[sdiy] Calculating cable capacitance effect
obl64 at ix.netcom.com
Sun Apr 18 04:24:18 CEST 2021
Asymmetric high frequency MHz oscillations can appear as "DC" offsets -
sorta like low pass filtered PWM.
On 4/17/21 8:41 PM, cheater cheater via Synth-diy wrote:
> Really interesting - why does the DC shift happen?
> On Sat, Apr 17, 2021 at 7:35 PM Bernard Arthur Hutchins, Jr via
> Synth-diy <synth-diy at synth-diy.org> wrote:
>> What about patch cord capacitance and those 1k series resistors often used on synthesizer module outputs: PATCH CORD CAPACITANCE may well matter.
>> The 1k series output resistor was not really for protecting the outputting op-amp from damage (very unlikely), nor was the automatic “mixing” (often touted) of much importance. [It did assure that something well-defined happened contrary to an ill-designed chance occurrence of two op-amp outputs being (inadvertently) directly connected! Also, a somewhat similar moderate output impedance (600 ohms) was common in audio signal work. And the 1k’s were obviously never suggested for the lines carrying a main-control-voltage (volts /oct.) .]
>> Two useful things: First, the SDIY experimenter frequently (typically) has his/her ”synthesizer” (finished modules) driving an external breadboarded module under test (MUT). For example, a finished VCO might be modulating a new VCO MUT. Things are not working – no surprise. Then to your horror you see that your good VCO has also now stopped - what have you done!
>> Well, if you had the 1k series resistor your finished VCO would at least be running happily regardless of what is going on in the connected VCO MUT. (Op-amps generally drive anything if isolated by 1k). Without the 1k, a wiring error (perhaps a breadboard short to ground) may well get back into your finished VCO, causing confusion as well as anxiety.
>> The second useful function of the 1k series resistors is that they “decouple” capacitive (typically shielded cables at perhaps C=100 pfd/meter) loads from op-amp outputs thus preventing spurious high-frequency (MHz) oscillations WITH ASSOCIATED DC SHIFTS. Connecting a cable (even just to a scope) directly to an op-amp output forms an RC low-pass (R being the inherent INTERNAL op-amp output resistance of perhaps 100 ohms). Such an oscillation is HF but low level (slew limited) and non-symmetric (non-symmetric slew limiting). The result is a fuzzy looking scope trace (looks out of focus) and is only there when a cable IS attached, and as a VCO control can cause a small but noticeable pitch shift.
>> The oscillation occurs because the R is internal to the op-amp and the RC is INSIDE any feedback loop and contributes excessive phase shift. With the 1k series the RC (R now 1k) is OUTSIDE the op-amp’s feedback loop. Fuzzy trace and pitch shift gone.
>> (previously posted – in large part here, or on MW?)
>> - Bernie
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