Starbucks of Berlin: East Side Mall

Today I'm heading to the Starbucks in East Side Mall. To get there, I biked through Görlitzer Park. Drug selling was at minimal, but I did see a pink haired lady burning some paper on the ground in the style of crack glass preparation. In pensive retrospect, I think she was trying to incense the nearby drug dealers into some sort of submission. Approaching Skalitzer Strasse, I yielded to a car of cool dudes who nodded back to me. The elevated train tracks, a colossus of iron, guided me to the Oberbaumbrücke, a bridge completed completely out of bricks, with its gingerbread spires.

On the other side of the bridge, a breezy industrial strip lines the river. I biked over to Tamara Danz Strasse, and followed it to the back entrance of East Side Mall. I shot this picture of the mall, where it looks like a vented plastic shoe, next to a glass leg that is the new tower thereby. My encampment for the afternoon is the Starbucks below urban outfitters and next to a souvenir store.

As usual I ordered a filter coffee in a huge mug, and sat outside amidst the potted bushes under the black branded Starbucks canopies. A man in short womens clothing walked by, yelling about immigrants in German to a conservatively dressed woman. I would now like to settle down to the research task at hand.
 

Today I would like to read up about the Butterworth filter. Before we listened to juicy filter sweeps in acid techno, this was always more important, to signal processing and the military. Its not about juice, but more about brick. That is, a brick wall. It smoothly cuts away frequencies like butter, without adding extra drip. As I started computer music programming, in Csound, the so-called butterlp was always my favorite, and it never dripped. It seems to me that ongoing electronic experiments neglect it too much, in favor of the "easy" drippy biquad. 

To read up on the Butterworth filter, I sipped my huge mug of filter coffee and oat milk, and opened up some datasheets. The lm324 datasheet, by Texas Instruments, as usual has a trove of information. You can see the Butterworth, entitled "DC Coupled Low-Pass RC Active Filter," on page 16. The typical formation is a resistor, with a wide feedback capacitor that looks like regeneration, then another resistor and a capacitor to ground, followed by a simple follower, usually with unity gain. 

It's a cool day at Starbucks by the river. Nevertheless, I am swatting wasps away from my huge mug. The next step is to investigate how the Butterworth acts in a transconductance opamp, so the usual reference is the datsheet for lm13700, page 12. The challenge of this filter topology is that it consists not only of integrators purely, but a hybrid configuration of capacitors. To realize the "Voltage Controlled 2-Pole Butterworth Lo-Pass Filter," these designers put capacitors on the current outputs, before the darlington followers.

That is going to be a challenge for adapting the Butterworth to a Norton mixed circuit. I would define this circuit as a configuration of differential PNP transistors, feeding a Norton opamp with a combination of resistors and capacitors coming in from the side. It's quite easy to adapt a biquad filter to this topology, in fact it isn't even mixed, consisting of purely transistor sources, but here I have to be creative. One could say Butterworth is a challenge if it's not simply rc and opamps. The hip-looking cross-dresser is acting quite aggressive to what he perceives as immigrants. 

I feel rushed by his escalated cussing, as I sit outside in the bushes of Starbucks. So I take down a quick sketch of how this mixed Norton Butterworth might play out with voltage control. I drew it with integrators, nested, and because it is modelling resistors, each stage needs negative following feedback to the transconductors. Now, because of the huge mug of coffee in me, I will be riding the escalator to the third floor food court, to pay fifty cents to visit the bathroom, which is always dark to save lighting expenses.

As I peruse the food court, without intention to buy anything, I try to memorize the names of each slot in the panopticon there. The result is like a poem, for eaters sitting lonely under upturned plants staring at a column:

• fresh rolls
• chutney express
• abudan grill
• asia river
• slice
• bigburger
• sigmadog
• gochu gang 

 

 

Now I am sitting inside the Starbucks, because it is ever so lightly raining outside. I had asked the colleague if she could save my coffee and now I am resuming the session at a warm oak table with my huge mug of cooled coffee. I realize I had not summarized the two datasheets I reviewed earlier, so on this warm oak rondette, I draw with a coarse pencil, the simple rc buffered filter, and the voltage controlled, transconductance version.

 

Note that there is a loop but it is hardly in the shape of a classic integrator. It is the regeneration that makes the brick wall possible without ringing. We will see how I take my Norton sketch and try to adapt it without relying on the simple integrator formation. Maybe I will try capacitance to ground, or between shunted between version and inversion, when I sit down at a bread board and oscilloscope, at home. As such, I won't be showing the result since this series is only about what happens to and at the Starbucks. Listen up for buttery passing in future instruments!

Next on Starbucks of Berlin: Thinking about cathode coatings at the church of remembrance.