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Homebrew Equipment A place to show, design and discuss the weird and wonderful electronic creations from the hands of individual members. |
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18th Mar 2010, 10:14 am | #1 |
Hexode
Join Date: Apr 2009
Location: Naples, Florida, USA
Posts: 255
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FM Converter.
This may have been covered before, although I couldn't find it in any search. Most of the old radios I have cover the VHF band from 88 to 100 MHz. Some time ago I came across an article on RMorg written by Philippe Annic about a circuit which would convert stations in the 100 to 108 MHz band down to 90 to 98 MHz. You can find the article here:
http://www.radiomuseum.org/forumdata..._converter.pdf I finally got around to the building the little circuit and tested it with a Murphy A242 that I've got. It didn't cost much, probably less than 10 quid, although some parts came from the junk box. The SA602 chip is readily available from various sources, I got mine from Farnell, which is where I also got the crystal. The good news is, as I said, it works. And you get some extra gain, depending how you couple the circuit to the existing radio. I didn't bother with the VHF ferrite ring described in the article and from the way I lashed it up, probably introduced more mismatches. But the SA602 gives you +17dB gain and even with my poor lash up, I still got some gain, which the Murphy can use. I like the way it was design so that even when you're listening to the lower band, the SA602 is still in the circuit (the 10 MHz oscillator is turned off), so you still get the extra gain. The down side is the converter has both the sum and minus products of the 10 MHz conversion, so depending on the spacing of stations low in the FM band with respect to stations in the upper portion of the FM band, they can end up on top of one another or close enough to interfere. I don't see any good way (meaning cheap) to filter out the sum product to avoid this. Even so, it works, it's cheap, and it brings those upper stations down to where they can be listen to on these old radios without modifying the radios. Here's a picture of the converter built on a little perf board. I don't know how critical the circuit is, I just tried to keep everything short and tight.
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Jim Mac |
18th Mar 2010, 1:28 pm | #2 |
Rest in Peace
Join Date: Sep 2008
Location: Solihull, West Midlands, UK.
Posts: 4,872
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Re: FM Converter
A useful little circuit, although the original designer seems a bit confused. The poor matching at the input will directly degrade the noise figure, although this won't matter if only strong local stations are listened to. Poor matching at the output is less important. Switching off the oscillator will leave the balanced mixer balanced, so it will attenuate signals as it passes them through - you won't get 17dB gain. Again, this might not matter if only local signals are involved. There may be a way to deliberately unbalance the NE602 mixer - someone on here is bound to know!
Filtering out unwanted signals is tricky, as the wanted and unwanted ones are only 10% apart in frequency. If there are only a few interferers you could try using coaxial cable stubs to notch out individual stations. A piece of cable exactly one quarter-wave long and open circuit at the far end behaves like a short circuit at the near end, so just connect this across the aerial input and short out the unwanted station. The actual length of cable depends on the frequency and the velocity factor of the cable. Veocity factor will be about 0.67 for solid dielectric, and about 0.8 for semi-airspaced. Cable length in cm will be VFx75/f, where VF is 0.67 or 0.8, f is freq in MHz. Cut it a bit too long and then try shortening a bit at a time. If you overshoot start again, or use it for a slightly higher frequency interferer. |
18th Mar 2010, 2:41 pm | #3 |
Hexode
Join Date: Apr 2009
Location: Naples, Florida, USA
Posts: 255
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Re: FM Converter
Where I live the interferes aren't too bad, but that would be depending on where you live. And that would go for coaxial cable stubs too. The Murphy is good only for strong local stations anyhow (with the internal antenna), so using it for just strong local stations is no problem.
It might be useful to house it in its own separate box and power it with a 9v battery. That way it could be moved to radio of choice (assuming all radios have the same type antenna jack, which isn't true) and truly leave the radio undisturbed. I've thought of a double conversion where the first stage would shift the frequency down, say 40 MHz, then through a 10 MHz wide bandpass filter, and finally back up to 90 to 100 MHz. Each conversion would use a NE602 mixer, which are cheap, but the bandpass filter might be difficult. Any suggestions on how to better couple the antenna to the front end of the NE602?
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Jim Mac |
18th Mar 2010, 3:21 pm | #4 |
Rest in Peace
Join Date: Sep 2008
Location: Solihull, West Midlands, UK.
Posts: 4,872
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Re: FM Converter
Two options for input matching: use the type of transformer he used for the output, or put a tuned circuit there - which will give some filtering as well as impedance matching.
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19th Mar 2010, 12:35 pm | #5 |
Octode
Join Date: Jun 2006
Location: St Osyth, Nr Clacton, Essex, UK.
Posts: 1,482
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Re: FM Converter.
How about putting a little variable capacitor across the crystal? You could then pull the frequency down a bit and find a quiet spot.
Graham |
23rd Mar 2010, 1:03 pm | #6 |
Nonode
Join Date: Mar 2010
Location: Dukinfield, Cheshire, UK.
Posts: 2,034
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Re: FM Converter.
You can unbalance the 602 by applying a wee bit of current through pin 1 or 2. A 100K up to supply or down to deck is a good starting point.
The SA/NE602/612 is a cracking little chip and I use it whenever I need a balanced modulator or mixer for my radio projects. It's been officially obsolete for several years now, but you can still get them, usually at around 2 to three £. |
25th Mar 2010, 12:10 pm | #7 |
Hexode
Join Date: Apr 2009
Location: Naples, Florida, USA
Posts: 255
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Re: FM Converter.
I've played around with it a bit more and tested it with a Saba Meersburg 7. The Saba has a fairly sensitive front-end making it more susceptible to any extra products being generated. I actually need to kill the gain for it to be useful with the Saba. I want to try Dave's suggestion above and match the input with a transformer and then use a passive matching filter on the output that will remove the gain on the backend. But with the coming of Spring, I'm finding it difficult work on all my fun projects with the one-who-must-be-obeyed managing me.
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Jim Mac |
25th Mar 2010, 2:15 pm | #8 |
Heptode
Join Date: Apr 2008
Location: Eastbourne, East Sussex, UK.
Posts: 671
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Re: FM Converter.
You can get cheap converters from online auction sites designed for imported Japanese cars where the radio tunes from 76-90MHz, has anyone tried using one of these?
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25th Mar 2010, 3:12 pm | #9 |
Hexode
Join Date: Apr 2009
Location: Naples, Florida, USA
Posts: 255
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Re: FM Converter.
I hadn't seen them before so I didn't know they existed. I saw one for 6 euros on ebay, which is cheap. Some method of coupling it to radio would have to be developed.
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Jim Mac |