The Vackar VFO oscillator
Jiri Vackar [Jiří Vackář], (correctly pronounced as "Vatzkaarz")
invented his VFO oscillator during late 40s. It is probably the most stable VFO oscillator known. Thanks George!
The Vackar oscillator configuration is rarely used because of known reason.
( NIH, the not-invented-here syndrome ).
The frequency tuning range is above 2.5, not observable in any other type of oscillator. The Coupling ratio is fixed; typical range is 1:4 up to 1:9. The frequency tuning is provided independently of coupling. Transistor's parametric variables are isolated from the resonator. The transistor input is not overloaded as Clapp or other circuits. The collector output is at low impedance providing low gain just to maintain the oscillation. The feedback division ratio is fixed. Even if the VFO is tuned, the impedance divider is fixed. The stability is close to XO - crystal oscillator.
Jiri Vackar published his work in a book, providing theory and analysis of each type of his oscillator. What was the last model, V66? Who knows?
A while ago, I thought about fine tuning oscillator for PLL based VHF synthesizer. Few designs failed high expectations. I tried Colpitts, Clapp, Hartley, Pierce, and Seiler. Junk. The Butler is better than single active component oscillator, but it is not good enough. Commonly used oscillator configuration does not guarantee good performance. The signal clipping by diodes guarantees additional phase noise and thermal frequency drift. The best oscillators use two or three active devices. This is valid for the VCO, TCXO, and OCXO. The second active device acts as an impedance converter, isolation amplifier, AGC circuit, series RF power dissipation device, and a phase shifter. Be aware the articles in QST very often copy old mistakes, and limiter diodes. U.L.Rohde from U of Washington wrote few articles about the poison of limiter diodes in oscillators. Kenwood TS-950 use limiter diodes in oscillators.
Ordinary oscillator has poor tuning range, the output voltage swing is unstable, and the frequency stability is poor as well. The industry tries hard to make its sale pitch, to replace single oscillator with 50 ICs, digital dividers, approximation registers, thermostats, and other junk. Now what?
I checked the Vackar. First measurements with frequency counter were quite positive. The Vackar VFO was running in freezer at -30°
C. That is about -22°F. Not bad for Yukon Territory climate. Stable. Can it run more stable? The ferrite and iron-powder tuning slugs went out, down the pipes. There is a strong public belief the iron powder tuning slug cores are good. It's not. In thermal stability, they are better than ferrites. They can take higher maximum magnetic flux than heavy ferrites.
Micrometals cores use iron powder technology from the 50's, nobody use it any more. The iron powder cores are lossy in terms of resulting Q. The grains are not properly bound together. You will have "brown fingers" from handling these cores. Generally, ferrite is based on NiZn or MnZn alloy.
Quite good are Ferroxcube-Philips ferrite cores, presently manufactured in Spain. The Spanish sampling service was good. The reps for US are fine. The Ferroxcube's sales reps in Poland asked $230 for delivery of $1.0 stuff. Sort of robbers, is that right? Wrong people and management.
Iskra-Feriti from Slovenia manufactures good ferrite products, toroidal cores and double hole cores, excellent for transformers (1C material ur = 900). I recommend this company.
The experience with Epcos ferrites is weird. Epcos is not able to deliver anything in time, delivering "we forgot". RIP, rest in peace Epcos.
There are few important components - good caps of known properties, inductors, voltage regulator, and the transistor. It will run with dual gate MOSFET, JFET (low 1/f noise), as well. Low noise MAC01 voltage regulator will do the job. Even LM78L06 with 50uV/Hz of noise is fine. The LM317 is good for car lead battery charger. The best generic voltage regulator of all times is maybe the LM723 :-) Considering the input/output voltage range, features, flexibility, output noise, stability. Ignore the +6V Absolute Max input Voltage technology.
For the oscillator, expect 80dB spur free spectrum. The TFT plastic multilayer caps from AVX are very good.
The mechanical design has to be stable. Tuning cap needs reduction drive. The coupling with buffer is loose, and at low impedance. The varactor is coil tapped, or cap-divider tapped. The fine tuning with varactor (± 1.0 kHz) will slightly change the frequency-temperature coefficient. Direct tuning with varactor will ruin temperature characteristics and phase noise of every oscillator. Watch how many designs with single varactor and a 500 kHz tuning range ruined all advantages of the oscillator. Varactor behaves as a non-linear resistor with variable capacitor. All parameters change with temperature, DC voltage, and RF voltage. Varactor is for designs with e.g. Kvco = 150MHz/V. Direct varactor tuning of low noise oscillator is nonsense.
I used ceramic coil form with diamagnetic tuning slug for better temperature stability (brass, aluminum). The brass slug works as a single short turn, and tunes the frequency up. Teflon coil form is good, or Plexiglas will work as well. Keep the Q high, and shield the whole box. Stability of 2Hz at 7MHz was measured. Under 1ppm? Here I stopped. "The VFO can create stable beat with crystal oscillator, and it will stay like that for hours".
The concern was why to use another PLL loop? The oscillator phase noise is lower than any synthesizer use to have. The reference multiplication adds noise, the dividers add noise, the phase detector adds noise, the buffers add noise, the filter and regulators add noise. Outside the DC regulation loop you will get similar VCO phase noise as before, but worse. The controller used for synthesizer burns power, radiates heat, and generates broadband spur spectrum.
The DDS requires a huge battery to power up the device. The DDS phase noise and residual noise floor is so bad, only moron will use it for receiver application.
The VFO is better solution than the AD9850, AD9891 DDS - direct digital synthesizer chip. The DDS chip has fine-tuning of fractions of Hz. With X-Tal tolerance of 25 ppm. Major performance limitations are digital clock tree radiation, crystal multiplication (more spurs and more power dissipation), DAC aliasing, discrete spurs, discrete dynamic spurs; very rich grass type of parasitic spectrum, finite level of residual flat background phase noise, and the output frequency is never a round number (5,000.000 kHz). The phase noise parameter is the worst performance issue. It takes 10-14 bits to create single sine wave. The reconstruction filter is not there. If you want phase noise of -80dBc/Hz @10kHz and worse, use the DDS.
Careful observation of the oscillator performance reveals well-known issue, the limited dynamic range and S/N ratio of HP Spectrum Analyzers. Compare with Rohde & Schwarz, but they have different issues. The oscillator circuit works well with JFET (square law). Components with cubic transfer characteristic (dual gate, tunnel diode) have good inherent frequency stability and performance.
Links related (UK):
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The genuine Vackar oscillator circuit by G3PDM.
With C1/(C4+C6) and C3/C2 = 6. Use a high-quality variable capacitor with ball bearings, two-wheel transmission. Adjust feedback control C2, an air-dielectric trimmer, so the circuit just oscillates. Use a strong box from solid metal. C1, C3 and C6 are silver-mica or ceramic types glued to a solid support to reduce sensitivity to mechanical shock. The buffer amplifier is essential. Circuits using external gate-to-ground diode suffer from high phase noise and instability. The diode loads the circuit. The signal is rectified by the diode, and dynamically shifts the operating point of the JFET. Single-point grounding is fundamental. The inductor used a ceramic form. Use thick solid wires (#16 to #18 gauges) for mechanical stability. The coil is always mechanically fixed by paint. My choice is transparent nail polish with lacquer thinner. Mechanical 20:1 reduction gearing with anti-backlash may do the trick. Take out the Zener and replace it with voltage regulator. Clean all components and the box in ultrasound cleaner.
The bias and feedback caps ratio changes the close-in phase noise. A wonderful nice sine wave output doesn't mean it is - a low phase noise oscillator with excellent stability. Calculate a T or PI type, five element low pass filter with 1dB ripple. Place it on the output. Tchebyshev structure is fine. Don't forget the termination resistances. Think about the frequency plan and frequency dependency, and different types of pulling. The buffers are essential. You can divide down, but there is no need.
What is the TEMPCO compensation?
The ceramic capacitors are manufactured with different Temperature Coefficients of Capacitance (Kc).
It means, by choosing the right combination of capacitors you get zero thermal frequency drift. The cap combination you have to find out by thermal measurement. Get the numbers, calculate the caps. Standard TEMPCO values for capacitors are
Kc = +44(white), +33, -47(NPO,COG), -125, -470, -750(purple), -1250, and whatever.
The resulting cap value is:
C' = Co *(1 + deltaTemp*Kc*1E-6). [pF, C or Kelvin]
Tuning caps have high negative Kc. Coils have positive Kc. Capacitor manufacturers design caps with different Kc. There are about fifty manufacturers. AVX, Murata, ATC... Expect lead time of few weeks. The TEMPCO compensation is done for a single frequency. By modification of the schematic diagram, it is possible to reach zero TEMPCO at two frequency points.
It is handy to keep a set of caps with different Kc. A throughole ceramic cap from TV tuner can solve, what you can't find.
DDS? That's a big question. Welcome to the aliasing world and phase noise background. The DDS chips were not developed for receiver applications.
After ten years, this article still evokes questions to think about.
QST: The old man rotten air
Iskra-Feriti, ferrite materials for broadband transformers 1C, and R-1F070506-02 two hole ferrite.
 Ceramic Magnetics Inc, NJ
 TSC International.com, IL
 Ferroxcube, a Yageo company
 Elnamagnetics, NY
DSP aliasing, clock leakage, clock slip?, mixing?, glitching, broadband noise
Measured DDS: sinewave AD9833, 10-Bit DAC with load 100pF @9MHz. Spectrum analyzer span 24MHz/10dB scale.
The 48MHz span shows even richer spectrum. It is not bad for audio applications, good for checking audio circuits and crystals.
The anti-aliasing filters can clean most of the spurs. Not the noise floor.
I'm not sure what the hams want to achieve with DDS master oscillator for receiver.
Maybe they want to hear the whole RF spectrum at once.
There is no free lunch, nor the master oscillator.
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