A Picaxe-controlled VLF receiver (16 - 80 kHz)

kranenborg

kranenborg

Senior Member
Wanting to pick up a nice Picaxe project again, I found a very suitable but also (at least for me) challenging application ;).

Here is a project in which a Picaxe-28x2 is a proud controller of a special receiver for the Very Long Wave area. It tunes to the 16kHz - 80kHz interval, where the famous DCF 77.5 kHz time signal (Mainflingen) is available (here is the recording of the signal with this receiver), but also the similarly well-known Grimeton/SAQ transmitter signal at 17.2 kHz. Also, communications with submarines take place here, and maybe more ... .

The architecture of the DC (Direct Conversion) receiver shows the role of the Picaxe:

VLF-Controlscheme.JPG

The Picaxe:
  • Generates the baseline frequency (16kHz - 80kHz) for the mixer/downconverter using a PWM signal and subsequent RC-filtering. In this low frequency area the frequency step size of the PWM (8-bit resolution) is sufficiently small to be applied here (min. 400Hz, max. 1600Hz).
  • Tunes the VLF ferrite coil resonant frequency using the varactor/varicap close to the PWM frequency, for optimal antenna sensitivity. The DAC signal output can be applied directly to the varactor.
  • Switches a relais, as two frequency bands are needed in the current setup to cover the 65pf - 1000pf capacitance range needed for the full 16kHz - 80kHz reception range.
The implementation is done with my Philips EE system of course ... .

The hardware is ready and the receiver works already very well (see the sound recording in the DCF link above), and the project is documented in detail at my website via the following link: A Philips EE VLF receiver (16 - 80 kHz) with digital control, including a link to the software. Some updates may happen as there are still optimization opportunities.

The picture below shows the receiver, with the Picaxe control area (including the PWM RC-filter) highlighted in the red box. The Picaxe board is just a simple, standard breadboard that I use for my projects. Regarding the antenna coil: I was lucky to have a set of Longwave coils myself (approx. 4mH each), but the VLF coil can be a home-made version too (and no secondary taps/coils needed).

VLF-overview1-PICAXE-HL-small.jpg

In the final version of the receiver yet another LW-coil needed to be added (the last one I had really ;), increasing the coil inductance from 52mH to 71mH), such that the required capacitance range (56pf - 1500pF) for the 80kHz - 15.8kHz range could be managed with band-switching between just two bands (so that only one relais is needed):

Coil-5.jpg

/Jurjen
https://www.kranenborg.org/maker
 
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On the Project page (but also my first posting above) I added the link to the Picaxe software that makes the HW actually a radio. It may provide useful information on how to control the ADC, DAC, the serial Display, deriving a frequency from a PWM setting etc. etc. .

Display-DCF77.jpg

DCF77 recorded time signal

/Jurjen
 
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A software update to V2 makes it possible to operate the receiver in two modes by the push of a button:
  • (Default) As a receiver, as before, with the tuning knob/dial to change the frequency in smaller/larger steps
  • Manual antenna coil tuning using the same knob to optimize receiver sensitivity for the current receiver frequency. The display shows both the DAC level for the varicap/varactor as well as the bandswitch relais status during updates ... .
The switching is facilitated by a simple pushbutton, generating an interrupt that changes the status.

The software (link) is documented in detail and may be helpful on aspects like DAC control, integer math, interrupt-based control etc. This version now uses the standard AXE133/133Y serial display interface (although one should change the execution speed of the 18M2 controller from 16 to 32 MHz - see this thread - to support the 4800 baudrate commands as sent by the receiver's 28X2).

All info is available via the project page as well.

/Jurjen
 
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You should version the file names then we would know that the latest is available. A bit of a read to find on your web page, but I found the code. TNX
 
I agree actually ;) . Any future updates will have explicit version nrs. in their name. I also updated the previous post slightly. Thanks for the interest!
/Jurjen
 
I didn't know that a PICAXE running a BASIC interpreter would be fast enough to act as a VFO to a mixer.

Right now my interest is VLF has been increasing with your project. I currently monitor VLF signals with an old Rycom 3136A Selective Voltmeter and 1m Loop antenna. It now has developed audio issue and I cannot hear the signals through the built-in speaker. The sound card (Spectran software) still can hear to low level output. It's now on the bench waiting to be serviced. If the IC amp is toast, I won't be able to source it as it's obsolete.

Yup, I missed the Grimeton/SAQ transmitter signal on Christmas. Would have loved to log it.
 
Nice to hear about the interest :cool:, and VLF is new to me, actually. Grimeton/SAQ was the motivator for me to investigate VLF, but I missed the Dec. 2024 transmissions so now I need to wait to June for a new opportunity ... .
But the project is fun anyway, in particular since indeed the PICAXE controls everything. Thus, nice things like on-the-fly antenna tuning, automatic bandswitching (enabling a broad frequency interval) and presenting station info are feasible as well and just implemented. I am still investigating to see what more can be done. VLF is actually interesting too. I am a radio enthusiast so this project fits nicely in my overall portfolio of radio designs (although they look somewhat different from yours which are truly beautiful!)

The reason that the PICAXE is fast enough is not the speed of the BASIC interpreter itself but the fact that the microcontroller has a separate hardware PWM module which runs independently, with the interpreter only instructing it what to do and then leaving it at peace to do the job. Although its resolution is quite "standard" (8-bit period spec) the delta at the highest receiver frequency (80kHz) is still very acceptable.
 
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Many radios (last count 76) are not listed on my site (need parts (tubes), re-capped, some are just falling apart). I just don't create much content. More photography then text.

Your site is always a good read. Love those spring boards. Had a Heathkit and Radio Shack spring boards as a kid, but never as nice as your Philips "EE" electronic experiment kits. Always made circuits from Popular Electronics Magazine (early 70s era).
 
A simple test transmitter at 17.2kHz simulating Grimeton/SAQ can be built quite easily, based on a Colpitts oscillator as published in the EE2003 manual (the coil has an inductance of 9.8 mH = approx 10 mH):



It can be extended to work as a CW-transmitter by modulation through a simple Picaxe controlling the base voltage of the oscillator:



All is documented on the project page.
 
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