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pcsalex

11/24/2012 10:32 PM EST

R4 =33 ohm Figure 2, is that the correct value? text: "The lower speed PNP ...

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anonymous user

11/2/2011 12:47 PM EDT

Just for the record, Edwards has been using a circuit
nearly identical to ...

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# Series-LC-tank VCO breaks tuning-range records

## 10/20/2011 10:00 AM EDT

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.

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.

WKetel

10/22/2011 10:07 PM EDT

This is a vey interesting design idea. And the included description of the operation is very good. Is thecircuit patented yet? It could become the basis for an inteestng all band receiver.

bcarso

10/24/2011 1:42 PM EDT

Nice work. It would be helpful to see some frequency-domain plots as it's difficult to estimate spectral content from scope photos. One would suspect a fair amount of 2nd, at least, particularly at the lowest varactor bias voltages where the signal represents a larger fraction of the bias.

Having said that, it would be intriguing as well to produce a "balanced" version with intrinsic suppression of even-order harmonics.

anonymous user

10/24/2011 3:44 PM EDT

These days, one might just subtract a frequency offset to create an arbitrarily large ratio. Or use digital techniques. But this is a great (!) circuit concept for when such typically power hungry digital techniques cannot be used.

LostInSpace2010

10/24/2011 6:03 PM EDT

Looking at the scope pictures, there doesn't seem to be much 2nd or 3rd order distortion there (to answer another question). A nice rule of thumb is that when you can just start to see distortion on the scope - it's about 40 dBc (a rule of thumb, subject to squinting, your years of experience, etc, etc, etc.... So don't flame me for sharing... Ha, ha, ha, ha... But I find it about right)

devassocx

10/24/2011 8:02 PM EDT

It looks to me that this circuit is dependent on
transistor leakage currents to even start...not exactly
a reliable approach.

anonymous user

10/25/2011 6:33 AM EDT

>>This is a vey interesting design idea. And the included description of the operation is very good. Is thecircuit patented yet?

The circuit is NOT patented, you are free to use it.

>>One would suspect a fair amount of 2nd, at least, particularly at the lowest varactor bias voltages where the signal represents a larger fraction of the bias.

Remember that with series operation, the current through the reactive elements is constant, but their impedance vary with frequency, and hence, for a constant output voltage, the voltage across the varactor is not constant.
For example at the max. output amplitude, that rms voltage is ~0.1V (0.3Vpp)for 0.3V DC bias, against ~0.4V (1.1Vpp) for 15VDC.
There is therefore a partial compensation.

>>It looks to me that this circuit is dependent on
transistor leakage currents to even start...not exactly
a reliable approach.

It is a perfectly valid point, but if you examine the circuit more closely, you will see that R10 combined with D1 and D2 address this issue in a deterministic way.

anonymous user

10/25/2011 12:07 PM EDT

You state in the 1st circuit that the capacitor is 4.7pf to 4.7pf should that be 47 or 470pf

bcarso

10/25/2011 12:21 PM EDT

Louis, thanks for your explication of the partial compensation. Have you examined the output on a spectrum analyzer yet? I agree with the one poster about being able to see of order 1% residuals, I'm just curious as to what the overall performance is.

anonymous user

10/26/2011 3:03 AM EDT

What was the application that prompted this design?

MITRONICS

10/26/2011 3:03 AM EDT

What was the application that prompted this design?

anonymous user

10/26/2011 5:31 AM EDT

>>You state in the 1st circuit that the capacitor is 4.7pf to 4.7pf should that be 47 or 470pf

It is 4.7pf to 4.7µF (microfarad): maybe the symbol doesn't show correctly on your screen.
There is also a small typo in the text, the capacitance ratio is not 106 to 1, but 10 to the sixth power to 1.

>>Louis, thanks for your explication of the partial compensation. Have you examined the output on a spectrum analyzer yet? I agree with the one poster about being able to see of order 1% residuals, I'm just curious as to what the overall performance is.

Under the worst conditions (max. amplitude, min. tuning voltage), the 2nd harmonic is 37.5dB down wrt the fondamental, 3rd is ~50dB down.
A screenshot of the spectrum is visible at: www.cijoint.fr/cj201110/cijkBiQxnB.png

>>What was the application that prompted this design?
It was the swept generator for a prototype cable discontinuity analyser.
A frequency ratio >3 was required, and this was a direct, uncomplicated solution.

bcarso

10/26/2011 2:09 PM EDT

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.

anonymous user

10/26/2011 3:20 PM EDT

>>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

bcarso

10/26/2011 4:02 PM EDT

Ah that one on imgur comes right up for me --- very pretty!

WKetel

10/29/2011 11:21 AM EDT

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.

anonymous user

11/2/2011 12:47 PM EDT

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

pcsalex

11/24/2012 10:32 PM EST

R4 =33 ohm Figure 2, is that the correct value? text: "The lower speed PNP creates greater phase difference and encourages oscillation" AAccording the data sheets the PNP BF450 has fT =350MHz, the NPN BF240 has fT = 150MHz so the NPN is the slower...