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107 lines
4.1 KiB
Plaintext
107 lines
4.1 KiB
Plaintext
Arbitrary-precision frequency counter
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=====================================
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Theory of operation
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-------------------
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The arbitrary-precision frequency counter counts clock cycles of a
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frequency source that is assumed to be free from drift. It compares the
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count with the host's system clock. If the system clock is synchronized
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with an accurate NTP reference, measurements with arbitrarily high
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accuracy can be obtained.
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In practice, this is limited by the the frequency source's drift and
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the time one is willing to wait. If NTP maintains the system time
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with an accuracy of +/- 100 ms, obtaining measurements with an
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accuracy of +/- 1 ppm would take about 28 hours.
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Additional error sources, such as the round-trip time when requesting
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a sample from the microcontroller, are also considered in the accuracy
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calculation.
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The counter consists of a board based on a C8051F320 microcontroller
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and the control software on the host. The microcontroller counts
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events in a free-running 16 bit counter that is regularly read and
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extended to 32 bits. The 32 bit counter is periodically queried by
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the host.
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The microcontroller's counter can count at a frequency of up to 3 MHz.
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(SYSCLK/4)
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In order to protect against transmission errors not detected by USB's
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CRC, which are occur relatively often, each packet is protected by a
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CRC-32 and an inverted copy of the payload. Corrupted packets are
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rejected by the host.
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The 32 bit counter wraps around at most once very 21.8 ms. The 32 bit
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counter wraps around at most every 1431 s. The host extends the 32 bit
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counter to 64 bits, and calculates frequency and accuracy from the
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count and the run time of the measurement application.
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Performing a measurement
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------------------------
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To perform a measurement, connect the CNTR board's probe input to the
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clock source and then run the "cntr" application on the host. An
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accuracy goal (in ppm) can be specified on the command line (see
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below).
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The host polls the microcontroller every 100 ms and displays the run
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time (in seconds), the measured frequency, and the accuracy achieved
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so far.
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Measurements can be stopped by pressing ^C or by specifying an
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accuracy goal. At the end, the total number of events counted and
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communication statistics are displayed.
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Updating the firmware
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---------------------
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The protocol revision and the build date of the firmware of the CNTR
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board can be queried with "cntr -i".
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To update the firmware, run
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cntr -r; sleep 1 && dfu-util -d 0x20b7:0xcb72 -D cntr.bin
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Known issues (version 2 hardware)
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---------------------------------
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- the input circuit only works up to about 1 MHz. The problem is that
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we discharge too slowly though the base of Q1, which in turn keeps
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the transistor turned on too long.
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An alternative design that sets R2 to zero, adds a 47 Ohm termination
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resistor in parallel to VR4, and places a 1 kOhm resistor between
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VR4 and Q1 works up to about 2 MHz, but accepts a lot of HF noise
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and is very sensitive to the signal amplitude.
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Some test results with a ~1.8 m RG-174 cable, square wave bursts
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with a 50% duty cycle and ~ 5 ns raise/fall time:
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Design Frequency Source amplitude Input amplitude
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(nominal) (nominal) (measured)
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------------ ---------- ---------------- ---------------
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Original 1 MHz 700 mV ~700 mV
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Alternative 2 MHz 1.6 V ~800 mV
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1 MHz 1.5 V ~750 mV
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The test consisted of setting the frequency and adjusting the nominal
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source voltage in increments of 100 mV for the lowest voltage at
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which which ten consecutive bursts of 50000 cycles each were all
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received correctly.
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The source has an output impedance of 50 Ohm, so voltage at the probe
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input (indicated in the table) is roughly half the nominal source
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voltage in the alternative design.
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- the MMCX connector is hard to solder because of its large thermal
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capacitance and surface
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- the lateral pads of the MMCX connector could be wider
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- manufacturing tolerances can push the USB connector a bit inside
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the board, making its signal pins overshoot the pads
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- the ground fill under the LED causes a secondary cathode contact
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(which is harmless on this case)
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