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Rx AMP or filter

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trennonix

New Member
Hello,
i got a Tx and Rx that i removed from my radio controlled car and i seperated them from the decoder and the encoder in order to place my own.
My question is: How do i remove the white noise in order to get a clear zero or a clear one, just so that a PIC could decode the signal.

Peak to Peak, the signal is at 1.5V
And what sort amplification should i add?

How should i modify the signal in order for the PIC to differenciate between a one and a zero???
 

mvs sarma

Well-Known Member
Hello,
i got a Tx and Rx that i removed from my radio controlled car and i seperated them from the decoder and the encoder in order to place my own.
My question is: How do i remove the white noise in order to get a clear zero or a clear one, just so that a PIC could decode the signal.

Peak to Peak, the signal is at 1.5V
And what sort amplification should i add?

How should i modify the signal in order for the PIC to differenciate between a one and a zero???
note the company that made them and their model numbers, search for datasheets
as these datasheets speak a lot. generally Rx can work at -100 dbm input signal. so it is not noise alone. you may get these datasheets or near by ones at
http://www.rentron.com/PicBasic/RemoteControl.htm
 
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trennonix

New Member
it's a cheap chinese toy, i found datasheets for the decoder and the encoder which i intend to replace. No details about the quality of the signal is mentionned in the datasheets plus they're in chinese :S
 

trennonix

New Member
i was thinking of sending a high freq signal (when one) and connect the receiver to a resistor-inductor circuit (high-freq pass) and then, send the treated signal through a capacitor removing the DC current , after that a diode will keep one polarity and this final signal will be set in parallel with a capacitor in order to get a continuous one.

When i'm sending nothing, the RL circuit is stopping the noise from penetrating thus a clear zero.

Would it work?? and what about the values, i'm a total newb when it comes to values
 

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trennonix

New Member
i'm not interested in the encoding\decoding part; that i already have.
i need the filter and the amplifier to treat the brute signal that i get from the receiver of a radio controlled car.
I want to turn this analog and alternative signal into a digital one\zero signal which could be then decoded by my PIC that i have already programmed.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
i'm not interested in the encoding\decoding part; that i already have.
i need the filter and the amplifier to treat the brute signal that i get from the receiver of a radio controlled car.
I want to turn this analog and alternative signal into a digital one\zero signal which could be then decoded by my PIC that i have already programmed.
You certainly don't need a filter, which would be completely useless - if the signal level isn't high enough, then a simply transistor amplifer/inverter is all that's required.

However, I suspect the problem is probably with your PIC program, you DO need encoding and decoding - what is it currently doing?.
 

trennonix

New Member
i tried using an inverter and right now things are working ok
thanks guys :D
btw, i haven't connected my pic yet, i was just comparing noise to a standard unmodified toy transmitter
 

marcbarker

New Member
i'm not interested in the encoding\decoding part; that i already have [in my PIC program].
Does your PIC program include a software PLL, to exclude noise? In the manner of a cassette tape interface i.e. as done in the ZX Spectrum 'TapeLoader'? I suspect that Nigel's PIC has.

The way that 80's cordless telephones did it in the hardware instead of in software was by transmitting a 6 kHz tone that was AM modulated by the encoder IC output, and at the receiving end, the 6 kHz tone + radio noise was AM demodulated into a pulse train again.
 

trennonix

New Member
to be honest, i have no idea what's PLL. :confused:

googled it and i was referred to an IC called LM565CN
tried to understand what it does but i couldn't get it,

the thing is that i'm still in high-school, so ...

but like i already said, i haven't connected my PIC yet, the receiver is connected to a virtual oscilloscope driven by the soundcard of my PC
(just to give an idea on my professional working environement) :p

but please do clarify on how this works
 

marcbarker

New Member
In the cassette tape interface of the ZX spectrum computer, there was a PLL done in software to clean up the noise and jitter, that's why I mentioned it.

The PLL probably evolved from analogue television Line Sync. The horizontal line scanning oscillator carries on 'flywheeling' at the same frequency, in the presence of noise or dropouts in the incoming line sync.

AFSK is just one of many PLL apps, i.e. Tone decoders/ FM demodulation / frequency synthesisers and on and on......

NE565, that's a blast from the past....
 

trennonix

New Member
hey, i'm interested in tone decoding since i'm willing to send commands through the telephone line. Now this is a project awiating other projects to get done, but still, i'd like to know what i'm in for.
How complex and what should i look for when i'm trying to send commands through the line, or better yet how does those answering machines with built-in menus work?
 

marcbarker

New Member
What, you mean those answer machines you send DTMF tones from the phone keypad with? Originally tuned circuits arranged together on a 4x4 matrix, then a DTMF decoder IC with audio in & digital out, now a clever microcontroller can generate and/or decode the DTMF in software.

What you have descibed post #1, I think is the serial data such as for cordless phone handset identification recieved by the base unit. Used to be in 80's done with separate IC's like MC145026, HT640 or HT12E /D.

I know it's not apply to the orig. question, but tones are great over really noisy links, because the longer the tone is, the easier it is to detect it. (like 1 baud data rate I mean) A 567 PLL tone detector is brilliant, if the tone is present the output goes logic low, simple as that. If it's listening for a tone in amongst white noise, it can lock on easily. I did a 567 detector that heard a feint tone in amongst noise, I couldn't hear it at all, but the 567 did OK. If I remember I cheated a little, because I preceded the 567 with a phase-shift oscillator that didn't quite oscillate, that narrowed and amplified the band of interest that the 567 was listening inside.

Luckily, phone lines are quiet, but still you cant easily send slow serial data unless it's on top of a tone of some kind. Tones are great over noisy links. Digital data is great between IC's on a PCB. The art is the compromise between these two extremes. Does this help?
 
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trennonix

New Member
the LM567 seems perfect for the job.
but if i got 10 tones from ten different keys, does it mean that i need 10 LM567 ?!?!
or is there a chip with multi tones and consequently multi outputs?

what about connecting a binary counter to a transistor which will play the role of the variable resistor; now the counter would be synchronised with the pic micro so as it scans through the desired range of frequencies ;)

thanks for your help, but before this, i have another project to finish
 

mvs sarma

Well-Known Member
the LM567 seems perfect for the job.
but if i got 10 tones from ten different keys, does it mean that i need 10 LM567 ?!?!
or is there a chip with multi tones and consequently multi outputs?

what about connecting a binary counter to a transistor which will play the role of the variable resistor; now the counter would be synchronised with the pic micro so as it scans through the desired range of frequencies ;)

thanks for your help, but before this, i have another project to finish
you may use MT8870 - download a datasheet
it is perfectly decodes the telephone dtmf codes
and gives 4 bit nibbles as output
 

trennonix

New Member
nice, exactly what i want; and it's widely available right?
i know we're way off-topic, but is there's a chip that handles the answering part? (not the talking part but just the picking up)
 

mvs sarma

Well-Known Member
nice, exactly what i want; and it's widely available right?
i know we're way off-topic, but is there's a chip that handles the answering part? (not the talking part but just the picking up)
yes
called DTMF decoder made by MITEL and many others
 
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