tirsdag 24. oktober 2017

Juno filter analysis

I intend to use the Jupiter 6 multi mode filter for the XM8. As this is based on the IR3109, just like the Juno 6/60/106, which is better documented, I took a closer look at that filter first.

Analogue renaissance delivers a great clone of the 106 filter. I am not trying to build a drop-in replacement for anything, so exact component values to match the Juno's are not important, but the idea of using LM13700 in place of the IR3109's OTAs seem to be the go-to solution these days.

In fact, for the filter part (ignoring the resonance and output VCA), part values are mostly the same between the clone and the original Juno 6/60 circuit - 560R and 68k resistors all the way. The caps are 270p in the clone and 240 in the Juno 6/60. I have done a thorough comparison between the various IR3109 filter circuits made by Roland, most of them use 560/68k resistors. The caps have changed from 240p in earlier designs to 330p in later. It probably only affects the exact cutoff-to-CV; I'll test this before I decide on the cap value to use.

I wondered what the linear CV response of the filter would be, so I simulated the circuit in LT-spice. NB: This is the first time I do anything like this, so my findings may not be correct. However, it seems that the current is the case:

For every 2.5kHz increase, the OTA control current increases by 75uA. This means that the LINEAR control current to frequency ratio is (100 * 10^6) / 3 Hz per A. It does not seem perfectly linear however, it drops off somewhat as we go up the scale (an increase from 75 to 150uA increases the frequency by 2,54kHz while an increase from 300 to 375uA increases the frequency by 2.31kHz.

Of course, to properly control the filter, an exponential converter or a DAC output with a high enough resolution is needed. A cutoff of 10Hz requires 0.3uA while 10kHz requires 300uA, a factor of 1000.

Similarly to the DCO frequency calculations, the feasibility of this has to be tested properly, and we may get issues with opamp inaccuracies at the lower end of the scale, though they may not be as severe as with the DCO because the error is not accumulated over time.

PS: The Juno 106 has non-polar caps in line with the signal both before and after the filter. These act as a highpass filter with cutoff around 10Hz.

Filter response with cutoff around 2.5kHz, with HPF included

Update: Replacing the 270p caps with 330p caps changes the control current-to-cutoff frequency from 2.5kHz/75uA to 2.1kHz/75uA, with the same slight dropoff as the frequency increases.

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