I have performed some experiments to see what might be the cause.
First, I will tell the exact setup:
DDS is AD 9834 driven by Arduino RedBoard, there is also encoder and two buttons and AD 9834 is on my own board. There is also I2C controlled LCD with 4 rows. (4x16 I think)
The receiver is SESE 80 (worked fine with VFO!)
Okay, so the experiments:
1. I replaced my AD 9834 board with the simple one from china. No difference, just maybe high pitch is a bit louder because my AD board has extra elements at AD output.
---> so my AD 9834 board fault ruled out
2. I changed my code to just apply signal without doing anything:
Code:
#include <AD9833.h>
#define FNC_PIN 4 // Any digital pin. Used to enable SPI transfers (active LO
//--------------- Create an AD9833 object ----------------
// Note, SCK and MOSI must be connected to CLK and DAT pins on the AD9833 for SPI
// ----- AD9833 ( FNCpin, referenceFrequency = 25000000UL )
AD9833 gen(FNC_PIN); // Defaults to 25MHz internal reference frequency
void setup() /*----( SETUP: RUNS ONCE )----*/
{
gen.Begin();
gen.EnableOutput(true);
WaveformType type = SQUARE_WAVE;
gen.ApplySignal(type,REG1,6390750);
gen.SetOutputSource(REG1); // Output 1000 Hz waveform
}
void loop()
{
}
---> No changes, the high pitch noise is the same, so it's not my code fault?
Interesting fact: the high pitch tone is constant and dependent on the DDS frequency. For example, at 6390750Hz it's a bit louder then at other frequencies.
3. I measured signal levels and realized that original VFO signal is at lower amplitude than DDS one. I tried adding various resistors and voltage dividers but no luck. When too high resistance is added then mixer simply stops working.
Futhermore, I noticed that original VFO signal amplitude changes while tuning, while the DDS amplitude signal is constant.
---> not sure, but that's probably not the cause?
4. I tried adding simple high pass filter with 1nF capacitor and 10uH coil. According to rfSim, it should pass frequencies above 3-4MHz
--> No changes.
5. I tried temprary disconnecting my LCD4x16 and I2C LCD driver module, totally away from the circuit (it's connected by 4 wires, GND, VDD, SCL, SDA)
---> No changes, so it's not I2C LCD noise.
6. I disconnected my antenna BNC from receiver
---> No changes, so it's not antenna noise...