Stephen Woodward, a prolific circuit designer with far more than 100 printed Design Concepts (DIs), had his “80 MHz VFC with prescaler and preaccumulator” [1] printed on October 17, 2024, as a DI on the EDN web site.
Upon studying his article, I used to be desperate to simulate it and attempt to push its operation as much as 100 MHz, if attainable, whereas sustaining its primary simplicity and accuracy. Nonetheless, Stephen Woodward bought there earlier than I did [2]! For the report, I had virtually completed my design earlier than I noticed his newest one on the EDN web site.
I gained’t talk about the small print of the circuit operation as a result of they’re so just like these of the above-referenced DIs. Nonetheless, there are added options, and the performance has been examined by simulation.
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Options
My voltage-to-frequency converter (VFC) circuit (Determine 1) has a excessive impedance enter stage, it could function reliably past 100 MHz, it may be operated with a single 5.25-V provide (or a single 5-V provide with a couple of added elements), and it has been efficiently simulated. Additionally, changes are supplied for calibration.
Determine 1 VFC design that operates from 100 kHz to past 100 MHz with a single 5.25-V provide, offering sq. wave outputs at 1/2 and 1/4 the primary oscillator frequency.
This circuit supplies sq. wave outputs at one-half and one-fourth the primary oscillator frequency. These alerts will, in lots of instances, be extra helpful than the very slim oscillator sign, which will likely be within the 2 ns to five ns vary.
The NE555 (U8) supplies a 500 kHz sign, which drives each a unfavorable voltage generator for a -2.5-V reference and a voltage doubler used to generate a 5.25-V regulated provide that’s used when a single 5-V provide is desired. TLA431As are used as programmable Zener diodes, NOT TL431As. In contrast to the TL431A, the TLA431A is steady for all values of capacitance related from the cathode to the anode.
Two changes are supplied: Each a optimistic and a unfavorable offset adjustment are supplied by R11, and R9 adjusts the acquire of the current-to-voltage converter, U2. I recommend utilizing R11 to set the 100-kHz sign with 5 mV utilized to the enter and utilizing R9 to set the 100-MHz sign with a 5-V enter. Repeat this process as required to maximise the accuracy of the circuit.
Attainable limitations
This circuit might not give extremely correct operation under 100 kHz due to diode and transistor leakage currents, however I count on it to function on the decrease frequencies not less than in addition to Woodward’s circuits. Operation right down to 1 Hz or 10 Hz is, for my part, principally for bragging rights, and I’m not involved about that.
I count on this VFC to be helpful principally within the 100 kHz to 100 MHz frequency vary: a 1 to 1000 span. Minute diode/transistor leakage currents within the nanoamp vary and PCB floor leakage might trigger linearity inaccuracies on the decrease frequencies. The capacitor charging present supplied by transistor Q1 is within the a number of microamps vary at 100 kHz; under that, it’s within the nanoamp vary. Having had some expertise with environmental testing, I believe it could be tough to construct this circuit in order that it could present correct operation under 100 kHz in an atmosphere of humidity/temperature of 75%/50oC.
Some particulars
When simulated with LTspice, the Take Again Half circuit [3] with 1N4148 diodes didn’t present acceptable outcomes above about 3.5 MHz when pushed by a sq. wave sign with 2-ns rise/fall occasions, so I used Schottky barrier diodes as a substitute, which labored nicely past 25 MHz, the utmost frequency seen by the Take Again Half circuit [1,3]. The Schottky diodes have considerably larger leakage present than the 1N4148s, however the 1N4148 diodes would require the very best frequency sign to be divided down to three.5 MHz to function nicely on this software.
I used two 74LVC1G14s to drive C4, the ramp capacitor, as a result of I used to be not satisfied one in every of them was rated to constantly drive the height or rms present required to reset the capacitor when working at or close to 100 MHz. And utilizing a 25-pF capacitor as a substitute of simply utilizing parasitic and stray capacitance permits higher operation at low frequencies as a result of leakage currents are a smaller proportion of the capacitor charging present. (Clearly, extra ramp capacitance requires extra charging present.)
The op-amp
If you wish to use a distinct op amp, verify the specs to make certain the required provide present shouldn’t be larger than 3 mA worst case. Additionally, it should accommodate the mandatory 7.75 V with some margin. Critically, the so-called rail-to-rail output should swing to inside 100 mV of the optimistic rail with a 1.3-mA load on the most working temperature.
Be suggested
Have a look at famend Jim Williams’ second model of his 1 Hz to 100 MHz VFC for extra details about the trouble required to make his circuit function nicely over the total frequency vary [4][5]. See reference 5 and have a look at the notes in Determine 1 and Desk 1.
Jim McLucas retired from Hewlett-Packard Firm after 30 years working in manufacturing engineering and on design and check of analog and digital circuits.
References/Associated Content material
- 80 MHz VFC with prescaler and preaccumulator
- 100-MHz VFC with TBH present pump
- Take-Again-Half precision diode cost pump
- Designs for Excessive Efficiency Voltage-to-Frequency Converters
- 1-Hz to 100-MHz VFC options 160-dB dynamic vary
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