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

[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>
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> 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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