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Old Newbee with radio question

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Old 08-04-2004, 09:04 PM
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afineman
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Default Old Newbee with radio question

I am just re-entering this hobby after MANY years of build and crash, I have been trying for 30 years to fly ( my brother too). Well over the last month we both were able to fly some cheap Chinese made RTF’s. the 2 motor with no moving rud/elv, then we both went to a 3 channel slope diver ( AB for short) and are doing well and having a BLAST, now for the question.

I’ll start by saying I am an engineer in electronics, but am confused about the + /- shift thing.

I will be buying a new radio for the first time in 20 years ( any one want a Cox/Sanwa 2ch and a 3 ch Hee Hee Hee).

What is the difference? Does a dual conversion Tx work with any Rx? Does a dual conversion Rx work with any Tx? Do ESC or the servos care if it is a +/- shift? Is there an advantage to one over the other? What one is the most used/compatible?

I’m sorry if this question has been asked before but I have searched here and at RC Groups and really can’t find or missed the answers.

From all I can gather ( price to function ) I’m looking at the Hitec Flash X 5 with the slim 8 Rx and or the 555, I am also looking at the GWS ECS.

Because of this +/- shift thing I have held off upgrading because I haven’t had any frustration with my re entrance in to the hobby ( unlike the previous 3 decades), and I’m looking to keep it that way, buying incompatible parts because of this +/- shift thing would really make for a BAD day.

Any help you offer will be greatly appreciated


Thanks
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Old 08-04-2004, 09:38 PM
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JohnBuckner
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Default RE: Old Newbee with radio question

Dual conversion and single conversion refers to a different type filtering senario in the RX. The Tx does not care what type of filtering is usesd in the RX. Generally single conversion Rx's are used in the park flyer type aircraft and may not have the selectivity of the dual conversion. The exception to this is JR's proprietary single conversion filtering used in some of their Rx's and in this case they perform equally as well as DC systems.

In regards to the modulation shift I am not an engineer and will not try to explain the technical end but can tell you that, in FM systems to have interbrand compatbility both the Rx and the Tx modulation shift must be of the same shift (with a couple of exceptions I will explain in a bit).


Hitec uses negative shift
Futaba uses negative shift

JR uses positive shift
Airtronics uses positive shift

GMS I have no idea but it would,nt hurt to ask

The exceptions are in some cases better Tx's and some aftermarket Rx's have a shift select feature that will make them compatable with a Tx or Rx of either shift.

The modulation shift selected by the manufacturer has no effect on the performance and I believe this selection is driven more by marketing decision and what their compeitors are doing.

In the case of PCM systems there is no interbrand compatability between Tx and Rx's.

The Flash five is not a bad radio and I use to recomend it to some folks but there is a new six channel out now and I have forgotten the name but the Hitec adds are all over this site. Any way I beleive this is a better choice for not much more money. It is more flexable and it has the shift select. Airtronics also has some not to expensive radios with this feature.

John
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Old 08-05-2004, 09:47 AM
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Rodney
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Default RE: Old Newbee with radio question

Positive shift and negative shift work equally well. Positive shift just means that the transmitted RF frequency is shifted upward when modulated with a data pulse and negative shift means the data pulse decreases the transmitted RF frequency. This excursion is usually about 5 kilohertz from the nominal frequency. While most people call the modulation technique used by RCers FM, it is really frequency shift keying. True FM varies the modulation frequency somewhat linearly while RCers do it in a single step frequency shift, thus frequency shift keying. Dual conversion and single conversion is not related in any way to the transmitter. Dual conversion is when the incoming signal to the receiver is converted to a second frequency, usually 11 megahertz. This signal is conditioned and then mixed again with a second frequency with a resultant frequency of usually 455 Kilohertz. This signal is then conditioned and converted to a series of pulses which continue on into the decoder chip in the receiver. Single conversion receivers omit the 11 mhz stage, going directly to the 455 khz stage. This makes them more apt to be interfered with by third order harmonic distortion and somewhat more suceptable to other interference. We RCers are quite blessed with very good equipment now. Any of the major brands perform admirably.
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Old 08-05-2004, 08:40 PM
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Default RE: Old Newbee with radio question

Thanks for the resopnse ALL.

I now have a better understanding of the +/- shift, I do not have to rethink my choice of radios.

Thanks ALL
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