9
\$\begingroup\$

I'm looking to better understand basic RF receiver circuits, and I thought building a basic AM 77.5 kHz receiver seemed like a good challenge. I do however have had quite a challenge in finding any resources on this. Do you know any good starting points for the basic circuits involved in building this, in discrete components if possible, as I would like to understand the details of the system?

If I understand correctly it should be possible without any kind of phase locked loop, and just with "simple" envelope detection as it is low frequency amplitude modulation?

Don't worry about the actual demodulation, I'm going to sample the signal and do it digitally. This is what I know how to do. ;) So I'm only looking for the RF to baseband part.

Thanks in advance.

\$\endgroup\$
4
\$\begingroup\$

I was looking into it some time ago and it seems that the 77.5kHz band is very noisy (SMSPs and stuff) and it makes it quite difficult to build a good receiver for DCF77. Most projects are based on ready-made modules.

If you want to make something simple try building an analog AM receiver for local radio broadcasts. It should be easier (the signal will be stronger) and there are plenty resources on the net.

The Radio Spectrum Monitor may also be interesting to you albeit it is much more advanced project.

|improve this answer|||||
\$\endgroup\$
  • \$\begingroup\$ Thanks for the input. Maybe I should just start with an ordinary AM broadcast receiver. I'm just a lot more interested in something where I can receive some data. :) \$\endgroup\$ – bjarkef May 17 '10 at 5:38
  • 1
    \$\begingroup\$ You may start with an AM receiver and then try to convert it to a 77.5kHz receiver with digital output. For some more links on the DCF77 you may check my Delicious links: delicious.com/jpcl/Projects%2Fdcf77 \$\endgroup\$ – jpc May 17 '10 at 9:32
  • 2
    \$\begingroup\$ Build that op-amp circuit and tune into a strong AM broadcast first, then adjust the values and move to 77.5 kHz when you've got it working. \$\endgroup\$ – endolith May 17 '10 at 14:41
  • \$\begingroup\$ @jpc: Your link is dead :/ \$\endgroup\$ – bjarkef Jul 10 '15 at 8:54
  • 1
    \$\begingroup\$ @bjarkef I've changed the tag name to dcf77 so it should not cause problems anymore: delicious.com/jpcl/dcf77 \$\endgroup\$ – jpc Jul 28 '15 at 8:49
2
\$\begingroup\$

Well, a crystal radio is the simplest possible AM receiver, which just uses simple envelope detection. http://en.wikipedia.org/wiki/Crystal_radio#The_naive_circuit

You've probably already seen this: http://www.giangrandi.ch/electronics/dcf77/dcf77.shtml

Very simple AM receiver using envelope detection: http://www.next.gr/inside-circuits/Simple-Coil-less-AM-receiver-l6022.html

But this is just regurgitating Google results. Can you give more details about what kind of circuit you're looking to build and what information you need?

|improve this answer|||||
\$\endgroup\$
  • \$\begingroup\$ Simply, my goal is to learn about receiver design. So I want to start with something as simple as possible, that I can build with discrete components and that I can measure on and understand the workings of. Wikipedia provides a naive circuit, that will probably not work. The second link just uses some IC that does the work. The last link looks promising. Although it is focused on receiving AM stations which as far as I remember is in the 150-250 kHz spectrum. Will this also work as low as 77.5kHz? \$\endgroup\$ – bjarkef May 15 '10 at 14:52
  • \$\begingroup\$ Yeah it should work fine for 77 kHz, but either the pots or the capacitors connected to them will need to be higher value. The op-amp is just configured as a bandpass filter. If you're aiming for a specific frequency, you don't really need the pots. Use a simulator or find a calculator to get the right values for your frequency. \$\endgroup\$ – endolith May 17 '10 at 14:33
1
\$\begingroup\$

Please be aware of DCF 77 reading some introduction text. Indeed DCF 77 it's a specific modulation protocol (AM) were the modulation signal have a subordinated phase shift as the way to transmit the digital information. Due to the working frequency (77,500 Hz) the propagation tends to follow earth curvature so the signal reach long distances. Although the modulation complexity there are some ease circuits to extract hours and minutes with discrete components to differentiate or integrate the pulses. The results are conditioned by distance, eight and electromagnetic environment. I'm at about 2,500 Km (line of sought) with an acceptable signal in summer time after that depends of climacteric conditions. I've two DCF and two GPS. If you need an acceptable compromise price vs features I would suggest you start with a LIDL DCF clock.

|improve this answer|||||
\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.