Converting Positive to Negative Shift at Tx

[Pinecone]

Hmm, wondering, can a Tx be switched from Positive to Negative shift by putting an inverter into the signal line between the encoder and the RF deck?

Would it be that simple? And which inverter to use?

My situation is this, I an currently flying a Hitec Eclipse 7. I picked up a JR 6102 and Spektrum DX6 to make a 6102DX. I also bought a JR 9303, so the Hitec is redundant. BUT, I have some negative shift only Rxs, and I alos have a Blade CP which has a negative shift 4 in 1. So I was thinking, why not make the DX6 Tx a nagative shift PPM Tx for the Blade and a couple of other things.

[pilotpete2]

Hi Pinecone,
You have an Eclipse 7, you should be very happy that you bought a Hitec, as that is one of your programming options for each model setup, a beautiful thing :cool: Now I hope I've made your day :)
Pete

[Pinecone]

Nope, the idea is I am going to sell the Hitec Eclipse 7. That will leave me with a JR 9303, a Spektrumized JR 6102 and a 72 Mhz DX6. Making the 72 MHz DX6 (6102 RF deck) negative shift would make my day.

[pilotpete2]

Hi Pinecone,
I glossed over your OP :o
Now, I'm in the process of converting some Airtronics receivers to neg. shift as I am planning on replacing my aging Infinitty 660 Tx with a Futaba 9C Super. With dual conversion receivers it involves changing the 2nd IF crystal to a different frequency, I received great help from some nice folks on the DIY electronics forum, and am in the process of "horsetrading" crystals with them :cool:
I would think changing the shift in a Tx would be easier, in fact I will ask around on that question, since if I converted my old Airtronics TX to negative shift, I could then still use it as a backup Tx :) If and when I find out something on that I will let you know.
Good luck,
Pete

[Pinecone]

Thanks. Since I don't have a scope I can get my hands on, I can't check the pulse trains.

If it can be done that easily, it would be nice. IN fact, it shouldn't be hard to switch it in an out to make a shift selectable Tx.

I just don't kno where the shift direction is done, encoder or RF deck.

[Pinecone]

Wow, over 100 views, and nobody can say where the shift is introduced. Encoder or RF deck?

[davekra]

DISCLAIMER: I’m not an EE. I don’t even play one on TV.



Many moons ago I was investigating how to incorporate a Spectra module into my old Futaba 7FGK gold radio. I lost the website but it was talking about the old MP8000 using the Spectra and being able to switch shift.



Here is the pic and text verbatim.

If you intend on using the information I posted please check the validity with someone who knows about such things.

Good luck,

davidk


*************beginning of text********************
Here is a message from D. Burnell regarding his way of doing the same type of Spectra mod:

I have a Spectra module - what did you do to tweak the modulation ?

It may have not been a problem for me. I am using a 74LS86 to do an
exclusive or which allows me to switch one input of the gate from ground to
+5 which alows the pluse train on the other input to be inverted or normal
on the output of the gate goin to the RF board. So another switch on the
luchbook gives low going high or high going low which matches anyones
modulation.

<end>

One problem I faced when I finally finished the modification (for the Spectra) was that I
could not get the whole thing to work. As it turned out, the Spectra was working but was
not sending any in "information" even though it was sending RF out the antenna. George
Stiener (GSP [916]362-1962) eventually identified the problem that the Spectra was not
getting enough modulatioin signal from the encoder. He made two minor tweaks to the encoder
that increased the modulation strength going to the RF deck, causing the Spectra to function
properly. Your transmitter may or may not have this problem when you make this mod. Be
sure you do a thorough range check prior to your trying to fly this setup. Thorough testing
is a must! If you run across this problem, you may find that it tries to work, at times,
but is generally jittery and unstable. Let me know if you get this and I will point out
which trim pots George turned to get the modulation to be increased.

Be forewarned that I have experienced weird anomolies when I turn on the MP8000 and then
switch from my other RF deck (AM) to the FM Spectra module. It tends to "Wig-out" my computer.
I have to make sure the Spectra module is selected prior to turning on the TX.

Side note: I discovered later that George modifies some/certain (I'm not sure which)
Futabas with a "Spike" button, causing a minor electrical surge to the Spectra forcing it to
modulate the information to the Rx. This surge can also be accomplished by removing the
module and turning on the Tx, then plugging the Spectra in again (while Tx is on) causing
it to be "spiked". However,this is inconvenient! I don't know how he rigs this spike
button, except perhaps to use a momentary-normally-closed switch(on the +9.6Vdc) where you
push the switch, then turn on the Tx, and then let go of the switch causing the power to go
to the Spectra (Purely speculation). I'll ask him how he does it next time I speak with him.

Good luck! and fly safe and do a good range check (150' minimum)!

Cabi~
(cabi@jps.net)

****************end text**************************

[pilotpete2]

Davidk,
thanks for the information, it looks like it could easily be implemented in the Tx, before the signal is fed to the RF module, or the RF deck in a non module rig.
Pete

Hey, I'm not an EE either, and I also didn't stay at a Holiday Inn Express last night :p

[Pinecone]

Cool, that is what I wanted to know.

And I had not thought of a XOR gate to handle this. Then you can switch modulation by putting the second pin to power or ground. Neat.

[AndyKunz]

I was working on an 8-channel computer radio encoder for FMA back in the mid 90's, one of the features being positive/negative switchable (yes, using an XOR).

One of the things to watch for is that it isn't simply a sharp edge that you feed to the RF deck. Proper shape of the "digital" signal going to the deck was important for maintaining FCC requirements with the decks we were using (Ace, which was designed by the owner of FMA). A square edge might work, but you might also find yourself outside the narrow band window.

A lot depends on the deck. Your best bet is going to be matching the shape of the wave you are feeding it now with the factory electronics.

Andy

[Pinecone]

Wouldn't an XOR or the posted circuit do that? Just invert whatever voltage is present at the time?

EDIT--------------

Now that I am awake, I understand. An XOR will switch on the edge and produce its own waveform.

Any better way to handle this?

[AndyKunz]

Yes - put the XOR _before_ the shaper. There's a digital signal coming from somebody - just invert THAT before it's changed into the almost-digital form.

Somehow I vaguely remember something about a Miller Integrator as part of the discussion.

Andy

[Pinecone]

Hmm, how about an EASY way to do this? :)

Is there some device that will invert and maintain the same wave form?

[AndyKunz]

An op-amp.

Andy

[Pinecone]

OK, cool.

Any suggested part numbers?

[Daniel7866]

I was working on an 8-channel computer radio encoder for FMA back in the mid 90's, one of the features being positive/negative switchable (yes, using an XOR).

One of the things to watch for is that it isn't simply a sharp edge that you feed to the RF deck. Proper shape of the "digital" signal going to the deck was important for maintaining FCC requirements with the decks we were using (Ace, which was designed by the owner of FMA). A square edge might work, but you might also find yourself outside the narrow band window.

A lot depends on the deck. Your best bet is going to be matching the shape of the wave you are feeding it now with the factory electronics.

Andy

+1

Daniel

[AndyKunz]

Excellent work. Thanks.

Andy