
William Ketel - 2011-29-10 08:21:36 PDT -
Ah that one on imgur comes right up for me --- very pretty!
Brad Wood - 2011-26-10 13:02:27 PDT -
>>The link you provided leads to an interesting website but lacking fluency in French I wasn't able to navigate to your photos.
Sorry about that, normally this site is the French equivalent of imageshack, etc, and no navigation should be necessary.
Here it is again, under imgur this time.
If it doesn't work, I'll keep trying other hosting sites!
(you have to add the usual h(tee)(tee)p plus double slash in front, as this is blocked in this comment)
i.imgur.com/cMeZt.png
Louis Vlemincq - 2011-26-10 12:20:16 PDT -
Thanks! Those two numbers are plenty. The link you provided leads to an interesting website but lacking fluency in French I wasn't able to navigate to your photos. But those are very good results for 2nd and 3rd, especially as worst-case and given the large tuning range and circuit simplicity.
The "mu" character did reproduce for me at least --- I suspect the poster just couldn't believe that you were talking about microfarads.
Brad Wood - 2011-26-10 11:09:50 PDT
