High Stability, Low Noise, Push-Push VFO

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

When is about frequency stability, building a VFO (Variable Frequency Oscillator) most of the time we live to the mercy of the quality factor of the components. Looks strange, but today the Q-factor of inductors and capacitors used in the industry is going down, due to high requirements on their miniaturization. For example an air core inductor from a tube receiver built 70 years ago can have a Q-factor a hundred times greater than an SMD inductor from a receiver of a cellular phone.

 

With the Push-Push VFO circuit presented I was thinking to an oscillator that gives good frequency stability, mainly by its design topology, and secondary by the quality factor of the components used.

 

The Push-Push oscillator concept was introduced in 1956 by Larson E. Rapp, W1OU, one of the greatest visionaries in radio design at that time.

 

Initially the symmetrical Push-Push oscillators were developed to use their behaviour of cancelling the fundamental frequency fo and getting high 2nd harmonic (2fo) at the output.

The two active devices (Q1 and Q2) of the symmetrical topology are operating in 180° anti-phase (odd mode) at the fundamental frequency.
In this way using the circuit symmetry, you can generate stable frequencies twice higher than the resonant frequency of the resonator, without using a filter at the output.
          In the same time a Push-Push oscillator could be designed to operate at both, the fundamental frequency fo and the 2nd harmonic frequency 2fo. In this situation if you feed fo into a frequency divider instead of 2fo lowers the divider efforts.
         
Third option, depending by the output coupling topology you chose, the Push-Push oscillator can operate only at fundamental frequency fo, which is the topology used in the VFO circuit presented here.

In general, the Push-Push concept offers several advantages over single-ended designs:

The signal produced by two identical Clapp oscillators has equal frequencies but they are forced 180° out-of-phase by the connection of the transistor drains. As an innovation of the circuit, the source capacitors of the two oscillators use the same feedback capacitor (C1).
The signal at the common drains has a frequency of 2fo, but in our circuit this line is decoupled with capacitors to the ground, so the 2nd harmonic is rejected at that point.

The VFO use two J310 JFET’s (Q1 and Q2) with a high Idss (60mA) which is required for good output power and reducing the phase noise. The source resistors (220 ohms) serve the dual purpose of an approximate current source and stabilizing JFET’s drain currents ID below Idss. Because they are placed on the output path of the oscillator the output noise is also reduced.

Drains currents flowing in the sources resistors biases the sources at a positive voltage relative to the gates, and in this way effectively reverse biasing the gates. Have to remember that the symmetry of the built circuit is important to get good performances, as frequency stability and phase noise.

For tuning the circuit uses a dual variable capacitor (Cv1 and Cv2). You have to ensure that the difference between Cv1 and Cv2 capacitances (during tuning) is in +/- 20% range. Greater circuit asymmetry increases the 2nd harmonic at the output (otherwise decoupled at the drains in this topology), and inherently degrades the frequency stability and phase noise.

The resonator inductor L1 was made on a ferrite toroid, and to get good performances this should have an unloaded-Q higher than 200.

The output transformer Tr1 was made on a ferrite toroid using bifilar wires for the two windings L2 and L3 (2x10uH), and separate winding for L4 (0.5uH). I tried to avoid using a trifilar winding for this transformer, minimizing the coupling between the main oscillator and the output buffer (Q3).

The output JFET buffer (Q3) use at the drain a 1:1 transformer followed by a steep Elliptical Low Pass Filter to cut all the remaining harmonics.

The VFO can operate up to 30 MHz and even up to 100 MHz, if you change in concordance the resonator values, the feedback capacitors, and of course the LPF values at the output.

Using JFETs in oscillators have some advantages compared to bipolar transistors (BJT) as follows:

 

 

 

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