Very Low Phase Noise Vackar VFO

for HF Transceivers

 

Iulian Rosu, YO3DAC / VA3IUL, http://www.qsl.net/va3iul/

 pdf version

 

Before making this circuit I followed a few basic rules selecting the right transistor and  the right circuit topology for an optimized oscillator design:

 The VFO topology was chosen to be a Vackar oscillator, and below are few of its characteristics:

Compared to a standard Vackar VFO, the modified oscillator use a DC coupled emitter follower (Q2) and a feedback bias RLC network (R2, C7, L2, C10, R5).

The feedback bias network samples the DC current on the emitter of Q2 up to some offset, and within the loop gain and bandwidth of the network is reducing the modulation and up-conversion that makes a noisy oscillator.

 

For oscillator transistors were chosen medium power audio transistors (BD135 and BD136), having the FT frequency approximately 190MHz, which works well as oscillators at HF frequencies up to 30MHz.

The two transistors were biased at very low current (both together, Q1+Q2 = 6 mA).

At this low bias current the 1/f flicker-noise is minimum.

 

The two-stage oscillator is followed by a buffer amplifier (Q3), which uses also a medium power transistor biased for linear amplification to get minimum spurious emissions and harmonics. This amplifier is LC tuned on oscillation frequency, and is followed by a steep 5 pole Elliptic Low-Pass Filter (cut-off at 10MHz) to further reduce the harmonics.
 

Tuning the Variable Frequency Oscillator (VFO)

 

The feedback trim capacitor (C4) will be adjusted just to start and maintain the oscillation.

The oscillator can be used for other HF frequencies modifying in concordance L1, C1, C2, and of course the output LC tank L5, C13 and the LPF cut-off frequency if is necessarily.

To increase or decrease the frequency bandwidth covered by the VFO, C2 will be adjusted and if is necessarily a capacitor will be added in series with the variable capacitor C1.

Good quality inductors and capacitors will be used for good performances as frequency stability and phase noise.

The output power of the oscillator is approximately +7dBm, ready to drive a diode mixer as SBL-1 or other mixers, adjusting the level in concordance.

     R7 will be adjusted for proper input level of the buffer amplifier, below compression point, for perfect sine-wave and minimum harmonics at the output.

 

In conclusion a few criteria’s for minimum phase noise were met:

Below are a few simulated measurements of the VFO:

 

Phase Noise

Offset from Carrier

-109 dBc/Hz

10 Hz

-128 dBc/Hz

100 Hz

-148 dBc/Hz

1 kHz

-165 dBc/Hz

10 kHz

-167 dBc/Hz

100 kHz

-164 dBc/Hz

1 MHz

 

References:

1. LC Oscillators and their Frequency Stability - Tesla Technical Reports, Dec. 1949 - Jiri Vackar

2. Oscillator Design and Computer Simulation -  R.W. Rhea

3. RF/Microwave Circuit Design for Wireless Applications -  Ulrich L. Rohde and David P. Newkirk

4. Microwave Circuit Design using Linear and Nonlinear Techniques -  G. Vendelin, A. Pavio, U. Rohde,

5. Oscillator Basics and Low-Noise Techniques for Microwave Oscillators and VCOs - U.Rohde

6. Low Noise Oscillator Design and Performance - Michael M. Driscoll

7. California Eastern Laboratories - AN1026 - “1/f Noise Characteristics Influencing Phase Noise”

8. Infineon Technologies - AN023 - Designing Oscillators with low 1/f-noise

9. Mini-Circuits – VCO Designers Handbook 2001

10. Applied Microwave and Wireless, 1997-2002

11. Analog Devices Application Notes

12. RF Design, 1993-2009

13. Microwave Journal, 1997-2009

14. Alpha Industries - VCO Application notes

 

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