Evolution of my QCX-SSB (PE1NNZ-SDR)

First attempt in May 2020 to build my QCX-SDR with a home made PCB V.1A

This transceiver, based on Guido PE1NNZ firmware, which I have been granted permission to use, has a built in battery, is home brewed with 3 Chinese breakout boards which populated on a ground plane of a double side PCB, while the other side is etched for a Tayloe Quadrature Detector and a low noise op-amp.

From left to right: PAM8403 3W class D audio amp, Pro Mini 328P and SI5351. Those boards have to be modified for proper functioning. I use a home made V.1b with ground plane to shield noise from those modules.

Schematic Diagram

A class D audio amplifier module PAM8403 and low pass filter were added in the circuit.
All tracks are 0.5mm width
Layout with low pass filter for PAM audio amplifier

Remove the pot, trim and cut a hole to the PAM 8403’s PCB for reducing profile height of the 470uF capacitor. Beware of 237kHz switching class D.


Update: 20210824

The latest firmware let me choose 16 or 20 MHz clocking for MCU and 25 – 27 MHz for SI5351. The Arduino Nano is my favorite due to all ports can be accessed and USB connection availability . Port D13 has to be modified by removing the Led or 1k resistor toward its ground path.

Spectrum of the Two Tone Test from my QCX-SSB (EER class E switch mode amplifier).

Spanning 28.100 – 28.110 MHz.
Poor result, the first lobe on the left is the opposite sideband. My prototype is only CW complying, it failed to pass the sideband suppression 43 dB down test.
3000 Hz Bandwidth Setting, spanning 28.100 – 28.104 MHz, lot of spikes!
Spanning 28.096 – 28.116 MHz, 3000Hz bandwidth at 28.110 centre frequency.
20 dBc sideband attenuation.
30 dBc 3000Hz bandwidth attenuation
Linear SSB Two Tone Test for comparison
3200Hz crystal filter, AN-GRC/106.
50 dBc sideband attenuation (first lobe, left hand side).
50 dBc bandwidth 3200Hz attenuation (lobes 7 & 8).


47 CFR § 97.307 – Emission standards


Only One Inductor and three Capacitors

NM0S (David Cripe) design formulas.


I am very impressed by a clever minimalistic LPF with only 4 components for impedance matching and harmonics filtering.

The only drawback for this filter is alignment, a VNA job.

Tailoring 10m band LPF, 3rd harmonic 84MHz matches with the dip S21 scan.
2nd harmonic 56MHz attenuation.

Back to square one, to drain modulation

It seems to me PE1NNZ spec is related to the drain modulation topology from QRP – LABS. It is quite well known in AM modulation.



  • Intermodulation distortion products (two-tone; SSB with varying envelope) IMD3, IMD5, IMD7: respectively -33dBc; -36dBc; -39dBc
  • Intermodulation distortion products (two-tone; SSB with constant envelope) IMD3, IMD5, IMD7: respectively -16dBc; -16dBc; -19dBc
  • Opposite side-band rejection (two-tone): better than -45dBc
  • Carrier rejection (two-tone): better than -45dBc
  • Wide-band spurious (two-tone): better than -45dBc
  • 3dB bandwidth (sweep): 0..2400Hz