There have been many articles published over the years describing converters to extend the range of otherwise satisfactory low frequency receivers, This article does not present a unique design, but rather a successful combination of" previously published circuits which, taken together, have the advantage of simplicity, dependability, and easy adaptation to a wide range of frequencies and a variety of applications.
If we consider the requirements for a good rf amplifier, it should provide efficient energy transfer from the antenna to the mixer; enough amplification to overcome internal mixer noise; a low degree of noise introduced by the amplifier itself; a low degree of cross modulation in the presence ot strong adjacent signals; and a moderate amount of selectivity (a requirement which will vary from one application to another). In practical design compromises are required.
We may fairly assume that most receivers to which we will want to attach converters use tubes rather than transistors; consequently, we may as well use tubes in the converter as well- Pentodes provide high amplification and gooil selectivity hut often with poor intermodulation characteristics and fairly high amounts of noise, ¡'nodes, in contrast, have good noise figure, less selectivity, lower amplification, and fair intermodillation characteristics.
The rf amplifier shown utilizes two similar triodes, cathode-coupled. This ;ias several advantages. Cathode coupling provides good energy transfer between the two stages and the circuit is not prone to oscillation so long as the antenna loading is adequate. With a typical amateur antenna system fed by a transmission line of fifty to three hundred ohms, it is easy to get good signal coupling to the first grid by adjusting the tap on the coil LL Raising the tap will increase the coupling and by loading the circuit it will broaden the tuning, which is desirable in many applications. Try one-third up from the bottom as a first approximation. Note that this coil is resonated with a variable capacitor, which allows for the easiest adjustment at the front panel when changing antennas or moving to different parts of the band.
The plate coil L2 is shown as slug-tuned. It may be necessary to shunt this coil with capacity to reach the proper frequency, and if I he response is too critical it may be neces-sarv to shunt it also with a resistance of ten thousand ohms or less to reduce the Q and thereby broaden the frequency response. Generally, it is a good plan to resonate L2 near the high frequency portions of the band to be covered and resonate LI near the low end. Some published versions of the circuit show an rf choke between ihe junction of the two cathodes and the resistor which goes to ground. This increases the efficiency, but T usually omit it. It is also possible to make the cathode resistor variable and adjust it for optimum gain, The 33( ohm value shown is generally adequate.
The resistors in the B + lines to the rf amplifier (here shown as one thousand ohms) are not critical as to value and can be omitted entirely if the power supply is located close to the rest of the components on the converter chassis* However, they do provide additional decoupling, if needed, and as they are hardly larger than a piece of wire and not very expensive it may be a good plan to include them.
This option does not apply to the 5000 ohm resistor feeding the oscillator, which is an essential part of the rf voltage dividing network. (It could, however,, be replaced with a suitable rf choke). The oscillator shown here is a particularly good one for developing output at some harmonic of the fundamental frequency of the crystal. I have used this with common surplus FT243 crystals to produce 3rd, 4th, 5th, and 6th harmonics, Output decreases as you go higher, Where the converter oscillator operates on a relatively low frequency, and a crystal for (he fundamental frequency is available, a simpler Pierce-type oscillator can be used, thereby eliminating a few components.
Note that the output of the mixer stage is not tuned. This means that images will be present in (his output and we depend entirely upon the front end selectivity of the tunable receiver which follows the converter to eliminate them. Image rejection can be improved by adding a tuned mixer plate cir* cuit, although this will restrict the bandpass characteristics beyond what is desirable for some applications. In many situations, the amateur may find it convenient merely to change the crystal if some one strong local image falls in a portion of the band which he wants to use.
Fixed capacitors shown should be either mica or high quality ceramic. Coils can be wound on any suitable material, including forms salvaged from old TV sets. The easiest way to make them is "cut-and-try/* checking as you go with a grid-dip oscillator. The circuit in general is sufficiently uncritical that layout imposes no great problem and any convenient arrangement of parts should serve. It is well to separate the coils so that they do not directly interact.
Tubes also are not critical The 6BQ7A connected as shown provides a satisfactory noise figure up through the six-meter band, although at this point, and on higher frequencies, improvement would be found by substituting nuvisters. There is nothing magical about a 12AU7 as an oscillator-mixer, but they are plentiful and work well on this circuit. Other triodes can be used if available. Power supply requirements are slight, and 150 volts B+ is entirely adequate.
In some units where I needed a converter of tliis description to be separately powered, I have salvaged the power supply from an old TV preselector, and found it entirely adequate. An alternative output circuit might be to put a resistor in the cathode of the mixer and take the output signal from the cathode. In such a case, the mixer plate load resistance would be reduced to zero or merely to a 1000 ohm decoupling resistor as in the rf stages.
Since this circuit is so simple, inexpensive, and dependable, it is a good one to pass on to the novice who is just getting started building equipment and needs the encouragement of success in each project. However, it is also good enough that T have several of these in operation in various kinds of gear and have virtually given up experimenting with any other circuit. Lifter having tried nearly all that I have seen published over the years. In short, it is not the best, but it will do a very good job.
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