All waves RF generator
built and modified by sv3ora


An RF signal generator is a special piece of test equipment, which allows you to test a wide variety of RF circuits and perform many other interesting RF experiments. For quite a few years, I was trying to perform the RF experiments that required an RF source, using purpose-built narrow band LC or even crystal oscillators. After the tests were completed, I dissasembled some of these since I needed the components and I had to rebuild them in the future to test new circuits. When the oscillators were not dissasembled, they ended up in the circuits junk box. More or less, I ended up with a whole bunch of narrow band RF oscillators, most of them not usable for my next projects that required other frequencies of operation. If you are really into RF, soon or later you are going to need a wideband RF signal generator.

Nowadays, with the improvements in DDS technology, a digitally-synthesized frequency-stable wideband RF signal generator with good characteristics can be made at very low cost. It almost makes no sense to try to build an analogue type RF signal generator, commonly composed of many resonators, so as to cover a wide frequency range. But for the homebrewer, the situation may be quite different. There are various reasons for that. The DDS requires a microcontroller in order to set it's frequency. The microcontroller requires a programmer hardware and software, in order to be programmed. A PC is also required for the programming operations. Quite a few of the homebrewers do not know how to write a program to control the DDS and learning MCU programming is difficult for many. Thus, they rely on programs others have built and they cannot alter their operation to their needs. The DDS also requires a stable very high frequency clock oscillator for it's operation although the situation becomes a bit better (lower frequency needed) if there are internal frequency multipliers within the DDS chip. Apart from these things, soldering a DDS chip is a nightmare for the homebrewer with little or no experience in SMD and it requires SMD equipment. The tiny pinout of the DDS chips can be proven to be challenging to solder, even for more experienced people and prototyping is almost out of question. Finally, a DDS chip is a "black box" module and there is no real satisfaction to the RF experimenter, since he is not building any RF circuit, but just using a chip to produce the RF without any chance to change it's RF characteristics. Take the time to read the list of requirements in this paragraph and you will immediatelly see why a DDS-based generator is not always the best solution, practically speaking. Even the cost of the DDS that might initially be thought as low, can be proven to be much higher at the end, with all those mentioned requirements.

An analogue RF signal generator instead, can be built with a few discrete components soldered together with a common soldering iron and no previous experience in SMD components or MCU-related things. Depended on the generator circuit and the way of building it, the total cost may be lower than that of a DDS generator and the resonators can be home-built. In a DDS, the output waveform result is always the same (guaranteed by the DDS chip specifications), if properly built. However in an analogue type RF generator, the quality of the output signal depends on many factors, such as the circuit topology, the RF shielding, the mechanical stability, the Q of the resonators etc. But you have a full control over these things and it is really up to you to decide the best way to match your requirements and that is the beauty of it.

There are poorly designed analogue RF generator circuits or poorly built, as well as good ones or properly built. I have seen and tested quite a few signal generators that appear on the net and on magazines but I wasn't really satisfied with the end result of any of these. It is ok to build a poor analogue RF signal generator if you only intend to do simple things with it, but if you are going to use it for more serious work, choose a circuit and method of construction that meet your tighter requirements. Since you are going to do the effort and spend your time in building an RF signal generator anyway, I would advise you to start with a better quality circuit and build it as properly as you can, based on your available time, your patience and the end result you try to achieve.

For the average home lab, the main characteristics that distinguish a good analogue RF signal generator from a poor one, as far as concern the quality of the generated signal, are:

This RF generator was presented in Elektor magazine summer circuits July-August 1980 at page 19. The schematic above, shows my modifications (not many) and this is exactly the schematic I built. I have built this circuit in different versions at the past and I was satisfied, but this version presented the best quality signal. The harmonics of the sinewave are -40dBc or better at all frequencies in this version. I did not build the AM modulator because I did not like the idea of modulating directly the oscillator. After all, modulation is only 30% that way. If I needed AM modulation I would do it better in a later stage, possibly in the final amplifier.

The power supply is my design. I happened to have a center tapped transformer so I made the power supply around this transformer. However, you can use a common 15v transformer and rectify the AC using four diodes in a bridge configuration instead. In that case you would need four 100nF capacitors, each placed in parallel to each diode on the bridge. The regulator is a capacitance multiplier circuit that has low noise and works well. The 13v zener gives an output voltage of 12.4v which is very close to the nominal (12v).

The built style I used was also a new one. This time I built the circuit on a piece of PCB with all the components surface mounted. I used a small cutter to cut channels onto the PCB copper, creating that way the pads and the PCB traces. This method takes a bit more time than just soldering the components in "dead bug" style, but the result is very neat as you can see from the pictures.

The trick for quickly engraving channels onto the pcb, is to make thin side lines on each trace or pad first, making sure that the copper is cut all the way through, down to the fiberglass. Then peel a small part of the copper out of the pcb at one side of the trace. Finally, use tweezers to pull this peeled part and the copper will be pulled all the way along the channel, isolating the trace from the copper ground around it.

Make sure you cover the red LED with a small piece of black thermal shrink tube, so as to create a lightproof enclosure for the LED. Heat the thermal shrink tube and when it is still hot, hold it's both ends with pliers and let it cool as you hold it. When it is cooled the shrink tube will stay in place with both ends closed, preventing any light from affecting the LED.

Here are more pictures from my implementation of the RF generator

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