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Advice for newbie please.

single / dual conversion.

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Phil Wood.30/08/2008 23:33:00
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  As a person who used to repair radio equipment I understand the technical difference between single and dual conversation but as a newbie ( Oldie coming back ) I've noticed that there are loads of receivers on Ebay that are either single or dual conversation. Will they both work with with the same Tx or is there a difference with the pulse width? I'm only talking ppm here not pcm.

  Is there a big difference between their performance especially with adjacent channel rejection?

  I remember from my days fixing C.B. sets that they used the same channel spacing as us (10 Khz ) and they suffered badly from bleed over even though they were dual conversion. ( 1st I.F. 10.695Mhz 2nd I.F. 455Khz ). Granted they were pushing out well over 4 Watts.

  Would I also be correct in assuming that the Rx's used for park flyers and small heli's are deliberately de- sensitized  to reduce range and interference, consequently they would not be good for much else.

ps. why the heli is my spell checker pulling me up on words like,

understand, flyers, and, (the word itself).

gangster01/09/2008 15:22:00
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Deleted by me  I made a mess of it!!!try the next one, maybe a mod can delete this completely please

gangster01/09/2008 15:37:00
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Sorry That last post got away, lets try again

The bleed over problem with CB was nothing to do with the IF frequency or even dual or single conversion. Difficult to explain without doing the maths, and that makes the answer confusing 

With regards CB the reason for dual conversion was, the second IF gave the selectivity ie it is easy to make a cheap filter that would give 10KHz channel spacing if that IF frequency was low ie 455KHz,  to prevent image problems ie from another transmission 2x455KHz away ( well within the passband of the early stages of selectivity ie front end filters)  A high IF was chosen to move  the image frequency 21.4 MHz away .

The adjacent channel bleedover was possibly due to the inadequacy of the 2nd IF filter and more widespaced bleedover due RF stage overload. 4 watts is not a high power neither is 40 watts. Interference is a "sods" law situation, if an unfortunate mix happens it happens.

So what has this to do with RC, this next point might cause debate.  I believe there is no advantage whatsoever in this country in using  dual conversion, I did know the calculation once but have forgotten it.  I believe an unfortunate image can exist when using the 72 Mhz band, do the sums on 35 MHz and they show no issue. I use DC/SC or PCM in equal quantities and am happy to put any one of them in any models. I have been flying for 30 years and am still waiting for genuine interference, ie that which comes down the RX aerial and not through the TX stick

To answer your other question  Yes they will work with the same TX indeed with 35MHz PPM you can easiy and safely mix Futaba, Hitec and JR combinations although you must use the correct rx crystal, ie a futaba xtal with a futaba rx etc etc . Obviously SC and DC crystals are different ie use a Futaba DC xtal with a Futaba DC receiver

Rick Tee02/09/2008 07:55:00
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What Gordan says is more or less what i discovered by doing some research on the Internet. With DC an additional decoder stage exists in the RX it's only purpose is to prevent the remote possibility of harmonic interference which may occur between transmitters on channels at the 2 extremes of the 35MgHz band. This is so unlikely that the extra costs and increased complexity (more to go wrong) is not warranted. At our site the upper frequencies 81+ are allocated to a near by helicopter flying site so the problem does not exist. Roughly though i can't remember the exact channels, 62 or under could produce/receive a harmonic to or from a RX/TX on channel 83 or over. Channels in between are unaffected as the frequency needed to produce the harmonic is outside the allocated bandwidth.

Eric Bray02/09/2008 12:39:00
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Rick, splitting hairs, but please use the correct teminology! - DC is Direct Conversion! ie NO intermediate frequency! Single conversion has one inter, and dual has two.

I too, have never had provable rf interference on my model flight gear, except of course when CB kicked off, and we were still using 27 Megs, but then - I went from 27 to 459 Megs, and apart from one other, had the band to myself!

Now, I'm moving into 2.4 G, when Futaba decide to exchange my tx. (I'm still waiting!)

Rick Tee02/09/2008 16:37:00
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Sorry about that Eric, was just following the previous posters abbreviation of the term.
gangster02/09/2008 19:32:00
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Eric

OK Guilty as charged m'lud 

Phil Wood.02/09/2008 23:02:00
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Thank's for the advice guys.   The old repair days are coming back to me now, I didn't realize how much I had forgotten.   I remember now that one of the problems with the CB's was swamping on the 1st RF stage and that everyone used to turn up the FM deviation thinking it made them louder.

I think it's about time I took a refresher course.

Phil.

Eric Bray03/09/2008 11:45:00
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And then they began shorting out the damping/limiting resistor in the power output! It raised the watts from 5 to 7, and made the tr's unstable!

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