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Zpět The VLF converter once more

Vydáno dne 08. 01. 2011 (13083 přečtení)


přepnout na verzi v češtině     version in Czech language

Almost immediately after the publication of an article about our listening to the SAQ Grimeton transmitter I got e-mail with a suggestion to design a printed circuit for the converter, which I mentioned there. Libor and I have agreed that for its future use, it would be better to remake it into a more compact and less vulnerable form.

I designed the PCB, as usual, in a free version of the program Eagle. A little problem was that the SBL-1 mixer is not included in any free library of components. So I had to create it. On the other hand, at least I was forced to learn how to create such a new component and add it to the Eagle library.

The following pictures show all you need for making the converter. The schematic, printed circuit and the board populated with components. The size of the PCB is about 6 x 4.5 centimetres. The picture in the middle is not to scale. It can only be used for checking. Pictures that are to scale and therefore can be used for making the PCB (or for exposure of film) are in the DOWNLOAD section of this web site. There are two PDF files; the picture in the first one is straight and in the second one it is mirrored.

schematic by G3XBM
PCB (not in scale)
populated PCB
Schematic by G3XBM
re-drawn in Eagle
From the copper side
(not in scale)
How to populate it
with components

    Used components:
  • You can probably use a mixer IE-500, UZ-07 or some others instead of the SBL-1, but we have not tried it. (UZ-07 was produced in former Czechoslovakia and can sometimes be available at flea markets).
  • You can use almost any silicon NPN transistors for general purpose. We used the BC337, since this bears higher collector current than the originally used BC546 and could therefore withstand somewhat less friendly treatment during any experiments.
  • We used a 10 MHz crystal (ordinary "Chinese quality") in the oscillator. Its frequency is actually not very important, providing we are able to listen to it using our receiver (transceiver). It can be necessary to change values of serial or parallel capacitors (C2, C13, C14) to adjust the frequency exactly to the beginning of a chosen band. (When using a 10 MHz crystal, make sure that your radio is able to operate at this frequency. The amateur band 30m starts at 10.1 MHz, so if the European version of the radio uses high quality band pass filters, it could have a worse sensitivity at this frequency.)
  • Inductance in the input LPF is an axial SMCC choke 100uH. (It looks like 1-Watt resistor)

almost finished converter
another view
It is ready for installation into the box.
Almost finished converter. View from the other side. It is ready for installation
into the box.

And finally, here are some photos of a functional prototype that Libor made using the above documentation.

Initially, we used the values of components as they were at the original web page. I recently noticed that the capacity of 100nF and impedance of 50 Ω (input impedance of the mixer) forms a high pass filter with a turning frequency around 30 kHz. If you want to listen to signals below this frequency, you should increase the value of C3 at least to 1uF. You may not use an electrolytic capacitor. Using a monolithic ceramic capacitor is probably the best solution.

Practical tests showed an improvement by several decibels (about 1S) at a frequency around 12 kHz.

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