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Two Tones, One Transistor

Discussion in 'Electronic Design' started by rickman, Jan 21, 2013.

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  1. rickman

    rickman Guest

    I seem to recall from long, long ago, that the DTMF phones that Ma Bell
    put out used a single transistor to produce both tones. I can't find
    any info on how this circuit would operate. How can you create a
    circuit that will resonate at two frequencies at once? How did Ma Bell
    do it?

    Rick
     
  2. 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
     

  3. 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
     
  4. Robert Baer

    Robert Baer Guest

    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.
     
  5. Robert Baer

    Robert Baer Guest

    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.
     

  6. Isn't this what the early touchtone generators did?

    --
    Many thanks,

    Don Lancaster voice phone: (928)428-4073
    Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552
    rss: http://www.tinaja.com/whtnu.xml email:

    Please visit my GURU's LAIR web site at http://www.tinaja.com
     
  7. rickman

    rickman Guest

    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
     

  8. 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.

    --
    Many thanks,

    Don Lancaster voice phone: (928)428-4073
    Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552
    rss: http://www.tinaja.com/whtnu.xml email:

    Please visit my GURU's LAIR web site at http://www.tinaja.com
     
  9. T

    T Guest

    Ma Bell used LC tanks - multi tapped too! There was one transistor on
    the DTMF pad, acted as an amplifier.
     
  10. rickman

    rickman Guest

    I was wondering if there was a way to tune a single circuit for two
    frequencies. I get that there are two tank circuits on one transistor,
    but what keeps the tank circuits from interacting with each other?
     
  11. rickman

    rickman Guest

    I appreciate the response, but that wasn't exactly what I was asking.

    I was wondering why the two tank circuits don't interfere with the tuned
    frequencies. Are the two circuits totally isolated? If so, how
    exactly? I don't get that from the schematic, although I don't have
    that in front of me at the moment.
     
  12. Guest

    If the tank circuits and the amplifier are
    linear, the superposition principle keeps
    the tanks from knowing about each other.
    Because stable oscillation requires a
    nonlinearity, to keep the amplitude
    finite, the trick (as John pointed out)
    is to have a *separate* nonlinearity for
    each tank.

    We`re planning to use bolometers
    as the nonlinearities. We also expect
    to open a can of worms there, for instance
    the harmonics created by the nonlinear
    element *will* be seen by other tanks. So,
    one has to arrange the frequencies
    suitably.

    Regards, Mikko
     
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