Can the Propeller be used to build a cheap 20MHz radio receiver for Jupiter signals?
ElectricAye
Posts: 4,561
I have a youngster interested in receiving signals from the planet Jupiter. Apparently these can be detected around 20.1 MHz, and there are kits available as part of a NASA "Radio Jove" program:
http://radiojove.gsfc.nasa.gov/telescope/rcvr_manual.pdf
I think the kits are about $125.
I know I've seen people on the forum mention MHz reception with some sort of Propeller configuration. I know nothing about radio, so I was wondering if the Prop could be used to make a cheap(er) version of a receiver of this sort, or if a kit would actually be a better way to go. I'm clueless.
Thanks.
http://radiojove.gsfc.nasa.gov/telescope/rcvr_manual.pdf
I think the kits are about $125.
I know I've seen people on the forum mention MHz reception with some sort of Propeller configuration. I know nothing about radio, so I was wondering if the Prop could be used to make a cheap(er) version of a receiver of this sort, or if a kit would actually be a better way to go. I'm clueless.
Thanks.
Comments
20 MHz is okay for the local I and Q oscillators, as long as the frequency is exactly 20 MHz, and this is the highest frequency that will work with a Prop-centric I/Q mixer. But diverging from that frequency with a PLL will introduce too much spurious jitter to be much good. There are three ways that I can think of to deal with this:
2. Use an external mixer/oscillator (like the SA602 in the radioJOVE project) to down-convert the signal to 10 MHz, say, tuned by a counter's DUTY-mode output, and use the Prop as a second mixer.
3. Fix the local oscillator at 20 MHz and reshift the resulting audio spectrum (up to maybe 100 kHz away) via DSP techniques to do the tuning virtually.
In any event, some RF construction will need to take place on the front-end. With signals at the 1 uV level, there's just no avoiding it.
-Phil
Thanks, Phil,
you are precisely the person I was hoping would answer this question. But... wow... I really need to sit down and study more about radios so I can understand what you're saying about all of this. Also, I learned that the 20.1 MHz is just one of the bands that these radio Jovians look at, though I don't think the kit can listen to anything very far away from 20.1 MHz.
This type of receiver is called a Direct Conversion receiver.
The oscillator, 20.1MHz in this case, is mixed with the input signal at the same frequency and converted directly to base band.
While this type of receiver looks deceptively simple they are not.
One major advantage is the high dynamic range capability. Essentially very weak signals can be detected in the presence of large signals close by.
A problem these have is micro-phonics. So no speaker in the case.
A very good DC receiver was an amateur radio design called the R-1. I made one of these along with the T-1 transmitter. These worked very well.
As Phil has taught us the Prop's NCO is just not good enough for a high performance oscillator without severe restrictions.
However, the audio section would be workable as long as the Prop is not inside the shielded enclosure of the receiver.
BTW, $125 for a kit doesn't sound to bad for a well built design.
Duane J
True. But if you can keep it to a submultiple of the clock frequency, the spectrum is pure enough for RF use. Also, the jitters are less of a problem for receiving than for transmitting, due to not having to conform to regulatory standards. But a PLL LO at 20.1 MHz in an 80 Mhz Prop for direct conversion would be a disaster, I fear. 20 MHz is okay: you get four samples per cycle, which is adequate for quadrature detection. Add that extra 100 kHz, though, and things get real slimy. That's why I thought one way to tune would be to load the crystal with a Prop-controlled varactor, since that sets the overall clock rate -- no PLL required.
-Phil
Yes, apparently it's NASA-approved, so I'm presuming it's fairly good. Rather than blow my mind with this, maybe I'll just buy the kit and bite bullets from there. I really need to learn about radio someday, though. If only I could do some sort of Vulcan mind meld with you radio gurus...
Are you thinking of directly sampling the 20.1MHz and doing the mixing in software?
Duane J
The sampling and mixing would be done by a pair of counters, with data accumulated at more leisurely rates in software.
-Phil
Ok, now what heck is a varactor?
http://en.wikipedia.org/wiki/Varicap#Substitutes_for_varicap_diodes
I can see I'm going to get anything else done today till I see see smoke
I have used this but it works best at relatively low frequencies compared to varactors.
Primarily because the capacitance is much larger than in varactors.
Duane J
Thanks again.
http://www.amazon.com/Experimental-Methods-Design-Wes-Hayward/dp/087259923X/ref=sr_1_1?ie=UTF8&qid=1352312721&sr=8-1&keywords=experimental+methods+in+rf+design
-Phil
http://www.radioclubs.net/herc/news.php?news_id=1584
Thanks, Phil. I gots me a not-too-terribly old ARRL handbook on order now. But, geesh, that other one looks pretty pricey. I'll have to drive to the library for that one, I spoze.
Thanks, Leon. I'd almost forgotten about those guys. Last time I saw a group of them down near the city, they all looked older than dirt. I bet they could help me if this kit doesn't work, though. As for setting up the antenna, it looks like it's time to break out the old moonbounce again:
http://forums.parallax.com/showthread.php?138720-Looking-for-suggestions-pertaining-to-a-UHF-VHF-antenna&p=1083270&viewfull=1#post1083270
-Phil
But there are the 1%ers that really get into the technical aspects.
Duane J
I could rant on and on about my struggles to get kids to do something other than play computer games, etc. When I was growing up, I was drawn to the mystique of science and technology. Maybe it was just a symptom of Space Race fever, but I was simply magnetically drawn toward anything that looked like science - glassware, coils of wire, vacuum tubes, machines, vials of chemicals, telescopes, microscopes, pyrotechnics, you name it, and books involving such stuff. My only limitation was a lack of cash and the density of craters I was allowed to leave in the back yard. Nowadays, I can't figure out how to compete with the Xboxes and whatnot. So few of the kids want to make anything or even do anything except click click click. It boggles my mind. So I can only imagine how difficult it is for the old ham enthusiasts to educate their replacements. How odd it must seem to a teenager to need all this bulky equipment just to talk to some dude on the other side of the globe that you could so easily Skype with. Can't Skype with anyone on Jupiter yet, so maybe there's a few places left for the imagination to wander around some. Maybe it's just me, but kids today sure seem apathetic about everything except their portable clicking devices.
Which reminds me, I should post something about this on the General Discussion board:
Looks like you can find small value X5R dialectic ceramic capacitors. Digikey 445-5496-1-ND is a 100pf 10v cap, but the size is a rather unfriendly 01005. (yes, 0.016 by 0.008 by 0.008 inches o_O don't sneeze...)
Lawson
To stabalize this VFO I thought of using a 28MHz Xtal being frequency counted and the error used to produce the lock. It did produce some strange things like 7/9maths, or was it 5/7th maths ??. It could have been a way of getting to those frequencies off of the 5, 10, 20MHz etc.
As always with me I didn't document the progress at all and now half a year on I remember very little. The whole receiver stuff got hi-jacked by SDR# and £12 USB sticks (but mostly my idleness!).
Alan
You could try a Ceramic resonator (no inbuilt caps) as the VFO ?
Murata specs show appx +0.05% to -0.1% for 3pF-25pF
or, paying a bit more money, I see MEMS oscillators claim up to ±1600 PPM
http://www.sitime.com/products/voltage-controlled-vcxo/sit3808
Even thought he internal PLL may be jittery, the external VCO won't exhibit that jitter, given a long enough time constant to filter the phase detector output. The tricky part occurs when the voltage-to-frequency relationship is highly non-linear. If that occurs, the phase correction "rate" has to change, depending on frequency. But that's not an insurmountable problem.
-Phil
The signals come from the planet itself. One source of the signals is the interaction between the solar wind and Jupiter's impressive magnetic field. The sound these signals make is really cool and recordings of them likely have value to astronomers.
I believe it's synchrotron radiation caused by electrons spinning around magnetic field lines. The frequency is determined by the energy of the electrons and the strength of the magnetic field.
Electrons, themselves, have a magnetic pole. As they spin around the field lines in a spiral path the electron pole direction rotates at a particular frequency. This rotating pole tends to loose energy as radio wave photons that can be picked up by a radio receiver.
Jupiter has a fairly high magnet field interacting with millions of amps of electron flow. There are several frequency bands in 20MHz to 30 MHz range. The strength of the signals are quite high. However, Jupiter is a long way away so the signal strength here is quite small requiring very good radio design and high gain antennas.
Duane J
To add a little to what others have already said, I think some of the signal is a result of volcanic eruptions on Jupiter's moon Io. Apparently these volcanoes spew material into orbit around Jupiter that interacts with the magnetic fields, etc. and this is the source of a powerful signal. The signal sweeps through space in somewhat of a beam pattern, so there's a periodicity to it. This year Jupiter is closer to the zenith than it has been in about 12 years, I think (for the northern hemisphere), so the signals should be as good as it gets in the US.