MAXXHEADROOM

My Feedback: (2)

No where in the book or on radio do they mention dual conversion.Is this a dual conversion and what do they mean by it.

Troy Newman

Hi Maxx,

Danny is away for a few days so I hope I don't steal his stage. I have spent some time in learning the difference. In doing that research I really came to appreciate the options JR gives us on our models. Choices are always a good thing.

Dual conversion is another way to filter the signal from the TX. The RX does all the work and there is nothing done differently in the TX. This is why there is not really anything in your TX manual about the D/C RX's.

Some other manufacturers use Dual Conversion RX's to do the filtering process of the signal. What it does is takes the frequency that comes in from your TX and mixes it with another frequency. The resulting frequency then tells the servos the commands.

You can see it as a filtering process.

JR uses a different system to filter its signal in the standard RX's. Instead of a additional frequencies that get mixed with the signal. The JR RX picks the signal and acts to amplify the good stuff and kill the bad stuff. Its just a different way to filter the signal before the servos are told what to do. Which is better at doing the job is a matter of opinion. But there are two different methods to get the clean signal and reject interference. JR is the only manufacturer in the USA that offers both as options. The reason is there are some environments that are not as good for D/C and likewise some environments are not as good for just single conversion ABC+W.


So JR offers a choice to modelers. By the way both 955,945 are both A,B,C&W even though the label doesn't say that all of JR's rx's are. Its Anti-Blocking-Circuit& Window. The best way to
describe it would be to call it an automatic gain control thats on the rx
and the circuit only letting through the "window" the correct freqs the rx
should be listening to and "blocking" off all other interfering signals. so in the JR D/C rx the ABC+W filters and so does the D/C system acts like another filter.


Basically the way to look at it is D/C adds another level of filtering in addition to the ABC+W system. But also in this same instance you are adding more complexity in the RX.

As for performance of the RX's I have run both for quite awhile and can't tell the difference between the two types in terms of performance and use.

One thing to keep in mind. D/C crystals are different from regular crystals. JR standard RX crystals have a gray plastic case on them. The D/C RX crystals are different and are denoted by red instead of Gray.

There is nothing different about the TX as I said before. JR only offers D/C option in the SPCM RX so you need to make sure that your TX is working in SPCM mode and not PPM. But other than this its a no brainer. And the TX will not do anything differently

Hope this helps answer your question.

Troy Newman
Team JR

Mac_Man_UK

The main problems with receivers are Co-Channel Interference, 3rd Order InterModulation, and Image rejection. Most of these can be minimised by careful front end design and expensive filtering. By compromising with decent design and cheaper filters the main problem is then Image rejection.

JR Single conversion receivers use a crystal running 455kHz below the channel frequency. This is fed to the amplified incoming RF and fed to a mixer. This mixer adds and subtracts the 2 frequencies and the resultant output is fed to a filter. This filter is centered at 455 kHz and passes this signal onwards for processing to the demodulator and decoder.

All well and good but if we get a situation where another transmitter is running at 2 x the IF ( 910 kHz ) this will also be accepted and ............... down goes your aircraft. ( Worse case scenario ! )


This is the image problem.



With Dual Conversion, an extra stage of frequency conversion is implemented. This time the crystal frequency is 10.7 MHz below the channel frequency. Again this is fed to a mixer and the resultant outputs are fed to a filter centered on 10.7 MHz. This is then mixed with a 11.155 MHz crystal, filtered at 455kHz and fed on to the demodulator and decoder.

This time the interference signal would need to be 21.4 MHz away which is going to be extremely unlikely.

This way, it resolves the image problem without costing too much extra over and above the Single conversion receiver.

I hope this explains the main difference between the 2 types, there are many websites that explain in greater detail.



[8D]

MAXXHEADROOM

My Feedback: (2)

Thanks Guys you have been very informative.As Iam a newbie and like to gain all the knowledge that I can.
Iam goin to be flyin a Hawk Sport using 8301 with a r700 and eventually I will also have a 60 size I will be flyin with a 549 receiver.I guess my other question is that I should make sure both receivers have the dual conversion crystals And secondly your thoughts on using the gear slot on receiver for my gyro and the aux 2 for my fma copilot.
Paul

Mac_Man_UK

The R700 and the R549S are both Single Conversion devices ! You cannot put DC crystals into SC receivers. They will just not work.


Sorry, but I can't comment on usage of where to put devices as I am not a flier, just the techie for the UK distributor.


[8D]

Troy Newman

Maxx,

The only Dual conversion RX's that JR has in the USA are the 10channel SPCM versions. Older 940 and the newer 945. Both excellent RX's and I would recommend them if you are interested. These RX's will work on the 8103 as will the 950 (older style) and 955 single conversion RX's

But the standard JR RX's that are just the single conversion ABC+W are extremely good and in my personal opinion better at interferance rejection than many if not most of the RX's on the market today in the USA.

The R700 is an extremely solid RX for the PPM use. I would say in 99.9% of the situations this will work with never an issue. The 549 RX is a little older design and I have also had good luck with it. But the 549 as has been documented in this forum is not the best solution when running several digital servos.

My suggestion to you, if you are concerned about rejection of interferance, go to PCM. You will get better protect going PCM in my experience than you will get Single conversion vs dual conversion.

I have been running PCM for many years since about 89-90' and in fact I only have a couple FM (PPM) RX's and these are the R700 and R610's I use in small sport models and foamies.

If the plane is worth more than about $300-400 I stick PCM in it. The reason being its way better at rejection of bad stuff.

Currently the R770 RX is like $90-100 and this is a really good RX and gives all the advantages of PCM


Just some advice its worth what you paid for it.

Troy Newman
Team JR