SN76477 Question

Mike Walters · Joined: Mar 24, 2005 Posts: 29 Location: Chapel Hill NC

I'm building a circuit based on the SN76477 chip, and I have a question derived from the "Sound Demonstrator Box" circuit that comes with the data sheet. I notice that many functions depend on two pins: with one a capacitor to ground, and the other a resistor to ground. For example, the LFO (or SLF it's called) is determined by pins 20 and 21. Pin 20 goes to a resistor, and pin 21 goes to a capacitor. I see there is an on/off switch on pin 20, but not pin 21. Can this switch be interchanged? If I don't have a switch on Pin 20, can I have the switch on pin 21 instead to disable the function?

Thanks!

v-un-v · Posted: Thu Apr 12, 2007 12:03 pm Post subject:

Mike!

Long time no see Very Happy

PM Scott Stites- he's a man who knows a thing or two about the SN76477 Wink

Scott Stites · Posted: Thu Apr 12, 2007 2:45 pm Post subject:

Can't recall if I've ever tried it, but I suppose one could put a SPST between pin 21 and the cap - I'd think without the cap connected, you wouldn't get any SLF oscillation.

Maybe if Thomas H happens across this, he'll have better info(?).

Why is it you don't want the switch on pin 20?

Mike Walters · Joined: Mar 24, 2005 Posts: 29 Location: Chapel Hill NC

Thanks all for the replies..

Well, the reason is I already have the capacitor sections wired to logic switches.. I could rearrange them the other way, but won't if that will work!!

I'll keep you all updated on the new project! I'm sure I'll have more SN questions soon Very Happy

Take Care,

State Machine · Posted: Fri Apr 13, 2007 7:01 pm Post subject:

Scott Stites · Posted: Sat Apr 14, 2007 7:53 am Post subject:

That thought did cross my mind, but I figured the connection would require one pad for the pin and a wire to the switch (there would be no need to connect the pad to anything else). Most of the capacitance would come from the wire, but it would have to be relatively long in this case to represent enough of a capacitance to worry about.

In any event, I would be rather use pin 20 myself.

Cheers,
Scott

Scott Stites · Posted: Sat Apr 14, 2007 10:00 am Post subject:

I tried it out with a simple application (the siren/raygun thing from the datasheet). I was using a skinny 76477 on a little carrier board, which would add even more capacitance, though I don't think it would matter much if it was on a PCB and soldered directly in- Yeah, stray capacitance is going to play a part.

In fact, it doesn't require a large value resistor on pin 20 to do it.

I buffered the signal at pin 1 so that I could observe it on a scope - with 2.1 M of resistance at pin 1, and no capacitor, just the wire going to the op amp buffer, the signal was 12.5 kHz, triangular.

If I changed the resistance at pin 20 to 100K, the signal was (get this) a whopping 71.4 kHz (far above the 20 kHz in the data sheet). This signal was a pulse wave with a period of 14 us; the duty cycle was largely on with a short off spike.

I was using a 100K pot at this point, so I dialed it all the way down to ground, and the signal at pin 21 increased in frequency and distorted in shape until it became mostly noise.

Long and short of it is, if there isn't infinite impedance at pin 20 (IE, no connected resistor), any minute bit of capacitance on pin 21 is going to have an effect. It's definitely not, in any case, going to cause the same effect as disconnecting pin 20. If you disconnect pin 20 from the resistor, the VCO pitch will stay the same. If you disconnect pin 21 from the capacitor, the pitch is going to rise.

Now, having said all that - I found the effect of it to be actually rather cool, and makes a great noise/bleep box application. An LFO at the above rates (well, it ain't an LFO any more) modulating the carrier turns adjusting the VCO rate into a series of steps rather than a smooth glide. Sounds pretty cool. The really neat stuff, if one made sure to take ESD precautions, would be connecting pin 21 to a touch plate through a switch.

Switching the connection to a touch place initially will raise the frequency of the signal output. Barely touching the touch plate will lower the frequency. and putting more pressure (IE, applying more surface area of the finger/unnamed body part) will cause a more dramatic bend in frequency. It can make the VCO quite expressive.

I had to wonder if that oscillation was taxing the 76477 - I measured the current draw of the circuit with the capacitor in place at 10 mA. Disconnecting the cap and throwing the LFO into high freq oscillation didn't move the digit on the DMM one bit (though the lowest reading it can take is 10 mA).

Cheers,
Scott

State Machine · Posted: Sat Apr 14, 2007 2:54 pm Post subject:

Well Scott your empirical evidence resulting from this experiment, and a wonderful experiment I think, proves my theory about the high frequency oscillation.

The good that came of all this is you stumbled on a really cool application by the possibility of using touch plates as variable capacitance control for frequency modulating the VCO ! Very Happy

A word about parasitics, your correct when saying that the wires and carriers and such add parasitic capacitance but also the device itself will have a finite amount of input capacitance from the bond out wires inside the chip and the semiconductor device itself. So you have:

C_parasitic_total= C_ext_wires + C_carrier + C_int_bond wires + C_chip_semiconductorr

Thus, for a known resistance and measured SLF Fout, we can very well calculate what this above capacitance by algebraically rearranging the simple equation and solving for C.

C_total = [C_ext_wires + C_carrier + C_int_bond wires + C_chip_semiconductor] = [0.64/(F_out * R_slf)]

I was surprised when you said you are getting about 70 KHz from the LFO (SLF) oscillator! Hmmm, another exponential converter and you have two oscillators on hand for two part harmony .... Wink

I suppose one can draw there own conclusions from all of this and expand even further using this IC; while they are still around ....... Wink

Chat later,
Bill

blue hell · Posted: Sat Apr 14, 2007 3:06 pm Post subject:

Scott Stites · Posted: Sat Apr 14, 2007 3:24 pm Post subject:

Knowing the top end of the SN76477 LFO was spec'ed at 20 kHz made me ignore the internal parasitic capacitance - generally it's a pretty low value, and would be beyond the range of 20 kHz in conjunction with even a quite large resistor.

I can only imagine the reason why it's speced at 20 kHz is that it most likely maintains a triangular waveshape up to 20 kHz. It obviously doesn't mean it still won't oscillate at higher frequencies than that.

I think the fact that it maintained a triangular waveshape below 20 kHz does prove that the extra capacitance of the breadboard, wire, etc did have more of an effect than I predicted, though.

I should probably not answer questions - I'm never sure of anything unless I breadboard.

State Machine · Posted: Sat Apr 14, 2007 3:43 pm Post subject:

State Machine · Posted: Sat Apr 14, 2007 4:02 pm Post subject:

State Machine · Posted: Sat Apr 14, 2007 4:08 pm Post subject:

ickystay · Joined: Nov 15, 2006 Posts: 142 Location: Oregon

In Thomas Henry's reprints, he mentions tapping a triangle wave from the lfo off pin 21. It shows up buffered and "beefed-up" in his Super Controller circuit.

CJ Miller · Joined: Jan 07, 2007 Posts: 368 Location: 127.0.0.1

State Machine · Posted: Sun Apr 15, 2007 7:25 am Post subject:

Mike Walters · Joined: Mar 24, 2005 Posts: 29 Location: Chapel Hill NC

Cool.. thanks for all the great info! I think I'll map the logic switches to the resistor pins instead Smile

State Machine · Posted: Sun Apr 15, 2007 3:57 pm Post subject:

Mike Walters · Joined: Mar 24, 2005 Posts: 29 Location: Chapel Hill NC

No, I didn't know about a new Raymond Scott compilation! That's great news.

A couple of years ago, my wife's grandparents gave me a bunch of 78's, one including an old Raymond Scott big band record. I finally recorded it and put it on a CD for them, and gave it to them last weekend. They really enjoyed it after not hearing it for 60 or 70 years!