Hi Guys,
I have send and received DTMF over air from the ground up, and would like to improve the receiver.
I have an AM receiver listening at 1700kHz, just off our AM commercial band here, and that frequency can carry DTMF just fine.
This is a surprise to me because commercial radios don’t seem to feature DTMF modulation until they get into VHF.

Beginning with a Motorola MC145436 DTMF decoder IC, it occurs to me a lot of the AM radio is an unneeded complication.
The minimum requirement (I believe) would be:
(1) An LC circuit tuned to ring at the desired frequency,
(2) A diode to strip the carrier (though I’m not sure it could be omitted when the chip runs at only 3.57MHz),
(3) “Perhaps” some transistor amplification if the signal is not strong enough for the IC to read the signal.
(4) “Perhaps” an antenna and Earth connection coupled to L in the LC circuit if L is not a good enough antenna.

Am I correct in all of this?
Cheers, Art.

 

dtmf requires a fairly low noise signal, hf frequencies have lots of atmospheric and terrestrial noise. the vhf and uhf radios with dtmf use fm modulation, and are not usually affected by the noise since they have shorter range and will not pick up a thunderstorm from the other side of the planet. also, dtmf requires a very low distortion signal, much more distortion with any distance on hf.

 

What kind of range are you expecting? I presume that this is for some sort of wireless remote control? How big can the antennas be?

 

NOTE grab and old DSL (phone line to Ethernet or wifi adapter), modem, answering machine, cordless phone with base - you'll have 99% of what you need for the phone line interface Decoding potion. Having said that for ham radio we use many commercial units for $ 20 or less, just add relay or similar.
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As little range as possible. Within my own property would be best

Not really for any practical use, although remote control and sending data were the ideas behind it.
I just wanted to verify sending data over the air when I did the transmitter and receiver from the ground up for experimentation.
I can’t say I’ve ever made a matching pair like that.

 

This is a surprise to me because commercial radios don’t seem to feature DTMF modulation until they get into VHF.

I have no idea what this means. DTMF is just a set of signals composed by two tones each, it is not a modulation type. Any carrier which can carry voice frequencies can carry DTMF frequencies.

At any rate, online you can find plenty of simple one or two transistor AM receivers and they should work fine. Look for "simple transistor regenerative receiver".

 

That was a transmitter and receiver in the video.
The DTMF is amplitude modulated.
When I say “DTMF modulation”, what I mean is the commercial radio with DTMF keypad
will modulate the carrier with DTMF directly without any audio coupling,
as opposed to modulating with voice through the microphone.
which is what I did here. The receiver doesn’t produce audio but only decoded DTMF.