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Martin_H wrote: »
Is that because direct digital synthesis made them unneeded?
Beau Schwabe wrote: »
Neat project!! ... and thanks for the comment "He Who Understands Inductors and Knows What to Do with Them"
It's funny, that while I was on vacation on the coast of Lake Michigan, I was working in Parallel apparently with what you were doing, but you beat me to it. lol
I knew what needed to be done, but did not get my head around a solution in code... Nice work!
Without going into a huge amount of detail... since the 330uH is by itself and not in parallel with an external capacitor to form an LC tank circuit (excluding the inductors self-capacitance) we will look at the impedance based on the frequency.
For the frequency range of 550kHz to 1600kHz the impedance of the 330uH inductor varies from 1.14K to 3.32K
The Length of the Antenna also makes a difference and contributes an LC component to the impedance value above to form an RLC tank.
You can get much better results and use a much smaller antenna if you use an RF transformer (two closely coupled coils or a single tapped coil) with a Capacitor to match the resonant frequency to something like 1100kHz (<-- approximate middle of the AM dial) and use that in place of your 330uH coil. The impedance of the antenna should also be matched to the RF transformer.
Edit: Actually a 63pF capacitor in parallel with the 330uF inductor would be "tuned" to about 1100kHz but there wouldn't be a good way to couple your antenna to it unless you could wind a few (2-5) turns of wire around the outside of the inductor... one end tied to ground, and the other tied to your antenna.· At first I would try a variable capacitor that is within·63pF +/- 10 pF at least, but you may find a fixed value that works well also.· In this case we would not necessarily want a high 'Q' value for the coil, and or the cap, so a·5 to 10·Ohm resistor in series with the capacitor might also benefit.· What happens with a high 'Q' is that it becomes·a narrow band tuner, and you may loose other stations.· To be able to tune the other channels digitally, we want a little bit of padding here, so a wider band·tuner is more desirable.
I built your circuit, and used a 500uH coil instead of a 330uH (1.73K to 5.03K impedance) ... I also used a 10Meg resistor for the ADC instead of the 470K resistor. This helps
to provide less loading on the antenna from the feedback I/O pin, allowing weaker signals to be picked up.
With 22 gage solid wire, I used a 6-foot piece of it for my antenna. The attached wav file is the result from a radio station a little more than 9 miles from my home.
Other useful references:
IC Layout Engineer
Post Edited (Beau Schwabe (Parallax)) : 8/15/2008 5:02:55 PM GMT
Phil Pilgrim (PhiPi) wrote: »
Thank you, Beau! And thanks for the detailed analysis of, and improvements to, the RF circuitry. I'm rather envious of your near proximity to an AM radio station, the closest strong station here being some 40 miles away. Between that and your RF improvements, the S/N of your recording is many decibels above what I was able to obtain!
One thing I'd like to come up with is an easy-to-fabricate coil (or transformer) with your recommended values, made solely from hardware store and/or Radio Shack materials. I have a real aversion to winding coils that likely harkens back to childhood fumblings with fine enameled wire and empty toilet paper spools. I'm sure I'm not alone in this, so making this part easier will make the project more approachable for those who are more comfortable with digital design and programming.
'Still some PropSTICK Kit bare PCBs left!
The P2 uses NCOs for frequency generation.