The "Super VFO"
As stable as a crystal!
 
by sv3ora


The front panel of the "Super VFO"

This "Super VFO" is a design of mine. It will give you rock-stable variable frequency operation at all HF bands and so it can be used reliably for your HF transceivers. I called it a "Super VFO" because of its features:



The back panel of the "Super VFO"



In the past I have built and tested many crystal oscillators for my projects. I have also built broadband crystal oscillators, in the sense that the crystal was the only component required to be changed for a different frequency or band of operation. These circuits work well and they are stable, but there is a need to obtain crystals for in-bands operation and these crystals are harder to find and expensive. Even then, you are restricted to operate on specific frequencies only, plus or minus a very small range around the crystals (VXO). These days, where fewer and fewer people seem to use shortwave, your chances to make QSOs when operating only on specific frequencies, may be very limited, especially when transmitting QRP. To be honest, I rarely had great QSO success when calling on a frequency, waiting for any answers. I mostly find good signal stations and try to reach them. These stations rarely transmit on the available crystal frequencies. Remember, in most cases the frequency of a VXO cannot be varied a lot especially in the lower bands. A few KHz out and you completely miss the signal. And you also have to allow for some tunning to be able to hear a comfortable tone when operating CW or to zero beat an SSB station. So despite crystals are very stable in frequency, their use is very limited.

The next frequency-stable and promising solution is to use a DDS oscillator. Nowadays, with the improvements in DDS technology, a digitally-synthesized frequency-stable wideband oscillator with good characteristics can be made at very low cost. It almost makes no sense to try to build an analogue type VFO, 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 oscillator is not always the best solution, practically speaking. Even the cost of the DDS chip that might initially be thought as low, can be proven to be much higher at the end, with all those mentioned requirements. A DDS oscillator project is also generally considered a complex project requiring quite a lot of parts.

Another solution for a stable variable frequency oscillator, is to use an analogue varicap-controlled VFO and phase lock it to a reference crystal oscillator. The old MC145151 is one of the simplest PLLs I am aware of. It requires a microcontroller for quick frequency changing or a set of a few CMOS or TTL chips, to change it's frequency. Some extra switching is needed as well, to switch the VFO resonators for different bands. This chip can be controlled by DIP switches as well, without the need for the extra chips, but setting a frequency requires some calculations from the user each time, so scanning a band for signals is out of question. Again it is considered a more complex project requiring quite a few parts.

The VFO stabilizer is another option
for a stable variable frequency oscillator. It uses an analogue varicap-controlled VFO and a frequency comparator to achieve the same effect as that of the PLL. There are two modes of operation, the slow and the fast mode. In the faster mode a stable high frequency clock is required and even then the stability is not guaranteed on higher bands. It is also considered a more complex project requiring quite a few parts.

The simplest solution, requiring the least number of parts, seems to be an unlocked VFO. But most unlocked VFOs are not stable enough for SSB/CW operation. That is the main reason why all the previously mentioned solutions have been invented. However, there are a few VFO circuits on the literature that claim to be very stable in frequency. Some of them, utilize temperature-compensation by using special components that compensate for the temperature-related frequency drift of the oscillator. Temperature compensation is difficult to achieve and it is not easily reproduced. Some other VFOs achieve frequency stability by other means. The oscillator I describe in this article, is such.


The deluxe version of the "Super VFO".

The full featured version of my "Super VFO" is shown above. This version includes all the features mentioned in the list at the beginning of this article. However, to better describe the operation of the core of the VFO, let's look at the simplistic bare-bones version of the VFO below.


The bare-bones version of the "Super VFO".

The circuit is built around the Franklin VFO topology. Any tunable oscillator consists in essence of two parts, a tuned circuit of high Q and a maintaining amplifier to replenish the losses in the tuned circuit. A basic advantage of the Franklin oscillator is that the maintaining circuit need to be only very loosely coupled to, and impose very light loading of the resonant circuit. Another practical advantage is the single two-terminal coil, without taps, which has one end at RF earth (also true for the variable capacitor), with no capacitive or inductive divider as in the Hartley or Colpitts circuits and most of their variants, which is frequency conscious. Because of the loose coupling, those changes affecting the maintaining amplifier, whether valve or solidstate, are arranged to have only very limited effect on the frequency.

It is important to remember that the stability of a Franklin oscillator depends upon the quality of the frequency-determining high-Q LC resonant tank circuit and the looseness of the coupling to it. Thus, the coupling capacitors must be the smallest possible that ensure reliable start up of the oscillator at all frequencies of interest.

The "Super VFO" is built around this topology. As can be seen on the top left side of the bare-bones version schematic, the coupling capacitors of the resonant circuit to the active devices are only 3.3pF each. These capacitors, but also all the frequency-determining capacitors in both versions of the VFO, must be of NP0 ceramic type, so as to provide temperature immunity for the oscillator, which leads to even more frequency stability. My previous experiments on oscillators, have proven that the cleanest point (purest sinewave) to extract the signal out of any oscillator, is the resonator point. Thus, in contrast to all the other Franklin oscillators I have seen, I extract the output of this oscillator directly from the LC resonators. This has the disadvantage of course, that the LC is loaded more. But practically speaking, I placed a 3.3pF capacitor at the LC to couple energy out and I did not find any significant loading of the LC. I guess it could have been done also without any coupling capacitor, using just a separate coupling winding near the L, but I thought the coupling capacitor method to be easier and more easily reproduced with similar results each time, because it does not depend on the style of the winding from the builder.

Close up of the VFO circuit and the resonator elements

The output level of any Franklin oscillator is very low and in my oscillator, where I couple the output from the LC using a very small value of coupling capacitor, this level is even lower. To be usable and also to ensure frequency stability under different loads, the VFO needs a good buffer. In my "cheap and easy all HF bands transceiver" I have designed a small buffer based around a 2N2222 or a 2N5109 transistor, that is easy to build, broadband, does not require a transformer, can drive 50 ohm loads and has a low harmonics content in the range of -35dBc to -40dBc. To bring the level of the VFO to around 5mW, I cascaded two such amplifiers, based on the cheap plastic versions of the 2N2222 BJTs.

To make the level of the VFO nearly constant at almost all bands, I tested an ALC loop that I had previously used in the audio section of my "broadband concept receiver", but now applied on the RF. It is essentially an automatic potentiometer, which works flawlessly in this VFO.


An overview of the internals of the finished "Super VFO".

It is important for the power supply to be able to provide a stable voltage to the VFO. This is accomplished by the use of two linear regulators. It could be made discrete, but linear regulators are very comon nowadays and I thought it would not worth the extra components needed for a discrete circuit. The 5V regulator can be the 78L05 (TO-92 package) or the standard 7805 (bigger package) but the 9V regulator must better be of the bigger package, as the buffer draws almost 60mA.

One of the most interesting things about this VFO is that for covering the whole HF (80m-10m) only a single untapped inductor is needed and a few range capacitors (which must be NP0). This makes it really easy to build, cheap and small-sized. Moreover, the frequency stability is more than adequate at all frequencies using this inductor alone. Build this inductor exactly as I did, with 13 turns of 1mm diameter enameled copper wire, evenly spaced around a T68-7 toroidal core. This will guarrantee you similar results as mine. Go ahead and experiment with other cores and coils if you like, but do not judge the frequency stability of your version of the VFO, based on your particular inductor, if it is not built as I describe. Keep the leads of the inductor short. You can also keep it firm by attaching it with a non-magnetic screw and a few pieces of plastic onto the PCB of the VFO, trying to keep it away from magnetic metals.

Close up of the toroidal inductor, fitted in place.

From now on, let's look at the deluxe version of the "Super VFO" to describe the rest of its features. An important thing when building a general purpose VFO that could be used in all shorts of circuits, is to be able to vary it's output level accordingly, while keeping a nearly constant output impedance. This can be done using switched attenuators, but for simplicity a potentiometer is enough. In the configuration shown, the output attenuators keep a relatively constant output impedance at all attenuation settings near to 50 ohms.

The variable attenuators (potentiometers) for both TX and RX outputs.

The frequency determining inductor of the VFO has been described earlier. In the bare-bones version, a band switching NP0 type capacitor and a variable capacitor are only used. However, to be able to operate the VFO without the need for mechanical reduction drives and to make the tuning convenient and within bands, a complex network of switching capacitors (all NP0) is needed. SW1, is a quad-section 6-position selector switch. Section SW1a is used for the band switching. Section SW1b is used for the band spread, i.e. to make the tuning non-touchy. SW1c takes care of the RIT/Fine-tune spread for each band. SW1d is used for the band selection control signals. These signals can drive external switched filters of other band-specific circuits if needed. Note that for 80m, SW1b and SW1c are shorted. Also on 30m,SW1c is shorted.

Because some of the capacitors used in this network are very small, depending on the parasitic capacitance of your own selector switch, you might not need them, or you might need bigger values. This is all a trial and error, but I provide you with starting values to begin with. You could use variable trimmers instead of the lots of the fixed capacitors combinations, but make sure these are all NP0.

Note that I have build the VFO for 6 bands because I only had a 6-position switch, but I have tested it and it can cover also the 12m and 10m with the same inductor. However, I could not test for the values of the band and spread capacitors in these bands, so this is up to you to determine it.

Close up of the selector switch and the tuning capacitors

On the DIN I/O connector used, apart from the band selection output signals, there is also a TX/RX I/O control signal available. This indicates the TX/RX state of the transceiver. The VFO switches this signal line to 12v to indicate to external circuits that TX is enabled. However other external circuits may also take over the switching of this line to 12v, to indicate the TX state of a transceiver. The VFO responds to this change as well and it automatically changes it's state. So this signal line is bi-directional. There is also a VCC line available on the DIN connector. This is optional and I left it there because I had a spare pin on the connector.

If it is to control a transceiver from the computer, an embedded keyer needs to be available. The Super VFO includes such a keyer which does several things. In CW mode, it is used as a manual keyer, allowing the external straight key to be connected to the ground. It also allows break-in operation, meaning that the RX is automatically enabled after any key-up. In CW and I/Q mode, it is also used as a VOX, allowing any audio signal from the PC sound card to key the VFO. Whether in CW or in I/Q mode, the keyer switches the TX/RX indicator and controls the TX/RX control line on the DIN connector. However, the keyer is not controlled by external TX/RX I/O control signals.

The keyer works in a totally different way than the clasic voltage doubler and transistor circuits. Here, the transistor works as an AF amplifier, amplifying audio out of the sound card without rectifying it, so that to make the circuit really sensitive in a single stage. The 100k trimmer resistor must be set so that the relay is switched off, just before it starts to switch on. This is not used as a sensitivity control, it is used as bias setting for the transistor. The sensitivity is controlled by your PC soundcard output level setting. This adjustment must be done only once, with no audio signal applied to the keyer. If the keyer is set like this, then an audio signal from the PC, will give it the extra boost needed to switch the relay on. However, the operation of the circuit in that way, may cause the relay to pulsate at the rate of the audio signal. To prevent this, a 10uF capacitor is used across the relay coil (note there is already an internal flyback diode connected to the relay coil I used, if your relay does not have a diode embedded you have to connect one). The 100nF at the VOX input, prevents any leaked RF from accidentally controlling the keyer. The automatic keyer operates from only 30% of the volume on my laptop sound card, which is something that can't be easily achieved by other single transistor keyers without voltage doublers or step up transformers. It operates reliably from about 100Hz to 3KHz of audio tone and from very low speeds up to very high ones (even Feld-Hell speeds) without problems.

The VFO has two modes of operation. The first mode is useful on CW direct conversion transceivers, where there must be offset of the local oscillator between TX and RX. The second mode is useful on transceivers featuring balanced modulators or phasing types (I/Q), which modulate audio. SW2 switches between these two modes. Of course you also need to connect your Super VFO properly, depended on the system you have.

In CW mode, the RIT/Fine-tune control, sets the offset of the oscillator during RX, whereas the TX frequency stays unchanged and can only be controlled by the frequency knob in this mode. The oscillator frequency can be offset below or above the carrier, allowing for USB or LSB operation respectively. If you connect an external frequency counter to the VFO, then you can find the zero-beat point (the point where the receive offset is zero), which is a different knob setting for each band, by noticing the TX and RX frequencies on the counter. Similarly, with a counter you can find out the exact offset frequency and direction (USB/LSB).

In I/Q mode, the RIT/Fine-tune control, is only used for fine tuning of the frequency of the VFO, since the offset is automatically taken care from the modulating sounds/tones. Being able to fine tune the frequency is very convenient and it is done without any extra circuits.

The picture above shows how the oscillator is to be connected in direct conversion transceivers in both modes of operation. The dotted lines are for connection of the oscillator to a CW receiver. The continuous lines are for connection of the oscillator to an I/Q transceiver (excluding its RF amplifiers). The RF power amplifier and preamplifier sections are common to both modes of operation. Hopefully, this diagram will show you how to properly connect your Super VFO to the rest of your direct conversion transceiver.

The Super VFO has been built in an ordinary metal project enclosure (it has to be all metal, do not use plastic or any other non-metal material). On the front panel, there is the ON/OFF switch, the TX/RX indicator LED, the frequency and band controls and the RIT/Fine-frequency control. On the back panel, there is the RX and TX signal connectors and their signal level potentiometers, the counter output connector, the DIN connector, the mode switch, the external key connector, the VOX adjustment and connector and of course the power connectors.

Near every control or connector, markings have been made, using a permanent marker. Markings are important, not only to make you remember what each knob or connector does, but also when you want your completed homebrew equipment to look more "commercial". It really makes a difference in the "look" of the equipment, so spend some time on it if you like good looking gear. The permanent marker is a good solution as it cannot be erased, unless you use alcohol or acetone. You can do really good markings with a steady hand and the help of some rulers. You can also use stencils for the various shapes and the letters (if you can find ones with small letters). Use your imagination to find out the best result. If you do not like it, you can erase it (or part of it) and then start all over again until you are satisfied. You can also use multiple color permanent markers if you like, either on the panel or on the knobs. For example, I used a white permanent marker, to draw some indexing lines and dots onto some black knobs that did not have any.

For the RF connectors, I used BNC ones. They do not need to be expensive varieties for HF. For the chassis I used these with the square flange, to prevent them form get un-screwed from the chassis. BNC connectors are an excellent choice. They are cheap, small (compared with UHF or N-type) they have the locking mechanism, they can be quickly inserted and locked or pulled out, they do not get un-screwed but they can be turned without disconnected. For the I/O signals connector I used the DIN type because it requires a circular hole on the chassis. A serial port or a VGA connector could have been used as well, but they require special holes for mounting.


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