[sdiy] Working with the ASM1 VCO.. weird MAT-02 behaviour...

[René Schmitz]

Because you steal from the high end what you gain in the low end.

Have a look at the VCO3 at my home page for an example of a circuit that 
runs well below the LFO range even (1 cycle per 15 Minutes!) and still 
uses regularly sized cap, and has a decent high range as well.

Am 17.01.2020 um 15:43 schrieb Ben Bradley:
> That's what I was thinking, why not increase the timing capacitor by 
> 100 or 1000 times.
>
> On Fri, Jan 17, 2020, 8:07 AM Spiros Makris <spirosmakris92 at gmail.com 
> <mailto:spirosmakris92 at gmail.com>> wrote:
>
>     It is true that a low charge current will be less ideal because
>     then leakage currents etc become significant. They usually cause a
>     rise to the minimum frequency the oscillator will still work at.
>     One can lower the frequency of any similar oscillator by
>     increasing the timing capacitor appropriately and that should not
>     impact the tuning range etc (referring to caps with similar specs
>     and different nominal capacitance).
>
>     Spiros
>
>
>     On Fri, 17 Jan 2020, 14:49 Magnus Danielson, <magnus at rubidium.se
>     <mailto:magnus at rubidium.se>> wrote:
>
>         Hi,
>
>         Recall that as you go for very low frequencies, you get very
>         small currents, as you have a Volt to exponential Current
>         converter followed by a Current Controlled Oscillator. For the
>         low frequencies to work, it needs to be very clean for
>         instance. It's not scaled to be a LFO, you could do that, but
>         care must be taken in such an effort.
>
>         I should dig up my ASM-1 board and measure this as I once
>         intended to do. I built the ASM-1, measured it with a scope to
>         span a large range. I then got a rubidium clock, a counter and
>         a couple of DMMs and intended to measure this more carefully,
>         learned more and now have a excessive time and frequency lab,
>         but still have not returned to the original project of measure
>         the ASM-1 VCO. Also got to repair the original rubidium clock.
>
>         Hmm, maybe I should resurrect the ASM-1 homepage I maintained,
>         just as a reference.
>
>         Cheers,
>         Magnus
>
>         On 2020-01-16 17:48, Jean-Pierre Desrochers wrote:
>>
>>         Hi list,
>>
>>         I'm working on the Elby-Design ASM-1 VCO
>>
>>         https://www.elby-designs.com/webtek/asm-1/vco/vco-asm1-cct.pdf
>>
>>         Im want to use it as a 1v/oct LFO (.05hz to 20hz)
>>
>>         I first mounted the circuit on my protoboard with success.
>>
>>         at first I used a matched pair of 2N3904 for Q1A & Q1B
>>
>>         instead of the MAT02 shown in the schematic.
>>
>>         I could reach very low frequencies (as low as .04hz).
>>
>>         But to do so I needed to replace R9(56k) for a 33k
>>         temporarily. Fine.
>>
>>         This morning I replaced both 2N3904 with a genuine MAT-02
>>
>>         and started the same frequency measurements as yesterday.
>>
>>         I bumped into something weird..
>>
>>         Here are the actual measurements I first did using 2x2N3904:
>>
>>         mVolt at 1k TEMPCO resistor vs Sawtooth period
>>
>>         158.0mV => 0.050sec
>>
>>         176.0mV => 0.100sec
>>
>>         193.8mV => 0.200sec
>>
>>         212.0mV => 0.400sec
>>
>>         229.8mV => 0.800sec
>>
>>         247.5mV => 1.6sec
>>
>>         265.5mV => 3.2sec
>>
>>         283.2mV => 6.4sec
>>
>>         301.7mV => 12.8sec
>>
>>         319.7mV => 25.6sec
>>
>>         (18mV steps, nice exponential response!)
>>
>>         Now here are the actual measurements with a MAT-02:
>>
>>         mVolt at 1k TEMPCO resistor vs Sawtooth period
>>
>>         157.0mV => 0.050sec
>>
>>         175.0mV => 0.100sec
>>
>>         193.0mV => 0.200sec
>>
>>         211.0mV => 0.400sec
>>
>>         229.0mV => 0.800sec
>>
>>         247.0mV => 1.6sec
>>
>>         271.0mV => 3.2sec
>>
>>         **** Now trying to go further increasing the TEMPCO mVolts*
>>
>>         **** the oscillator starts backing up and re-increase its
>>         frequencies !!!*
>>
>>         **** I could not go further lowering the frequency. *
>>
>>         I tried 3 different MAT-02's and even LM394's
>>
>>         and they all behave the same.
>>
>>         Using different matched pairs of 2N3904 worked perfectly..
>>
>>         I suspect  MAT-02’s minimum base to emitter current reached (???)
>>
>>         What am-I missing here ??
>>
>>         By the way, all the IC's I'm using here are genuine one's
>>
>>         (TL082, MAT-02, CA3140, LM311).
>>
>>         JP
>>
>>
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