amplitude at that point in the receiver signal path is too low to produce much output from the ADC
unless the received signal is very strong.
9.5 Frequency
4.21523 MHz
This is the frequency counter. The resolution is 10Hz, which results in an update rate of around 8
times per second approximately.
The frequency counter input is connected directly to the Timer1 input of the microcontroller. There
is no pre-amplifier or level adjustment. The ATmega328P processor operates from a 5V supply. It
expects the frequency counter input signals to be of an appropriate amplitude.
According to the ATmega328P datasheet, a “low” has a maximum input voltage of 1.5V and a
“high” has a minimum input voltage of 3.5V. Therefore, the minimum amplitude signal to drive the
frequency counter successfully would be 2V peak-peak centred on a 2.5V DC offset. Exercise
great care to ensure that the voltage levels are correct, to avoid damaging the ATmega328P input
pin.
Additionally, since the ATmega328P has synchronous timer inputs, the highest frequency which
can be counted is limited to about 40% of the system clock (20MHz).
Therefore, this frequency counter has range of 0 to 8MHz. The upper frequency end of this range
will only be achieved in the case of a nice tidy squarewave of adequate amplitude an 50% duty
cycle. Lower quality signals will result in a lower upper frequency limit.
9.6 Signal gen.
25,124,093
The signal generator function is switched off until you press the “Select” button to enable it. Then
the cursor appears under the 10MHz digit. You can then adjust this number just like any other
configuration parameter. Use the rotary encoder to increase or decrease the frequency. Press-and
hold, then turn the rotary encoder, to change the tune rate (move the cursor left or right).
As you adjust the frequency, the output frequency of the synthesiser is tuned to match.
The output frequency range is from 3.5kHz to 200MHz. The Si5351A Synthesiser chip datasheet
maximum frequency specification is 200MHz. However, in practice it has been found to still
operate reliably up to almost 300MHz.
The output waveform is a squarewave with amplitude about 3.5V peak-peak. The output is at the
Clk0 and Clk1 terminals (see diagram, above).
Note that the output is directly connected to the Si5351A Synthesiser chip’s output pin. Take very
great care not to short the output to ground or in any other way abuse this output! If you damage
the Si5351A chip it is very difficult to replace
QCX operating manual, firmware 1.07, manual edit 0.03
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