da Rock

I was told awhile back not every manufacturer uses dual conversion design to get longer range from their better RXs and that just because a RX didn't mention conversion meant that it was a single conversion, the design could also be different for those.

I've just been called to order about my understanding of the term "conversion."

So does every RX do conversion to filter the mass of incoming signals down to the desired one?

Any short explanation for the process?

I'm gonna be blushing over this one for weeks [sm=spinnyeyes.gif]

fizzwater2

My Feedback: (61)

ok..
Basically, it's harder to build a very narrow filter to pick out your channel the higher you go in frequency. So, at 72+ MHz, it's really difficult to make a receiver selective enough to pick out a single channel.

So - they use an internal signal in the receiver called a local oscillator to mix with the input signal, shifting it down to a different frequency. This is called a "conversion" or shift of the frequency.

A single conversion receiver typically mixes your channel down to 455,000 cycles per second, or 455KHz. At 455 KHz it's pretty easy to buy filters that are narrow enough to pick out your channel. A dual conversion receiver typically mixes your channel down to 10.7 million cycles per second, or MHz the in the first conversion, does some of the filtering there, and mixes again to 455 KHz where it's filtered again.

Receiver sensitivity has nothing to do with number of conversions, that's a matter of the design of the receiver input itself. There is this thing called an "image" frequency that can cause some trouble, though.

For example - say your channel is 72.15 MHz. If we mixed that with a crystal controlled local oscillator at 72.605 MHz, we would get two output signals - the sum (144.755 MHz) and the difference (0.455 MHz, or 455KHz) from the mixer. The high frequency is far enough away it's pretty easy to filter out. We put a narrow filter on the 455 KHz signal, and there's your channel, and nobody elses. But, if a signal 455KHz ABOVE the LO (local oscillator) comes along, it also mixes with the LO and produces 455 KHz - in this case that would be 73.06 MHz. This "image" frequency is filtered out by the input section of the receiver, but the closer it is the harder it is to filter out.

So - along comes a dual conversion receiver. Same channel, 72.15. Mix it with 82.85 MHz, we get the sum (155 MHz, easy to filter out) and the difference (10.7 MHz, which is what we're looking for). THe 10.7 is filtered, then mixed again with a second LO, (10.245 MHz or 11.155 MHz) to get the 455 KHz, where it is filtered again. The image frequency of the receiver in this case is 10.7 MHz ABOVE the LO signal of 82.85 MHz, or 93.55 MHz - which is far enough away that the receiver front end filter has an easier time of getting rid of it.

JR took a different approach - they use typically single conversion receivers (less parts) and don't use the channels below channel 15 - so the image frequency is well above our RC band. It takes either a very close transmitter or a very strong transmitter to cause an image problem with them.

I've used both - Futaba and Hitec dual conversion receivers, and JR single conversion receivers - and had good luck with either. The actual design of the receiver and filters is probably more important than the number of conversions, dual conversion just makes it a little easier to get selectivity.

Any help?

Campgems

Now, if they would have called it IF's I would have understood. Interesting that they still pick 455 as the last IF frequency. I guess it makes since as they are in ever portable radio already. Off the shelff engineering.

Don.

fizzwater2

My Feedback: (61)

455kHz IF is pretty common, and parts are fairly easy to come by. Same for 10.7, but anything very narrow at 10.7 is a crystal filter, and in order to get very narrow, it's several poles of crystal filter. The 455kHz filters are more rugged.

I did a receiver design many years ago with a 420 kHz 2nd IF. Had Murata custom make me ceramic filters at 420, and the off the shelf 455 parts are MUCH easier to deal with! It wasn't a RC receiver, it was a GPS receiver back before they were all so integrated and customized like they are these days. This was about 22-23 years ago now..

Lately, our newest receivers have taken the 2nd IF directly into digital signal processors, where we can do all sorts of fancy stuff with them!

da Rock

So one could say that some designers sell quality receivers for long range that are single conversion, just higher quality design and filtering. I've noticed that JR for example, doesn't seem to market "park flyer" receivers. And they are single conversion RX that are perfectly dependable at out of sight ranges.

So one could say that the designations "single" and "dual" actually only mean the manufacturer who uses both is telling his buyers which are less dependable and should only be used at park flyer ranges. While manufacturers who don't use the designations at all don't need to.

Any help? You betcha. Thank you very much. It's actually what I remembered as an idea, just couldn't remember the words. thanks again.

Bax

My Feedback: (11)

No. The manufacturers state whether they are single-conversion or dual-conversion because they've done so for years. All R/C receivers used to be single-conversion when the only channels were on 27 MHz. When the 72 MHz systems came out, 72.080 MHz could be "hit" by 72.960 MHz. Some makers then made dual-conversion receivers to alleviate that problem. They were more expensive to make at the time, and so were limited to the "premium" systems or were considered "premium" options for other systems. With the advent of the 50 R/C Air channels, and the need to reject other types of interference, the dual-conversion receiver became pretty much the norm for the majority of R/C systems makers. As the number of makers has been weeded-out over the years, they've kept the designations.

If a manufacturer calls a receiver "single-conversion", and other receivers they make "dual-conversion", it doesn't mean that one or the other is "better" than the other, or for lower ranges only. For example, Futaba has a 4-channel single-conversion receiver intended for park flyers, but also has one that is "full range". It has to do with cost of manufacture as much as anything.

da Rock

Well, one could say that he was speaking rhetorically. Or making a hard to understand point. One could. And did.

But thanks for the background. Hope that helps all the beginners who think if it doesn't say dual-conversion then the sucker is a lame little parkflyer cheapo. Or if anyone tells them it's single-conversion that it's just a lame little...... etc

Lots of excellent, out-of-site range RXs don't say anything. And darned if a bunch of experts pass judgement on that to mean they must be just lame little...... etc.

onewasp

(quote}

Well, one could say that he was speaking rhetorically. Or making a hard to understand point. One could. And did.

{quote}

Interesting-----------I found it very easy to understand.

I'm backing up Bax and I'm a JR/Spektrum flyer. Sooooo it is rather "not the usual" kind of reinforcement.

On 72MHz and on six meters JR offers a choice between their ABC&W single conversion Rx and a different dual conversion Rx. Under certain specified conditions each (different conditions) can show as being better at unwanted signal rejection than the other. But not under ALL conditions.

Base line explanation on DC you are processing the signal received twice. This CAN offer better reception under some circumstances----not under ALL.
On single conversion you are processing the received signal once.

Before you can say one is better than the other you have to define the usage and interfering signal expected.

I preferred DC when on those freqs. but that is all that it was --- a personal preference. You have to do a lot of defining before you can add the "better than" tag.

BTW type 'dual conversion' into Google and read to your heart's content----------lots of examples to choose from. Some more germane than the others.


P.S. Try this: http://www.bergent.net/SC-DC.pdf

da Rock

The problem isn't in defining the term.

The problem is that so many modelers have interpreted the absence of the label to mean more than it does.

onewasp

Also called human nature. ;-)

If our A/C flies JUST as we wish it to, under all circumstances and attitudes then who cares what the incidence is?
Exception would be if you were trying to duplicate THAT A/C for yourself.

Ivory Soap or Lifeboy???

rmh

JeeZ-I alwas thot dual conversion had to do with missionaries--
now you say it is lectrical thing

I always used JR single conversion - -once they got the Auto Window Blocking working back in 80's

AWorrest

My Feedback: (5)

There is a lot of good information in this thread but I think there is some confusion between the concepts of a receiver selectivity and its sensitivity. Selectivity is the receiver ability to distinguish your signal from all the others around you. Sensitivity is its ability to detect a weak signal. Single and dual conversions circuits are means to increase a receiver selectivity. Better selectivity does not mean greater sensitivity.

My first receiver had no conversion. It had no IF's. It was a super regenerative circuit. It had as good of range as any present day dual conversion receiver. But it had no selectivity and it would pick up everything in the band. At the time that wasn't a problem since we had only one frequency, 27.255MHz.

Allan

fizzwater2

My Feedback: (61)

I wondered if someone was going to bring up regenerative receivers..

Rodney

There is one more type, the monodyne. That is where the crystal is on the same frequency as the transmitter and the beat or difference frequency is the data. This has been successfully used with typical broadcast frequencies for audio transmit ions. An old issue of Popular Electonics featured just such a device.

fizzwater2

My Feedback: (61)

Also known as direct conversion. Takes a pretty good match between transmitter and receiver LO frequency.