Negative and Positive Shift?
 

I was wonding what negative shift and postive shift means?
thanks,
Richard

Negative and Positive Shift?
 

It means some radios don't work with some other radios.

http://www.torreypinesgulls.org/Radios.htm

Read the section "AM/FM". The rest is also well worth reading.

Steve

Negative and Positive Shift?
 

Steve,
great site to visit and learn,
thanks,
Richard

Negative and Positive Shift?
 

Steve;

Thanks for posting the Torrey Pines Gulls info on radios. Good stuff to know.

I also read through most of the info on the whole site and thought that the authors did a really good job of addressing a whole host of issues.

Best of luck,

Tom

Negative and Positive Shift?
 

PPM (Pulse Position Modulation) works by alternating between two frequencies. One frequency means ON, one frequency means OFF. The length of the on pulse is where the receiver gets the information for servo positions. Negative vs Positive shift just means that the on and off are reversed. It's a stupid marketing ploy by Futaba to try to keep more people using it's equipment, even though it's beyond trivial to change the shift on a receiver or transmitter (a simple 50 cent chip will do the trick)

Negative and Positive Shift?
 

If a picture is worth a thousand words, this post will exceed the limit :D

Negative and Positive Shift?
 

Accepting in order to be standard the Frequency shift has to be identical, otherwise someone using a positive shift on 72 would hit someone using a negative shift on 73. The core frequencies are identical only the pulse itself is reversed. I can't show you a picture to compare to yours to show what I mean because both sides would look the same unless you measured the pulse width with a ruler. Basically on positive shift the "shifted" frequency is considered to be the starting edge of the control pulse. On negative shift the "carrier" frequency is considered to be the starting edge. That picture is not accurate to what's actually going on, but it's the only way to graphically depict it. Try looking at the output on a scope and you'd see the difference.

Negative and Positive Shift?
 

Futaba makes a special effort to inflict this on its North American customers, since for the rest of the world it has to manufacture its radios to shift the same way as JR and everyone else.

HArry

Negative and Positive Shift?
 

Too bad more companies don't do what Airtronics has done with their RD6000 line and let you select which shift you transmit... then again, if everyone used the same system.....

Negative and Positive Shift?
 

Unfortunately, I don't have access to an airtronics radio at this time (only Futaba & Hitec), but here's a picture of a Hitec PPM signal from a scope. However, all the technical data I have read indicates the image I posted earlier is an accurate representation. The amount of shift is nowhere near enough to come close to the neighboring RC channel. Don't forget, between each of our channels, there is another channel for the primary frequency users of the 72Mhz band.

If I can borrow an Airtronics Tx, I will post a side-by-side comparison of the scoped signal.

Negative and Positive Shift?
 

Well what exactly is the shift frequency of RC then? I'd like to know the bandwidth.

Frequency shifting
 

Each channel has a 10Khz bandwidth, and if I recall correctly, the shift is a 5KHz shift.

For example, lets say you are on channel 16 - 72.110Mhz carrier frequency, shifted to either 72.115 or 72.105 depending on if you use positive or negative shift.

The neighboring RC channel 17 - 72.130Mhz is shifted to either 72.135 or 72.125.

That leaves a 10Khz gap (between 72.115 and 72.125) for the primary users of the 72Mhz frequency.

Bandwidth and shift are two separate things
 

Hopefully this is accurate. I'm an EE, but have never worked on RC systems other than as a hobby. If I'm wrong, I'm sure someone one RCU will jump in and correct me ;>).

The bandwidth is the deviation in frequency from the "center" frequency cause by modulation. Transmitters (simply speaking) transmit around the center frequency called the carrier. This is what channel you are on and the receiver filters out anything outside of the bandwidth. Single conversion receivers do it once and double conversion receivers do it twice and therefore reject a greater amount of interference. The slight change in frequency is then demodulated into the pulse trains seen elsewhere in this thread.

The joystick (servo) signals in the transmitter are modulated onto the carrier frequency and each channel has a period of time that is allocated to it in the pulse train, then it starts over again. The pulse can vary from 0-100% of this time period and that corresponds to the servo throw from end point to endpoint, however, one type of shift has the pulses going low for this time and the other has the pulses going high for that same time. The reason that they just don't reverse the servo throw is that the time relationship must be reference to the start of the servo signals and if the shift is reversed, the receiver picks the wrong time period for the servo signal.

Some receivers now can switch the waveform and switch their shift to allow use with both shifts. From an engineering standpoint the shift change is easy to put into a transmitter or receiver when it is designed, but would not be practical to retrofit.

Negative and Positive Shift?
 

Hello,

My knowledge of electronics and radios is very limited. Altough what I think to have understood is that a negative or positive PPM signal has nothing to do with negative or positive shift.

Having a Futaba radio (Robbe in fact) with negative PPM, I can use a Graupner receiver, also working with a Graupner MC12 transmitter that is working with a positive PPM.
But in North America a negative shift transmitter cannot be used with a positive shift receiver.
So to my humble opinion negative and positive shift is about the HF (high frequency) part and how the signal is transmitted, positive and negative PPM is about coding and can be read by any receiver...

Olivier

Shift
 

Actually, for PPM, it's the spacing between the pulses which represents the servo position information. Except for sync, the shifted pulses are of fixed time. For PCM, after the sync & format pulses, the TX sends out 1's & 0's as shifted or unshifted pulses of fixed width. The serial data stream is decoded back to a pulse width sent to each servo. In either case, it's called frequency shift keying. Just a matter of how the data is encoded/decoded. That's why its easy to use the same RF deck for either kind of system. The PPM/PCM encoder just sends the shift on/off times to the RF section.

Remember part of the narrow banding process was to guarantee no adjacent channel interference to either our channels or the industry ones in between so the shift frequency isn't much. I don't remember if its 5khz or 4 khz or even if it's the same brand to brand.

Negative and Positive Shift?
 

The shift change is in the PPM signal not the RF signal, otherwise you couldn't switch the 'shift' with just a simple transistor addition to an xmitter (which is all it takes) I've heard people say it's in the RF end, but no one's ever been able to show a link or shred of proof for it. Not to mention positive shift on channel 61 and negative on 62 would probably hit each other. The RF end of RC has been standard since 1991.

Negative and Positive Shift?
 

Lynx - I now regret not bookmarking the info I had which showed it as being a shift in frequency. The shift is 5Khz from the carrier frequency, which still leaves a 10Khz gap between neighboring channels, as I explained in my earlier post.If you start searching the sites which provide background for building your own RC transmitters and receivers, you will probably come accross the same info I read. Some designs were complete with block diagrams, schematics, and parts-lists. However, since I don't have a HAM license, I did not pursue it further.

Negative and Positive Shift?
 

I've done some searching and nailed it down. +ve and -ve are logic terms, and have nothing to do with the RF in reality. It doesn't change which direction it shifts, just what that shift is considered. +ve means that the shifted frequency is considered the positive edge of a pulse. and -ve means that carrier is considered the positive going edge. IE in reality it's just an inverted PPM waveform, on the exact same radio frequencies. Also, I don't believe the FCC would allow one way OR the other. That's 5Khz of wasted bandwidth, they're not going to allow that just so Futaba can sell a few extra units by being proprietary. Spectrum is too valuable to allow things like that to occur.

Negative and Positive Shift?
 

The reason that Futaba can't do negative shift on us is that we use 10Khz spacing so our regulations require the frequency shift to be the same direction. If it was just an inverted PPM data signal then the two frequencies used would be the same for +ve and -ve shift and Futaba would do to us what it does to North America. Futaba can do its negative shift because your regulations allow it because your 20kHz spacing is big enough to cope with frequency shift in both directions. It is the RF that is shifted irrespective of what the manufacturer does about the PPM signal inside the Tx.

Harry

Negative and Positive Shift?
 

Where did you find that interesting but inaccurate information ? The reality is that in both shifts the carrier i.e. the nominal channel frequency, is considered to be space(off). The different in shifts is whether carrier PLUS shift is mark(on) or carrier MINUS shift is mark(on). That's in the RF.

Since the bandwidth of the Rx is enough to handle either + or - shift a Rx on the wrong shift will essentially treat shifted freq as a slightly off channel carrier (i.e. off) and the real carrier as though it was a slightly off channel shifted freq (i.e. on). The result looks like an inverted PPM waveform so, provided you don't mind the fact that you are not running exactly in the centre of the crystal spec, if you invert it it sort of works.

But the RF signal out of each Tx is definitely different. It's not difficult to measure or at least it wasn't when I was in the US and had access to 72MHz gear. Over here we don't have all that fuss.

Steve

Negative and Positive Shift?
 

Interesting thread....now you guys are going to ,make me break out my o-scope and look for myself. Been a broadcast engineer for almost 10 years and never gave this a thought until now. Off to the FCC database and the test bench.

Negative and Positive Shift?
 

Ugh, you guys are giving me a headache....

Now I remember why I so hated EE. Why does an AE have to take EE anyway? Oh yeah, fly by wire- damn.

I think my 6-year old daughter's technical explanation will clear everything up:

"When Daddy moves the sticks, the plane flies."

What else is there to know?!!

Seriously, keep at it guys- you EE and RF experts could help answer this lingering question for the rest of us.

Thanks,
George

Negative and Positive Shift?
 

Not exactly. You have described FM and left out AM . PPM can use AM,FM,FSK etc as well .

Also , as far as I know the deviation is symmetrical from the FM carrier so the polarity of the encoding should not affect channel spacing.

Negative and Positive Shift?
 

http://www.aerodesign.de/peter/2000/...M_PPM_eng.html
http://www.mh.ttu.ee/risto/rc/electronics/index.htm

Negative and Positive Shift?
 

It may seem like we have 20KHz spacing, but there are industrial users in between each of our channels. Each channel is only 10KHz wide - just like yours (carrier freq +/- 5Khz).

The shift is only 5KHz, so there is enough room within a 10KHz channel spacing.

LF - if the deviation is symetrical as you suggest, why does the receiver have to match the shift of the transmitter?