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Zpět Another version of the VLF converter (I hope the last one on this web)

Vydáno dne 07. 05. 2011 (19139 přečtení)


The article is at the new address.

přepnout na verzi v češtině     version in Czech language

It is true, that the converter with SBL-1 mixer is quite an unnecessary luxury for such "nonsense", as listening to the Christmas message of the station SAQ Grimeton. So we decided to try a cheaper version, with the low cost mixer SA612.

I am sure, there was an e-shop, which offered a kit of the converter with this integrated circuit. Now, I can't find it, so it seems, that the sale has finished.

Integrated circuit SA612 (alternatively NE602, NE612) is a very popular low cost mixer. Unlike the passive diode mixer, this has, according to catalogue parameters, a pretty decent gain. In essence, each of its legs has its own function, so we don't have to be very inventive to be able to create the schematic of the converter.

  • 1 and 2: differential input
  • 3 : Minus of power supply
  • 4 and 5: differential output
  • 6 and 7: internal oscillator (with external crystal)
  • 8 : Plus of power supply

SA612 internal schematic
SA612 brief paremeters
Schematic of a converter with SA612 Block diagram
of the IC SA612
Brief parameters of the IC

I used a little impedance transformer to match the balanced high impedance output of the IC to the unbalanced 50Ω input of the receiver (transceiver). I have wound it on a yellow AMIDON toroidal core. Due to the used frequency of 10 MHz, the red core would probably be better. But I didn't have this one in my junk drawer.

It also would be fine, to use a similar matching and balancing circuit at the input side. Unfortunately I don't have any transformer, which would be suitable for this purpose. For a range of input frequencies of about 5 to 150 kHz it probably would be possible to wind it on a low-frequency ferrite pot-core. But it seemed to me too laborious, so I used the unbalanced input circuit.

At first, I tried it with a Low Pass Filter (LPF) at the input for the impedance of 50 Ω. But I achieved a much better result with a LPF designed for 1.5 kΩ, which is the input impedance of the IC (according to the datasheet). Values of parts in the schematic correspond with this second one. I used a substitute antenna for the reception - Inverted V for the 80m band. There might be necessary to use a different impedance of LPF when we use different antenna. If you want, you can try some experiments. It is possible to use Java script for quick design of LPF that is on the web site of Jarda OK1CJB. Since I was too lazy to wind the inductances, I used the axial or radial chokes, which can be bought in any shop with electronic parts.

VLF converter SA612 board
The copper side of PCB Populated with components Photograph of the populated board
(LPF matched to 50Ω)

As in previous articles, the picture of the printed circuit is not to scale and you can use it only for checking. I have saved two PDF files to the DOWNLOAD section. They include pictures of the PCB, both the straight one and the mirrored one, which are to scale. I designed it with wide connection lines and spaces, so it is easy to make the PCB even for inexperienced homebrewers.

I was quite afraid of a low intermodulation immunity combined with strong AM broadcast radios on nearby frequencies, so I put an attenuator (the resistance trimmer) to the front. However, as practical tests have shown, it was not necessary. The attenuator can be incorporated or bypassed using the jumpers that are visible in the photograps of the populated board (see also the schematic). The integrated circuit withstands up to 9 Volts, so I recommend using a voltage regulator of 8 Volts. We could use lower voltage, but it would unnecessarily reduce the intermodulation immunity.

The diode D1 is just the reverse polarity protection. If you are confident, you don't have to use it. The resistor R2 serves to discharge any static from the antenna in case that you don't use the resistance trimmer (attenuator). Value of the C5 depends on the crystal and on the range of the C11.

VLF conveter SA612 (LPF 1.5k ohm)
Different version
(LPF matched to 1,5kΩ)
From the other side

Practical tests showed, that this converter is a little bit more sensitive than the one with the passive mixer, provided that you matched the LPF to your antenna.

Finally, above is a link to a sample of a recorded signal of a Russian VLF navigating system. Sometimes it is called the Alpha navigation. It is a sort of an equivalent of the American system LORAN. You can hear signals of three transmitters on frequencies approximately 14.9 kHz, 12.6 kHz and 11.9 kHz.

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