Signal processing and demodulation is done at audio domain in Linrad Open Source software by Leif, SM5BSZ.
Linrad is available for Linux, Windows, Free BSD and other operating systems.
DDS's clock frequency is 40 MHz, so it is not recommended to work at frequencies higher than 1/3rd the clock frequency.
In the last picture you can see the waterfall on the CW segment on 80 m band.
A close up of the 74HC4052 mixer and 74HC74 by four divider.
You can see the trifilar winding FT-50-43 toroid at the RF input.
Service for models supporting firmware updates via network only will resume in the near future.
We will post a notification on our website as soon as the firmware update is available for download via network.
Jim Shaffer, WB9UWA sugested to use a voltage divider to rise the input level at the 74HC74 clock and improve sensitivity. At 500 KHz is only 88.28 ohms, certainly not enough.
74HC74 by four divider idea has been learnt from several WEBs on the internet. Softrock and Zeta SDR designs and others where studied: The idea is simple.
At the left there is a BC547 NPN bipolar transistor RF amplifier for the signal generated by the DDS. Receiver schematic can be seen on the following picture: L1 is a trifilar 8 turns winding on a FT-50-43 toroid. Reactance at 3.5 MHz is 2*pi*3500000*28.16e-6=619.27 ohms, 12 times 50 ohms.
Signal level was not enough to excite 74HC74 clok inputs. At 1.8 MHz reactance is 317.8 ohmios, 6 times 50 ohms.
The first steps in restoring and updating the old G2DAF linear amplifier commenced with the removal of the 2 x 6U4GT rectifier valves and the associated voltage doubler components / 300 ohm input circuit.