Categories: Electronics

80 MHz VFC with prescaler and preaccumulator

In 1986, famed analog innovator Jim Williams, in “Designs for Excessive Efficiency Voltage-to-Frequency Converters^”printed his 100 MHz “King Kong” VFC. If anybody’s ever achieved a sooner VFC, I haven’t seen it. Nonetheless, Determine 1 shamelessly borrows a number of of Kong’s velocity secrets and techniques and melds them with another easy methods to attain 80% of the superior speed-of-Kong. I name it “Child Kong.”

Determine 1 “Child Kong” VFC with take-back-half (TBH) pump and ACMOS prescaler can run at 80 MHz.

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What lets the Child work at a Kong-ish max output frequency with significantly much less complexity (about half the components rely) than the King’s? It’s partly the self-compensating TBH diode cost pump described in an earlier Design Thought: “Take-back-half precision diode cost pump”. It additionally will get assist from AC logic household power-thrifty velocity that was model new and simply turning into accessible in 1986. Jim used logic know-how that was extra mature then, primarily MECL.

The (considerably tachycardia-ish!) coronary heart of Determine 1’s circuit is the tremendous easy Q1, U1a, D5 ramp-reset oscillator. Q1’s collector present discharges the few picofarads of stray capacitance offered by its personal collector, Schmidt set off U1’s enter, D5 and (as little as attainable, please) of interconnections. U1’s single-digit-nanoseconds propagation occasions permits oscillation frequency to run from a useless cease (assured by leakage-killing R4) to past 80 MHz, (however not reliably as excessive as 100). So, the Velocity King’s crown stays safe.

Every cycle, when Q1 ramps U1pin1 all the way down to its set off stage, U1 responds with a ~5 ns ramp reset suggestions pulse via Schottky D5. This pulls pin 1 again above the constructive set off stage and begins the following oscillation cycle. As a result of the ramp-down price is (kind of) proportional to Q1’s present, which is (sort of) proportional to A1’s output, oscillation frequency is (vaguely) likewise. The emphasis is on vaguely.

It’s suggestions via the TBH pump, summation with the R1 enter at integrator A1’s noninverting enter, output to Q1 and thence to U1pin 1 that converts “vaguely” to “precisely”. So, what’s U3 doing?

The TBH pump’s self-compensation permits it to precisely dispense cost at 20 MHz, however 80 MHz could be asking an excessive amount of. U3’s two-bit ripple-counter issue of 4 prescaling fixes this drawback.

U3 additionally offers a chance (be aware jumper J1) to substitute a top quality 5.000v reference for the questionable accuracy of the 5v logic rail. Determine 2 offers circuitry to try this, with a 250-kHz diode cost pump boosting the rail to about 8v to be then regulated all the way down to a precision 5.000. Max U3 present draw, together with pump drive, is about 18 mA at 80 MHz, which fortunately the LT1027 reference is rated to deal with. Simply.

Determine 2 Rail booster and 5.000 volt precision voltage reference.

The 16x preaccumulator U2 permits use of microcontroller onboard counter-timer peripherals as sluggish as 5 MHz to accumulate a full decision 80 MHz VFC output. It’s described in an earlier DI: “Preaccumulator handles VFC outputs which are too quick for a unadorned CTP to swallow”. Please confer with that for a full clarification.

Stephen Woodward’s relationship with EDN’s DI column goes again fairly a great distance. Over 100 submissions have been accepted since his first contribution again in 1974.

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