In the UK radio programs are broadcast nationally on 198kHz Long Wave (LW).
A 25bit/s digital signal is also encoded, by phase-shifting, on the carrier.
Every 40ms the carrier phase changes by 45 degrees;
A change from -22.5 deg (retarded) to +22.5 deg (advanced) encodes a 0.
A change from +25.5 deg (advanced) to -22.5 deg (retarded) encodes a 1.
I would like to decode the digital signal.
I am confident I can do that if I have some hardware that receives the LW radio broadcast and presents me with an analogue signal (either hardwired or over a USB link) that represents the phase shift of the carrier.
Please note I have no electronics or radio experience so I am looking for off-the-shelf hardware that gives me the signal I need in a relatively straightforward way. I would like to use an Arduino to decode the phase shifting.
The “Si4735 AM FM Receiver Shield” looked really interesting as it can receive the LW broadcast and is an Arduino shield. However looking at the manual it does not look (to me) as if it can provide me with information about the phase shifting of the carrier. Am I correct about this or is there some way to use it to get the output I need?
If the Si4735 will not do the job is there any hardware that will - bearing in mind I cannot start building radio receivers, phase lock loops etc. (all well beyond me).
I hope you can point me in the right direction, thanks.
TinMan:
The “Si4735 AM FM Receiver Shield” looked really interesting as it can receive the LW broadcast and is an Arduino shield. However looking at the manual it does not look (to me) as if it can provide me with information about the phase shifting of the carrier. Am I correct about this or is there some way to use it to get the output I need?
I’m pretty sure there is no way to recover the carrier phase information using the Si4735. It’s a great chip and works extremely well for FM broadcast and is surprisingly good for AM, too, with a proper antenna, but no I/Q, LO, IF, or other signals are available externally. This also precludes its use for SSB reception, much to the disappointment of many.
Lots of fascinating info from one of the Si4735’s designers is available on the Yahoo Ultralight DX group. Scott mentions that prototypes of the chip included DSP firmware for synchronous AM detection, but it was removed to conserve memory.
Also, doesn’t much matter for hobby purposes, but the SI4735-C40 is now listed as “Not Recommended for new Designs” with the SI4735-D60 the recommended replacement.
Of course, you will be looking for transient distortions at the phase transitions.
If you use the 565 with a smoothing filter to provide a reference frequency at an average phase, you could probably use a phase comparator for a direct output.