QCX assembly Rev 1.08
122
Frequency Counter
The frequency counter test pin input is connected directly to the
microcontroller’s 16-bit Timer1 input pin.
There is a 1K series resistor (R55) to connect it also to the Si5351A’s
Clk2 output. This is used during the optional GPS-disciplined 27MHz
reference crystal frequency measurement. In this function, the Si5351A
is configured to route its raw 27MHz output, divided by 4, to its Clk2
output. This passes through the 1K resistor R55, to the microcontroller.
The 1K resistor provides protection to the Si5351A in the event you
accidentally have something connected to the FREQ input pin, and the
Si5351A is operating its Clk2 output at the same time.
Since the 3.3V peak-peak signal from the Si5351A is rather marginal for operating the Timer1
input, a 3.9K pullup resistor increases its centre point. Now “low” is 1.02V and “high” is 3.65V,
which is enough to trigger the Timer1 input reliably.
The frequency counter can operate from 0 to 8MHz theoretically, assuming a clean 50% duty-
cycle squarewave signal of sufficient amplitude. The 8MHz upper bound is a limitation of the AVR
processor which has a synchronous timer input, and can only count to 40% of the system clock
frequency (40% of 20MHz is 8MHz).
The signal to be counted must satisfy the logic thresholds of the ATmega328P, namely a “low” or
“0” is less than 1.5V, and a “high” or “1” is greater than 3.5V.
Signal Generator
The signal generator mode was also discussed in the operating manual section. The signal output
is simply taken from the Clk0 or Clk1 output of the Si5351A. Care should be exercised not to draw
excessive load (e.g. short circuit) from these pins, which could damage the Si5351A. It will be very
difficult to replace a damaged Si5351A.
Self-test of Signal Generator and Frequency Counter
Note that once the Signal Generator has been activated by scrolling to the Signal Generator menu
item, and pressing the left (select) button to enable it
– the Signal Generator remains active on the
set frequency, until you press the right (exit) button and leave the Test Equipment menu.
Press the left button (select) to enable the signal generator; an underline cursor appears under the
digit that will be tuned by the rotary encoder. Adjust the signal generator to produce a frequency
some way under 8MHz. Let’s say, 6MHz. When
you leave edit mode, by pressing the left (select)
button until the cursor disappears off the right side
of the 1’s digit, OR by pressing the right button
(exit), then the cursor will no longer be shown on
the display. Turning the rotary encoder 1 click
anticlockwise shows menu 9.5, the frequency
measurement. Now if you connect a wire from the
FREQ input to the CLK0 output, you will measure
the signal generator’s frequency. This is shown
here in the photograph (right); clearly the 2kHz low
reading indicates I have not calibrated the 27MHz
reference value and/or the 20MHz system clock
value!
Summary of Contents for QCX 5W CW
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