Two Tones, One Transistor

Put a negative resistance in series with two tanks, and put an amplitude limiter (like a varistor) across each tank.

I have the actual schematic here somewhere... can't find it at this instant.

Stromberg Carlson had them in ther 1A2 phone equipment repair manuals. I have no clue what I did with mine, when I stopped repairing the phones. I only know one local business that still have them, and gave them the spare phones I had on hand.

The original touch-tone DTMF schematic is online somewhere. I just have too many photos, can't locate my copy.

Aren't all the old BSTJ's online?

I've never looked for them. My telephone work was done as part of radio & TV station support. Those old 1A2 systems were pretty much RF proof, unlike a lot of electronic key systems.

Here ya go . . .

Funny that the approach pops up right now here in SED. We're right now in progress to use such a multitone oscillator to read out an array of radiation detectors, a detector per frequency. Kind of a multi-tone version of the HP200A.

Regards, Mikko

On 2013-01-22 13:42, Mr Stonebeach wrote:

This is fun. I threw together a simulation model based on John's hint, and it looks like it works.

I got intrigued by multi-tone oscillators because of a trick mentioned in the context of high-stability quartz oscillators, where Mike Monett decribed how the difference between the 3rd harmonic and the 3rd overtone could be used to stabilize the frequency to a few 1e-9. I have yet to apply it though...

Anyway below is my LTspice dual-tone oscillator model. Comments invited!

Jeroen Belleman

============================================ Version 4 SHEET 1 880 680 WIRE -272 -224 -336 -224 WIRE -128 -224 -208 -224 WIRE -48 -224 -128 -224 WIRE 48 -224 16 -224 WIRE -336 -144 -336 -224 WIRE -272 -144 -336 -144 WIRE -128 -144 -128 -224 WIRE -128 -144 -208 -144 WIRE -48 -144 -128 -144 WIRE 48 -144 48 -224 WIRE 48 -144 16 -144 WIRE -336 -96 -336 -144 WIRE 192 -80 192 -144 WIRE -336 -48 -336 -96 WIRE -272 -48 -336 -48 WIRE -128 -48 -128 -144 WIRE -128 -48 -208 -48 WIRE -48 -48 -128 -48 WIRE 48 -48 48 -144 WIRE 48 -48 16 -48 WIRE 48 0 48 -48 WIRE 80 0 48 0 WIRE -336 32 -336 -48 WIRE -288 32 -336 32 WIRE -128 32 -128 -48 WIRE -128 32 -208 32 WIRE -64 32 -128 32 WIRE 48 32 48 0 WIRE 48 32 16 32 WIRE 192 80 192 0 WIRE -288 128 -400 128 WIRE -64 128 -208 128 WIRE 64 128 16 128 WIRE 128 128 64 128 WIRE 64 208 64 128 WIRE 192 208 192 176 WIRE 192 208 128 208 WIRE 240 208 192 208 WIRE 272 208 240 208 WIRE 384 208 336 208 WIRE 192 240 192 208 WIRE 192 352 192 320 WIRE 192 480 192 432 FLAG -400 128 0 FLAG 384 208 0 FLAG 192 480 0 FLAG 192 -144 0 FLAG 240 208 e FLAG 80 0 0 FLAG -336 -96 tank SYMBOL res 176 224 R0 SYMATTR InstName R1 SYMATTR Value 1k SYMBOL npn 128 80 R0 SYMATTR InstName Q1 SYMATTR Value 2N3904 SYMBOL cap 336 192 R90 WINDOW 0 0 32 VBottom 2 WINDOW 3 32 32 VTop 2 SYMATTR InstName C1 SYMATTR Value 100p SYMBOL cap 128 192 R90 WINDOW 0 0 32 VBottom 2 WINDOW 3 32 32 VTop 2 SYMATTR InstName C2 SYMATTR Value 100p SYMBOL ind2 -80 144 R270 WINDOW 0 32 56 VTop 2 WINDOW 3 5 56 VBottom 2 SYMATTR InstName L1 SYMATTR Value 100u SYMATTR Type ind SYMBOL voltage 192 16 R180 WINDOW 0 24 96 Left 2 WINDOW 3 24 16 Left 2 SYMATTR InstName V1 SYMATTR Value 6 SYMBOL voltage 192 448 R180 WINDOW 0 24 96 Left 2 WINDOW 3 24 16 Left 2 SYMATTR InstName V2 SYMATTR Value 6 SYMBOL ind2 32 16 R90 WINDOW 0 5 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName L2 SYMATTR Value 25u SYMATTR Type ind SYMBOL cap 16 -64 R90 WINDOW 0 0 32 VBottom 2 WINDOW 3 32 32 VTop 2 SYMATTR InstName C3 SYMATTR Value 100p SYMBOL ind2 -304 144 R270 WINDOW 0 32 56 VTop 2 WINDOW 3 5 56 VBottom 2 SYMATTR InstName L3 SYMATTR Value 100u SYMATTR Type ind SYMBOL ind2 -192 16 R90 WINDOW 0 5 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName L4 SYMATTR Value 25u SYMATTR Type ind SYMBOL cap -208 -64 R90 WINDOW 0 0 32 VBottom 2 WINDOW 3 32 32 VTop 2 SYMATTR InstName C4 SYMATTR Value 120p SYMBOL diode -208 -160 R90 WINDOW 0 0 32 VBottom 2 WINDOW 3 32 32 VTop 2 SYMATTR InstName D1 SYMBOL diode 16 -160 R90 WINDOW 0 0 32 VBottom 2 WINDOW 3 32 32 VTop 2 SYMATTR InstName D2 SYMBOL diode -48 -208 R270 WINDOW 0 32 32 VTop 2 WINDOW 3 0 32 VBottom 2 SYMATTR InstName D3 SYMBOL diode -272 -208 R270 WINDOW 0 32 32 VTop 2 WINDOW 3 0 32 VBottom 2 SYMATTR InstName D4 TEXT 280 -32 Left 2 !.tran 10u TEXT 280 16 Left 2 !.ic v(e)=1 TEXT -96 208 Left 2 !K12 L1 L2 1 TEXT -312 208 Left 2 !K34 L3 L4 1

Yup. Each tank will try to oscillate from the negative resistance, but the one with the higher Q will hog the drive, and the other won't oscillate. Adding the individual limiters forces things to self-balance.

The FFT of your waveform is a little messy, not two clean lines. Maybe the hard clipping of the diodes is doing some mixing. It's much better if you move the frequencies farther apart, change C4 to 270p maybe. That's more like the Bell DTMF case, where the two frequencies are pretty well separated.

Interesting dynamics.

When Bell did the first DTMF, transistors were expensive and varistors were cheap.

"John Larkin":

The other approach could be one oscillator and two high Q filters to pick up harmonics of interest. Or, excite two resonant tanks by a pulse from keypad pushbutton and buffer signals by transistor. The Q of the tanks should be several dozens; not unreasonable. Just a different solution.

VLV

And if it will work for two tones, would you believe five? Bell had a modem, and now I can't think of the model number even though I designed equipment to use it, something in the 400 series. It used nine tones in parallel, generated by two transistors, eight tones for the 8 bits of data and one for clocking. The resulting system was cheap to transmit, expensive to receive, so was just fine for a data collection application where there are many outlying stations and only one computer polling them all. The number 408 seems to stick in my mind; maybe 408A for the sender and 408B for the receiver. Teletype made (somehwat) high speed paper tape equipment which Bell marketed as Dataspeed. Type 5 Dataspeed used these parallel modems at a speed of 75 char/sec. The transmitters were very popular for collecting data that the outlying stations had punched into paper tape. There was a receiver that could punch tape from the modem output; but many customers ran the modem straight into the computer and avoided reading paper tape at the receiving end.

Ma Bell had Fairchild make an IC to spec for DTMF; I otta know, as i did a lot of testing of the first fab ICs, and helped devise a way to test them in a decent time (that d*mn inductor takes a looong time to charge from zero).

It has been a rather long time (since around 1969 more or less), and do not remember the designation given; i believe it was in the uA700 series.

Digging thru the datasheets i kept from those daze, i found the uA745 which looks like that may be the specific IC for that purpose. ONE: It is a DUAL, and uses 10 transistors total; it is in a 14 lead flat cerpak (which matches my memory of the package) and the datasheet shows two test circuits.

#1 circuit uses 27 ohms in series with 0.75Hy(32 ohm) inductor and that network is paralleled with 340 ohms; this is the load to 12.6V; and used to test gain, Zout, bias current tests and supply current test. #2 circuit uses 68 ohms in series with 2.5Hy (68 ohms) inductor and that network is paralleled with a 660 ohm resistor in series with a 4uF capacitor; this is the load to 6.7V; and used for the THD test. The load was common to the two amplifiers.

Ma Bell had a rather tight series of test limits including THD,which obviously cannot be tested in a reasonable time for production purposes.

We (mostly me) characterized a thousand devices over temperature (5 temps) for all given specifications and scatter-plotted THD against everything else.

There was absolutely ZERO correlation; the BeeHive plots did not even allow limits on any given parameter to guarantee THD, much less in the spec temp range.

The engineer involved did some analysis and made some slight changes in resistor values so that biasing of the one transistor causing poor THD at -40C (worst case) would not have a forward Vcb greater than its Vbe and thus would not be in actual or practical saturation (only in theoretical saturation). In other words, it still acted as an amplifier and not a polarity-reversing rectifier.

Very dicey as Ma Bell demanded the design and values within some range.

Well, the Baby Bird (Goo Gull) presented only a few. In any case it seems impossible to find the ONE (of hundreds) that mentions DTMF much less gives a circuit.

Isn't this what the early touchtone generators did?

Pay attention, Don! That's exactly how this thread started.

I remember learning about phone phreaking when I was in college. I went directly to the library to find that copy and of course, it was gone, most likely taken by a student rather than removed by the school I expect. It had the exact frequencies published although at that point the horse was out of the barn.

Rick

The secret insider phone phreaking document was -- The Bell System Technical Journal!

It was originally much easier. All you needed was a safety pin and a bent coat hanger.

Or the rotor from a magneto >:-} ...Jim Thompson

I thought you iused barbed wire?