Single span, high spectral purity oscillator 3KHz-30MHz
A better alternative to DDS?


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An oscillator is the basis of all modern receivers and transmitters. In many circuits the oscillator characteristics (stability, phase noise, frequency range, output level, signal distortion, spectral purity, etc) define in a great extend the overal system characteristics. Designing signal sources has been an issue for many decades and different types of signal generating techniques have been proposed over the years, each one with it's own characteristics.

Basic oscillator types

HF radio amateurs pay specific attention mostly on two basic characteristics of an oscillator, frequency range and stability. The first one, defines the receiving range on a receiver and the transmitting range on a transmitter. The second one, defines the frequency stability of a receiver or a transmitter. Many techniques have been tried in order to achieve such good characteristics. Some of these include:

In a variable frequency oscillator the frequency is set using a variable coil-capacitor circuit. In most cases the range is limited and the stability is not adequate. Nevertheless, there are special VFO circuits where the range can be much extended.
In a voltage controlled oscillator the frequency is set using a coil and a varicap, which plays the role of the variale capacitor. In most cases the range is limited and the stability is not adequate. Usually, a VFO can be made into a VCO by replacing the variable capacitor with a varicap and appropriate circuicity. Nevertheless, The variable capacitor is more neutral and clean than the varicap.
Crystal oscillators use a crystal to define the frequency. These are very stable oscillators and if combined with crystal ovens to keep the temperature stable, they can be made into reference oscillators. Well designed such oscillators can also achieve very low phase noise. The drawback is that they can oscillate on only a fixed frequency.
Variable crystal oscillators is a try to achieve somehow extended range on a XO, by varying the crystal parameters, using combinations of coils and capacitors. Nevertheless, their range is very limited compared to other types of oscillators.
This is where analogue synthesys begins. Mixing of a XO with a multiple switched XO makes a stable oscillator that oscillates in more than one switched predefined frequencies. Old CB transceivers were using this technique to achieve a band of frequency channels.
Similar technique to the above, but this time the XO is mixed with a VFO or VCO to achieve a greater range of frequencies. By switching multiple XOs, a great range oscillator can be made using the same VFO.
This is where digital circuits combine with analogue, to stabilize them. A frequency locked loop monitors the output frequency of a VCO and sends the relevant voltage to it, in order to stabilize it, therefore greatly improve stability.
A phase locked loop is a similar technique to the above that locks to the phase of the VCO to stabilize it. Usually, the range of the oscillator affects the frequency step acuracy, so very wide range accurate PLLs may be difficult to build.
Whereas the two previously mentioned techniques are not actual oscillators, but mostly VCO stabilizers, the direct digital synthesizers are true digital oscillators. They are totally based on software to generate the output waveform.

Obviously, one or more of the above mentioned techniques can be combined, to achieve the desired specifications in a system. 

Oscillators spectral purity

Apart from frequency range and stability, there is a third parameter that is important in HF designs and this is the spectral purity of the oscillator. Every oscillator type of the above, will generate harmonics of the fundamental frequency. Especially on mixer and DDS based oscillators, the harmonics and spurious can be so many, that is difficult to calculate them without using an appropriate calculator software. The unwanted harmonics can degrade the system performance so much as to make it totally useless in some applications. The solution is filtering.


Filtering of harmonics is easy on VFO, VCO, XO or VXO oscillators. It usually consists of a LPF after the oscillator, to filter the higher produced harmonics. Since the range of these oscillators is limited, just one LPF is usually required per oscillator. However, on synthesized and DDS oscillators, filtering is much more difficult.

DDSs have harmonics as well as totally unrelated spurious tones, that can become huge as you approach an output frequency of around 1/2 the clock rate. Some radio amateurs use DDS as the LO in receivers, but this could have spurii all over the place, so one would probably want to filter as much as possible. This requires many different filters or variable ones which are difficult to make and tune and the unwanted signals could never be totally eliminated. The best solution, if such signals must not be present in the system, is to try not to use a DDS as LO at all.

In synthesized oscillators, fundamental and higher harmonics from both oscillators in the mixer input will combine and produce a wide range of output frequencies, not only higher than the oscillator frequencies, but lower as well. As before, filtering becomes very difficult at the mixer output. At least, in this case, there is something else that can be done to improve things.

The oscillator design

By carefully selecting the frequency ranges of the oscillators, just two suitable low pass filters can be built, to filter out the higher harmonics of the oscillators prior to mixing. Thus, the levels of the oscillators harmonics will be much lower and the mixer output signal will be much cleaner. Nevertheless, a third LPF is needed after the mixer, to filter out unwanted mixer products, as well as further attenuating any oscillators harmonics higher products that have passed the mixer.

The overal oscillator proposal is shown above. A fixed frequency crystal oscillator is mixed with a VCO to produce the wanted range of frequencies. Filtering is done before and after the mixer. Following the mixer is a set of broadband amplifiers to isolate it from the load and to bring the wanted signal into useable higher levels.

As far as concern frequency stability, the fixed frequency crystal oscillator can be very stable, especially if a TCXO is used. Thus, the main thing that defines output frequency stability, is the VCO stability. Especially if using high frequencies for the VCO, the stability could suffer. Therefore an FFL is used to stabilize the VCO. The FLL chosen, had an embedded frequency counter, so it was connected between the filtered mixer output and the VCO, to directly read and display the output frequency. Alternativelly, if an FLL without a frequency display is to be used, the FLL could be connected between the input and the filtered output of the VCO.

The variable oscillator

The VCO is based on the MC1648 chip. It oscillates on 48MHz-78MHz. The 3.9k sets the AGC so that the power output is as linear as possible throughout the frequency range and the sinusoidal signal is as much undistorted as possible. The power output is 0.35Vpp-0.43Vpp throughout the frequency range. The 11 windings on the T30-12 toroid core have been chosen so that most of the turns in the multi-turn potentiometer are used in the 48MHz-78MHz. This gives finer frequency tuning throughout this frequency range.

The fixed frequency oscillator

The fixed crystal oscillator used is a TCXO, to ensure great stability. The frequency of choice is 48MHz. This does not have to be a TCXO, but this will eliminate frequency drift due to temperature variations. The detailed characteristics of the TCXO are shown below.

The mixer

The mixer used is a Mini Circuits MCL SRA-6. This has a 3KHz-100MHz LO/RF and a DC-100MHz IF. Detailed specifications about this mixer can be found in this pdf file. If you can't find this mixer an SBL-1 may be alternativelly used with narrower lower range or you may build your own discrete mixer from a quad of diodes and a pair of broadband transformers.

The filters I have designed are shown below.

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