Categories: Electronics

100-MHz VFC with TBH present pump

Well-known analog designer and writer Jim Williams printed an superior design in 1986 for a 100-MHz voltage to frequency converter. He named this high-climber (image it on the roof of the Empire State constructing swatting biplanes out of the air) King Kong! He adopted Kong in 2005 with a considerably up to date successor, “1-Hz to 100-MHz VFC options 160-dB dynamic vary.”

I used to be fascinated by each of those spectacular designs as a result of they had been means quicker than some other VFC I’d ever seen! One other 20 years handed earlier than I made a decision to attempt for a 9-digit VFC of my very own.

Wow the engineering world together with your distinctive design: Design Concepts Submission Information

Right here’s the consequence (Determine 1).

Determine 1 This easy VFC borrows a few of Williams’ pioneering pace concepts and combines them with a couple of methods of my very own to succeed in the excessive altitude of a 100-MHz full scale frequency.

The Q1, D1, and Schmitt set off U1 make a sloppy however tight and speedy VFC which is then accurized by the suggestions loop comprising prescaler U3, take-back-half (TBH) cost pump D1-D4, and integrator A1. The preaccumulator U2 interfaces the 100 MHz depend price to reasonable pace (~6.25 MHz) counter timer peripherals with out dropping decision.

The core of Determine 1’s circuit is a quite simple Q1, U1, D5 ramp-reset oscillator. Q1’s collector present discharges the few picofarads of stray capacitance supplied by its personal collector, Schmitt set off U1’s enter, D5, and their interconnections (as brief and direct as potential, please!). U1’s sub-five-nanosecond propagation delay permits the oscillation to run from a useless cease (potential as a consequence of leakage draining R4) to past 100 MHz.

Throughout every cycle, when Q1 ramps U1 pin1 right down to its set off degree, U1 responds with a ~5 ns ramp reset suggestions pulse by Schottky D5. This pulls pin 1 again above the constructive set off degree and begins the subsequent oscillation cycle. As a result of the ramp-down price is (kind of) proportional to Q1’s base present, which is roughly proportional to integrator A1’s output voltage, oscillation frequency is likewise. The caveat is “roughly”.

The suggestions by the TBH pump, summation with the R1 enter at integrator A1’s noninverting enter, the output to Q1 and thence to U1 pin 1 converts “roughly” to “precisely”. One merchandise that lets this VFC work in Kong’s frequency area however with a significantly easier components depend is the self-compensating TBH diode cost pump described in an earlier design thought (DI): “Take-back-half precision diode cost pump.”

So, what’s U3 doing?

The TBH pump’s self-compensation permits it to precisely dispense cost at 25 MHz or so however 100 MHz would positively be asking an excessive amount of. U3’s two-bit prescaler addresses this downside. U3 additionally offers a possibility (be aware jumper J1) to substitute a top quality 5.000v reference for the seemingly lesser accuracy of the generic 5v rail.

Determine 2 reveals a 250-kHz diode cost pump boosting the 5v rail to about 8v which is then regulated right down to a precision 5.000 by U4. U3 present demand, together with pump drive, is about 23 mA at 100 MHz; U4 isn’t rated for fairly that heavy a load, so buddy resistor R6 takes up the slack.

Determine 2 A 250-kHz diode cost pump Rail booster bringing rail to 8V which is then regulated right down to a precision 5.000 V reference by U4.

The 16x preaccumulator U2 permits use of reasonable efficiency counter-timer peripherals as gradual as 6.25 MHz to accumulate the full-scale 100-MHz VFC output. That concept is described in an earlier DI: “Preaccumulator handles VFC outputs which can be too quick for a unadorned CTP to swallow.”

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.

Associated Content material

The publish 100-MHz VFC with TBH present pump appeared first on EDN.

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