Wide Range Current Pumps for Precision PLL

Wide Range Current Pumps for Precision PLL

The wide-range current pump for the precision phase locked loop circuit is a “semi-precision” circuit, and provides an output current proportional to -V1, give or take 10 or 15%, over a 3-decade range. The 22 MW resistors prevent the current from shutting off in case -V becomes positive (probably unnecessary if A4 is used). For best results over a full 3-decade range (11 kHz to 9 Hz), do use A4, delete the four 22 MW resistors, and insert the (diode parallel to the 470 kW) in series with the RG as shown. They will give excellent stability at all frequencies (although balance cannot extend below 1/1500 of full scale without extra efforts).  This is a picture schematic:

Wide Range Current Pumps for Precision PLL circuit

A5, A6, ARE LF351 OR 1/2LF353 OR SIMILAR

Q5, Q6, Q7, Q9 ARE 2N2907 OR 2N3906 OR SIMILAR

Q8, Q10 ARE Q8 IS 2N3565 OR 2N3904 OR SIMILAR

 

This PLL like circuit has been widely used in testing of VFCs, as it can force the LM331 to run at a crystal-controlled frequency (established as the F input), and the output voltage at VOUT is promptly measured by a 6-digit (1 ppm nonlinearity, max) digital voltmeter. With much higher speed and precision than can be obtained by forcing a voltage and trying to read a frequency. While at 10 kHz, the advantages are clearcut; at 50 Hz it is even more apparent. Measuring a 50 Hz signal with ±0.01 Hz resolution cannot be done (even with the most powerful computing counter-timer) as accurately, quickly, and conveniently as the PLL’s voltage output settles.

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