What kind antenna to use? A long-wire antenna has a very high impedance at these frequencies, unless it is several kilometres long. The finished projects should be mounted in a metal (preferably die-cast) box in order to minimise local oscillator radiation.The case of the crystal XL1 should be earthed by means of a short length of wire soldered firmly to the ground.If the oscillator is some distance from the input to the mixer then it should be fed via screened cable to avoid stray pick up.Tr5 and Tr6, should be laid out on the board as symmetrically as possible and leads kept as short as possible.Keep the input of Tr1l as far away as possible from the output of Tr2.Layout is not critical, as long as the following guide-lines are observed: StraightforwardĬonstruction is quite straightforward, and the unit can be built on Veroboard or similar. The number, and ratio, of turns could be experimented with, to give best overall results. In the prototype, T1 was wound on two ferrite beads, or you could use a T50-2 toroidal ferrite core. The secondary is 12 turns of the same wire. Transformer T1 primary is 24 turns, centre-tapped, of 0.27 mm (32s.w.g.) enamelled copper wire.
The up-converted output from the mixer, is taken from the collectors of Tr5 and Tr6 via the output transformer T1. The output of the oscillator is taken from the emitter of Tr4, and fed into the mixer via identical networks C20, R23 and C21, R24. Transistor Tr4 and its associated components form a crystal controlled, Colpitts style, local oscillator.Ĭapacitor C 13 may be a 60pF trimmer if precise control of frequency is required. The converter oscillator is crystal controlled, and almost any parallel resonance fundamental crystal in the range 3-5 MHz will do. The mixer, is symmetrical, but in addition, variable resistors, R25 and R27, improve the balance of the mixer, reducing l.o. The transistors, Tr5 and Tr6, form the balanced mixer, and must be matched for gain. This phase splitter stage, Tr3, provides two signals of the same level but 180° out-of-phase with each other at outputs A and B. Transistor Tr2 is another gain stage (C7 reduces gain above 150kHz), feeding the phase splitter Tr3. Stage gain is adjustable with R6, while capacitor C2 in the collector circuit of Tr1 reduces gain above 150 kHz. This, in conjunction with the input impedance of the first stage, forms a high pass filter from about 500Hz upwards. signal from the loop is fed into the base of Tr1 via C1. Points A and B couple to the similarly marked points in Fig. 2b: The local oscillator and balanced mixer section. 2a: The circuit of the pre-amplifier and phase splitter section of the project.įig. amplifiers may be broad band and rolled off gently above 150 kHz.įig. Since we are dealing with low frequencies, the r.f. Even the most elementary of receivers should resolve the signal. Using a balanced mixer, results in a great reduction in the l.o. The answer to the converter problem is to use a balanced mixer in the up-converter. signal being lost under an enormous blocking signal. The result of the strong nearby signal, is to overload the front end of the communications receiver (and probably the i.f stages as well if a mediocre filter is used). What we have done is to generate a very strong signal (the l.o.) only 10 kHz away from our wanted (and very much weaker) signal. With the converter local oscillator running at 3.5 MHz and a conventional mixer all hell is let loose. Converting A SignalĬonverting a 60kHz signal to 3.560 MHz presents no real problem, but consider converting 10 kHz up to 3.510 MHz.
Using the up-converter described here, will make them audible in the 3.5 MHz band as shown here. 1: Many, interesting signals are to be found below the long wave broadcast band. A standard communications receiver will then allow you to listen in on this rather strange, and intriguing part of the electro-magnetic spectrum. The converter described here translates frequencies, in the range 500 Hz to 150 kHz, up into the 3.5 MHz amateur band. There are also naturally occurring phenomena between about 1 kHz and 20-30 kHz. Signals in this band include radio navigation, standard frequencies and submarine communications. Transmissions below 150kHz, potentially, have ranges of many thousands of kilometres. was aroused some years ago, when I found an article on decoding time transmissions from the atomic clock signal on MSF (on 60 kHz). band, most do not cover the frequency range below 500 kHz. Whilst the majority of these sets perform very well in the h.f. Some readers will doubtless possess a communications receiver.
#VLF RECEIVER DIY HOW TO#
Adrian Knott G0KSN shows you how to make these signals audible on a Conumt meat ions receiver. Home - Techniek - Electronica - Radiotechniek - Radio amateur bladen - Practical Wireless - VLF Up-Converterĭown below the long wave band there are many interesting signals to he heard, both natural and manmade.
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