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Old Oct 19, 2006, 10:48 AM
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Converting positve shift Rx to negative shift

Any chance of converting positive shift 72MHz Airtronics (Sanwa) receivers to negative shift? Any suggestions would be appreciated, also I'm very experienced in precision soldering and equipt to do it, just clueless of what to do.
Thanks,
Pete
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Old Oct 23, 2006, 07:28 PM
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Quote:
Originally Posted by pilotpete2
Any chance of converting positive shift 72MHz Airtronics (Sanwa) receivers to negative shift? Any suggestions would be appreciated, also I'm very experienced in precision soldering and equipt to do it, just clueless of what to do.
Thanks,
Pete
I'm very interested in the same for Hitec receivers. That must be simple since they exist in both modifications and RCD3500 can even be shift-selectable. I guess just put low or high on a pin in the decoder. Does anybody knows this? Somebody who owns a shift-selectable model, can you take a blown-up picture of the PCB?

Nick
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Old Oct 24, 2006, 02:29 AM
PDK
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Hi,

By positive shift I am asuming that some radios have a 'from ground to 5V' signal pulse while some have a 'from 5V to ground' signal. Am I right? (my gear is all Futaba and I didnt realise that other manufacturers use something different).

If so I would use an inverter chip. Probably the 4069 would do it (should be easy to get). It is a hex (6) inverter chip so all you would need to do is wire up each of the RX outputs to an input of the chip (and connect the supply). Each output will be an inverted copy of the input. You might need a small filter cap across the supply (maybe 0.1uF). Other than that job done.

Might be worth downloading a datasheet and having a go!!
Good luck,

Phil
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Old Oct 24, 2006, 03:50 AM
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Phil,
You wouldn't have the shift problem, because it is only with 72mHz equipment. On 35 - 40mHz the shift seems to be the same for all manufacturers. The shift refers to the signal from the FM transmitter which shifts between two frequencies. On positive shift, the signal is normally at the lower frequency, and shifts to the higher frequency to send the pulses. On negative shift it is normally the higher freq and shifts to the lower.

The effect at the receiver is just like you said, an inverted signal. An inverter chip would probably do it, but you would have to know where to insert the inverter. It would have to be at a point before the sync separator.

Jeff
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Old Oct 24, 2006, 07:41 AM
PDK
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Ok,

Thanks for the info Jeff. Glad we don't have to worry about it!!

Yes what I was meaning is a making a separate module; not something to put inside the RX. As you said a single inverter before the sync separator would do it. My thought was to take each individual channel signal and run them through their own inverter (simply so you donít have to fool with the RX internals). Probably a little clumsy but it would work. (Any unused inputs should be grounded or pulled high).

Anyway, just a thought. All the best
Phil
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Old Oct 24, 2006, 07:49 AM
aka: A.Roger Wilfong
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Only problem with an external module is that it needs to be inserted between two circuits within the Rx. The location may be within a single chip.

OTOH, you can accomplish the same thing by inverting the signal at the Tx between the encoder and the RF module. Several circuits have been published for adding switchable shift to various Tx's.

- Roger
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Old Oct 24, 2006, 08:33 AM
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Quote:
Originally Posted by PDK
Hi,

By positive shift I am asuming that some radios have a 'from ground to 5V' signal pulse while some have a 'from 5V to ground' signal. Am I right? (my gear is all Futaba and I didnt realise that other manufacturers use something different).

If so I would use an inverter chip. Probably the 4069 would do it (should be easy to get). It is a hex (6) inverter chip so all you would need to do is wire up each of the RX outputs to an input of the chip (and connect the supply). Each output will be an inverted copy of the input. You might need a small filter cap across the supply (maybe 0.1uF). Other than that job done.

Might be worth downloading a datasheet and having a go!!
Good luck,

Phil
Right, we mean the shift in the frequency radiated by the TX when the signal puls is on.

Inversion would be most simply accomplished after the amplifier before the decoder, and this apparently can be done with one transistor and one resistor only. But soldering even this into
the modern receivers, especially micro is a task, for those who don't have equipment and experience.
Breaking a PCB line or adding a connection would be much easier! I am suer this can be done, at leastwith Hitec. To check this idea I only need positive-shift versions of the receivers I have.

Nick
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Old Oct 24, 2006, 08:48 AM
PDK
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Maybe I am not understanding the problem correctly. Is the output from each channel inverted (i.e. the RX works but does not send the correct pulse to the servo) or does the positive shift RX simply not work with a negative shift TX? (presumably because the sync separator is expecting a long high signal to reset and getting a long low).

If the latter is the case:
A couple of the RX's that I have seen use a simple shift register to separate the signals and a transistor enabled RC delay for the sync separator. If you changed the type of transistor (i.e. pnp to npn or the other way round) and ran the signal from here? Anyway before I get too carried away I suppose we should ask what RX is it and what does the inside look like?? Maybe we can get a better idea of the circuit.

Then again maybe I am thinking too hard!! Its late, I'm off to bed.
All the best
Phil
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Old Oct 24, 2006, 09:02 AM
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beat me to it Nick,

So the RX works, it is just inverter right? Yes it should be easy to put a transistor in there to invert the signal. As you said the size might make it difficult.
As I just asked, Maybe if we can see the inside it might be clearer. Might be easier than we think.

Phil
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Old Oct 24, 2006, 10:55 AM
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Originally Posted by PDK
beat me to it Nick,

So the RX works, it is just inverter right? Yes it should be easy to put a transistor in there to invert the signal. As you said the size might make it difficult.
As I just asked, Maybe if we can see the inside it might be clearer. Might be easier than we think.

Phil
Let me clarify things but specifying what I know from what I am note
sure but assume.

Yes, the "shift" simply means the direction of frequency
deviation. That is, if it were an FM broadcast station, it would
simply mean the inverted amplitude of the modulating signal, which of
course simply changes the sign of the signal after RF, IF, mixer,
detector... all that high frequency part of the receiver.

And, yes, I believe the receiver itself works fine with either shifts.

What can be stated definitely is that if you use positive and negative
shift TRANSMITTERS on the same receiver, the signal which is output
into the low-frequency ("audio") part of the receiver will be just
inverted.
A small reservation: this strictly applies if the spectrum of the
signal does not span beyond the bandpass of the receiver. In theory,
if a manufacture produces the receiver for a definite shift and for
use only with the standard FM (PPM) modulation, they could have made a
narrow asymmetric bandpass. Say, for the TX on channel 15 where the
carrier is at 72.290MHz regardless of the shift, they might have do
(by the filters in the IF section) the passband from 72.282MHz to
72.293MHz without sacrificing the reception, for a Hitec transmitter
(negative shift). The signal from a JR transmitter on such a receiver
would probably have more distortion (wider fronts) since a part of the
allowed spectrum above the carrier it populates, will be
suppressed. For a positive shift system one would then place the
asymmetric passband, say from 72.287MHz to 72.298MHz.

I do not know for sure how in practice these receivers are
constructed. If you ask my opinion, I would strongly doubt that such
an asymmetric design is implemented, at least not in the commonly used
receivers. One needs to be selectivity-paranoic to undertake such
measures for maybe a couple dB signal to noise ratio. The adjacent
frequency suppression seems to be already high enough for the existing
designs. For instance, we checked with our Club president who ran
Futaba on channel 32 and me Hitec on channel 33 that no interference
appears even when another transmitter is 15 feet away from the plane.

Now, back to the concrete receiver. It is natural to expect that the
positive and negative shift receivers output the yet PPM-undecoded
"audio" signal of the opposite polarity. However, this is assuming
they use the same chips and the similar design. Clearly this must be
the case for the same brand receivers. I would expect this to be
universally true, but who knows for sure. What is definitely true is
that to change the shift orientation of a particular receiver one
simply needs -- or it is sufficient -- to invert the "audio" signal,
before it undergoes all that shape modification, decoding,
demultiplexing or whatever is involved and called. And that can be
done, I assume with one transistor and one or two resistors.

The polarity and all other signal details on the output of the decoder
must be the same, since this should match the servo standard. If they
were inverted (which I understand was the case early on with some
brands), the servos would not be compatibe with the receiver.


I just am back now from a place where I took a closer look at my
recently bought micro Electron 6 by Hitec, negative shift. It is so
compact that even to look at it I had to use a strong magnifying glass
or alike. I can't say I now know its schematics completely, but it looked
probable that the low-frequency signal output from the high-frequency
part, TA31136 passes through two transistor amplifies. If I had the
transmitter and the oscilloscope, I would have checked this
directly. If this hypothesis is right, then it makes sense that the two
cascades are just for the flexibility with the shift we discuss, and
simply bypassing one of them would change the shift. However, in this
case I am not sure such superfluous addition would be found on the
receivers which are not produced in the different shift
modifications...

Well, I still think it would be simpler to implement the shift change
at the decoder level because I think in the modern receivers it is
something better than a simple shift register -- although I may be
wrong in this. I am going to look into the chips, apparently the two
used in this Electron 6 is TI 23 358 (I don't know which one of these
is the chip model), and the decoder proper is something with CBO15B
1BK AJXJ on it.

Nick
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Old Oct 24, 2006, 12:32 PM
aka: A.Roger Wilfong
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Quote:
Originally Posted by Volante24
Let me clarify things but specifying what I know from what I am note
sure but assume.

...
Now, back to the concrete receiver. It is natural to expect that the
positive and negative shift receivers output the yet PPM-undecoded
"audio" signal of the opposite polarity. However, this is assuming
they use the same chips and the similar design. Clearly this must be
the case for the same brand receivers. I would expect this to be
universally true, but who knows for sure. What is definitely true is
that to change the shift orientation of a particular receiver one
simply needs -- or it is sufficient -- to invert the "audio" signal,
before it undergoes all that shape modification, decoding,
demultiplexing or whatever is involved and called. And that can be
done, I assume with one transistor and one or two resistors.

The polarity and all other signal details on the output of the decoder
must be the same, since this should match the servo standard. If they
were inverted (which I understand was the case early on with some
brands), the servos would not be compatibe with the receiver.

I just am back now from a place where I took a closer look at my
recently bought micro Electron 6 by Hitec, negative shift. It is so
compact that even to look at it I had to use a strong magnifying glass
or alike. I can't say I now know its schematics completely, but it looked
probable that the low-frequency signal output from the high-frequency
part, TA31136 passes through two transistor amplifies. If I had the
transmitter and the oscilloscope, I would have checked this
directly. If this hypothesis is right, then it makes sense that the two
cascades are just for the flexibility with the shift we discuss, and
simply bypassing one of them would change the shift. However, in this
case I am not sure such superfluous addition would be found on the
receivers which are not produced in the different shift
modifications...
...

Nick
You've got it. All that needs to be done is add or remove an inverter between the detector and the decoder (a transistor and a couple of resistors would work fine). Everything else is the same.

- Roger
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Old Oct 25, 2006, 11:01 AM
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Many thanks to all those who responded, unfortuantly without the schematic for the receivers there's not much hope of working it out.
I talked to the folks at Radio South and they don't think it can be done (well at least they don't do it)
This is an annoying problem here in North America on 72Mhz, and one would reasonably expect the biggies (brand "F" and"J") to rethink their marketing strategy and have shift select on their computer radios (hold over from the days when they could "force" you to buy their receivers, long gone thankfully )
Will be taking a serious look at the new 9Ch Hitec when it is introduced
I would very much like to go with a 9C Super, but a JR 9303 may cost less in the long run when being able to use my existing receivers is factored in, oh well .
Regards to All,
Pete
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Old Oct 30, 2006, 06:11 PM
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Quote:
Originally Posted by pilotpete2
Many thanks to all those who responded, unfortuantly without the schematic for the receivers there's not much hope of working it out.
I have not given up yet, but I deal only with Hitec (so far). Well, I'll probably get a positive-shift Electron 6, but by that time my negative-shift one will be already installed in my son's plane... I am sure with the Hitec receivers this must be simple.

Nick
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Old Oct 31, 2006, 09:28 AM
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Hi Nick,
If you are buying Hitec's why not go with one of their auto shift models, the O5S is great for parkflyers, and for bigger models the new Supreme IIS is only a few of bucks more than the Electron and is auto shift
Pete
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Old Oct 31, 2006, 12:53 PM
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Quote:
Originally Posted by pilotpete2
Hi Nick,
If you are buying Hitec's why not go with one of their auto shift models, the O5S is great for parkflyers, and for bigger models the new Supreme IIS is only a few of bucks more than the Electron and is auto shift
Pete
Well, Pete,

Where I need this to fly in summer, using a single conversion receiver
is often out of question. What precisely happens on airways there is not
completely clear to me yet and what kind of RF radiation produces it, but the
effect is often terrible with a SC receiver. So, I decided to deal only
with the Double Conversion whenever I put in the receiver myself. Besides,
I don't really need shift select, I just want to have all the
receivers on a negative shift. Adding a shift select, on the other
hand, is a potential source of a problem; odds are small, but an extra
contact for sure may not add reliability.

The reasons why I am interested in changing shift is not fully
rational. In fact, I already have quite a number of standard-size
receivers. By now most of them are from eBay, only the first few were
brand new. I got interested also in electric planes and am thinking
about a sailplane, and have realized that micro receivers would matter
here.

One of the driving forces for this quest into shift select is just a
challenge. Once you have one, it is easy to find justifications. Say,
you have more opportunities on eBay. For instance, recently I got a
positive-shift Electron 6. In principle, I have Optic 6 and I can
change shift on it. But I want to be able to use the receiver with
other cheaper (and more reliable) transmitters, Laser 4 which I
personally like.

After all, challenge often drives us. One can go with DX6 or soon DX7
and do not have most of the problems behind discussions here, yet people
do. RC flying itself is often a sort of challenge whether we
appreciate this or not.

Nick
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