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Dual vs Single conversion JR 945 vs 955

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Old 08-27-2003, 01:43 AM
  #1  
HardDeck
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Default Dual vs Single conversion JR 945 vs 955

Hello all. I have followed quite a bit of the discussion re manufacture radio wars on various threads - so I do not wish to stir that pot. However I am JR user and would like to know more about single vs dual conversion. What is the difference, functional significance of difference? JR and supporters say JR single conversion receivers are as effective as other options - and I assume this is true because JR (as I understand it) only makes one duel conversion rx - that being 945. I am completing first large project and trying to decide 945 vs 955 - both PCM. I asked Danny at JR site, and his reply suggested not much difference - that one might be better than other in certain situations???
Comments about where I can learn more in general, and comments re 945 vs 955 much appreciated.
Rj Miller
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Old 08-27-2003, 03:01 AM
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Lee Belew
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Default dual or single conversion

HardDeck

I think the 945 & 955 are single conversion.
The 940 and 950 are the dual convversion.

The 955 and 950 are the latest version of the JR top of the line receivers. The 945 & 940 are a little older. Not much difference otherwise.
Good luck
Lee
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Old 08-28-2003, 01:06 AM
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Default Dual vs Single conversion JR 945 vs 955

The R950 is the older single conversion JR receiver, replaced by the R955.

The R940 is the older dual conversion receiver, replaced by the newer R945.

The R950 and R940 are no longer available through Horizon/JR because they have been replaced by the R955 and R945.

Two major criteria for judging how any device receives Radio Frequency signals. Sensitivity and selectivity. The more sensitive the receiver, the weaker or farther away the transmisson source (i.e. your transmitter) can be and still be "heard." The more selective the receiver, the better it is at filtering out interference and locking on to the transmitter.

I judge receiver performance by looking at recevied signal through my old Hewlett Packard occiliscope. I broadcast a signal using my old signal generator.

What I am looking for is how weak a signal the receiver can receive before the signal breaks up.

The farther I can turn down my signal, the better the sensitivity of the receiver.

With the R950 single conversion recevier I usually see a good signal down to about -50 dB. With my friends newer R955 receviers I have tested I can get down to -60dB.

"Ground Noise" is the common clutter created by the sun, earth, and assorted mild RF interference. Ground noise on a clean channel here in Missouri is about -55 dB to -60 dB. Thus, the newer R955 is sensitive and selective enough to "hear" signals that are weaker then the earths own radio noise.

With the older R940 dual conversion, I can see my signal down to -63 dB. With the newer dual conversion R945, I get -74 dB.

This is quite a improvement over the already impressive JR single conversion receivers.

I would translate these results to mean that the R945 performs about 15-20% better than it's single conversion brother, the R955.

There is a reason why so many Jet folks fly the JR R940 and R945!!!

Most of my JR receivers are single conversion, but when $$$ allows, I like the R940 and R945.

I have went back to mainly just training the neigborhood kids with cheap trainers. I have good results with cheaper JR R600's and R700's. I use a Dynamite Race Guard failsafe on my throttle.

It has often been said that JR single conversion receivers perform as well or better than
other manufacturers' dual conversion receivers. I am not sure that is true across the board. The R700 and R955 probably do.

I feel quite safe flying Futaba and JR receivers. They are surely competitive of each other. The JR R945 seems a bit better than all the others I have tested. It is very expensive. I have had good luck with Hitec too. I don't have enough experiance with other brands to offer any judgements.

JR single conversion with ABC&W is kind of like a psudeo-dual conversion. The ABC&W seems to remix the signal back into itself and also controls antenna gain. This is my theory at least. I can't prove it and I could be wrong. I have never yet found a person who could tell me for sure. Must be a secret. This ABC&W is also in JR dual conversion receivers and may be the reason they re so good.

Dual conversion uses a second occillator and mixes the signal twice. It is better, but some dual conversion receviers are better than others.

There are better ways than I have described to test receivers, but they are far too complicated for me to explain.

I hope my explanations helped.
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Old 08-28-2003, 02:07 AM
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Default Dual vs Single conversion JR 945 vs 955

Thanks Mo. I appreciate your detailed and thoughtful response. It definately helps, although I am beginning to see that the "rabbit hole goes pretty deep" here.
Thanks
RJ Miller
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Old 08-28-2003, 02:50 PM
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Default Dual vs Single conversion JR 945 vs 955

Mo,

Great explanation.

Thanks

Mark
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Old 08-28-2003, 02:56 PM
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Default Dual vs Single conversion JR 945 vs 955

MO_Radio_Tech - have you run similar tests on some other receivers?

I use a mixture of FM receivers on my glow stuff (and sailplanes) - several R600s, one R226, an R327, an ancient R529, a bunch of Hitec 555s, one Airt 92777 and one FMA Fortress.

There's one R610M in the mix here - somewhere. Updated version.

I've got nothing but R649s for my gas-powered planes.

If you've run sensitivity tests on any of these receivers I'd love to read your findings.
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Old 08-28-2003, 10:55 PM
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Default Dual vs Single conversion JR 945 vs 955

This subject is a lttle hairy. JR receivers seem to get better each time I see a newer one.

Especially with the R649's, etc. that they still produce. I don't know if thier crystals or other components are getting better or just what. Here are some of my findings.

The best receiver of the bunch you listed is the R649. You are doing well too use JR PCM with spark ignitions.

Testing is a bit subjective depending on the quality of the tuning of the individual receiver, the accuracy of the crystal, and any damage the filter might have. Look at my numbers as being realitive to each other. Actual numbers will be different at each flying field depending on interference.

R649 sensitivity -50 dB to -55 dB for older ones. Newer ones, -60 dB or about as good as their newer R955.

The R327 and other similar receivers from its era did not have the best sensitivity. They did have good selectivity. Probably due to the ABC&W. They were great at surviving hard landings and crashes with no damage. If your R529 has ABC&W written on the front it will be about like the 327. IF not, it is a wide-band paper-weight. Sensitivity -47 dB to -50 dB.

The R600 and R233 are part of a whole family of JR receivers using the same receive portion.
R226, R600, R236, R233?) They would design different decode portions depending on PPM or ZPCM. I wonder how many of these they sold as the R600. The R600 seems to have been packed with most JR radios for 10 years straight. These receivers were good for their day and performed well considering how small they were sized for thier time. Sensitivity -45 to -52. JR still makes the ZPCM versions (R236, R233).

The R610 seems to be a problem child. It has less circuitry (less coils) inside of it than any oher JR airplane receiver and seems to be more prone to problem interference. I like it in small hand launch gliders and park flyers. Sensitivity -55 dB to -60 dB. However selectivity is lower than other JR receivers. If the aircraft has a big motor or is worth $$$, I would go with he far superior R700.

I do not know much about FMA.

I have used Sanwa/Airtronics but never tested any of it. I have never heard much about in problems with the receivers. Airtronics seems to be of high quality like Futaba/JR/Hitec. Not as popular anymore.
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Old 08-28-2003, 11:11 PM
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Default Dual vs Single conversion JR 945 vs 955

The ABC&W seems to remix the signal back into itself and also controls antenna gain. This is my theory at least. I can't prove it and I could be wrong. I have never yet found a person who could tell me for sure. Must be a secret.
The ABC&W uses a windowing technique. It is based on an old Motorola patent. It is all disclosed here:Patent 4,739,518
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Old 08-29-2003, 03:01 AM
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Default Dual vs Single conversion JR 945 vs 955

Thanks for the info!

BTW-my 610M is indeed installed in a HLG, and has performed well in that application.

Any idea how the R549s compare to the R649s?
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Old 08-29-2003, 05:21 AM
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Default JR R549

First, thanks to RC_CAM for the link. This patent does indeed sound much like what I thought ABC&W to be. I must study this further.

I have had great luck with the R549. It seems to have similar selectivity and sensitivity to the SPCM R649.

However, as I learned for the first time right here on RCU, it seems that the R549 does not work well with servos of tighter "dead band" (correction rates), specifically digital servos. I don't have any digital servos, so i had to borrow some from guys at the field to test it. It seems to be some kind of high frequency interference caused by the fast, high current loads of the digital servos. The more digital servos used at once, the more glitching. I am not sure this could ever cause a crash, but I don't like it.

I experimented adding caps and coils to an R549. The best way to solve the problem was to power up the servo independently of the receiver. Just run ground and signal from the receiver itself. Use a separate large battery for servos. Might also add a coil to the ground wire.

Better yet, just don't use the R549 with newer digital servos. I was unable to duplicate this problem with my R700's. They seem to be immune.

Also, I should have mentioned that my sensitivity tests were performed with 72 mHz on a clear channel. Other bands would be different because of higher or lower noise floors.

Hope all this tech stuff is helpful.
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Old 08-29-2003, 09:24 AM
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Default Dual vs Single conversion JR 945 vs 955

I used multiple of the 945 and 955, also have the 950 receiver, as a hand user I don't the difference, they are used in different airplane but all Pattern with lots of Carbon fiber, and two with Carbon fiber fuselage....to ME there is no practical difference, all three models (receivers) perform without any hit. For larger model I would go with two receivers for sure....either one!
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Old 08-29-2003, 03:53 PM
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Default Dual vs Single conversion JR 945 vs 955

I don't want to step on MO's toes nor defuse his enthusiasm but the figures stated in dB for JR receivers are totally meaningless and misleading without some reference value.

The method by which such tests were done is not stated and such tests must be taken as a generalisation.
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Old 08-29-2003, 05:00 PM
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Default Dual vs Single conversion JR 945 vs 955

Originally posted by MO_Radio_Tech
...However, as I learned for the first time right here on RCU, it seems that the R549 does not work well with servos of tighter "dead band" (correction rates), specifically digital servos.
I also learned about that issue here on RCU - that receiver had some sort of problem when multiple digital servos were connected.

Horizon was offering to update users' 549 receivers in order to alleviate the problem.

I took them up on it. Sent them my cache of 9 R549s.



After a month of waiting, I received a box from Horizon containing 9 R649s and a note from the tech.

"We weren't able to update your receivers, so we've exchanged them for 649s. We apologize for delay and any inconvenience this may have caused you."

:stunned:

I am now a *huge* fan of Horizon's Service Dept.

(hmm. I have a Saito .72 here which needs new bearings. If I send it in - I wonder if they'll swap it for a 1.80?)
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Old 08-29-2003, 10:26 PM
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Default Dual vs Single conversion JR 945 vs 955

.
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Old 08-29-2003, 10:30 PM
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Default Dual vs Single conversion JR 945 vs 955

-----------------------------
"The method by which such tests were done is not stated and such tests must be taken as a generalisation."
--------------------------

That was the intention, a generalization. I did say that the exact sensitivity would be different at individual locations because of different RF conditions at different locations. Different locations will have different "noise floors".

The testing is not "meaningless". It shows relative performance of the receivers tested at my location.

It is far from perfect and there are more accurate ways of testing as you correctly say, but this gives folks a good idea of receiver performance.

I do think I described the method of testing. I could get more detailed, but only other radiomen would have any idea what I am talking about.
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Old 08-30-2003, 02:54 PM
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Default Dual vs Single conversion JR 945 vs 955

MO
Please get more detailed, I'm very interested to know your precise test procedure and any instrument values obtained.
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Old 08-31-2003, 04:55 AM
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Default Dual vs Single conversion JR 945 vs 955

I am getting with David privately on these tech details.
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