ANOTHER DIRECT CONVERSION HF SSB TRANSCEIVER
|PLL VCO||DDS VFO|
|Tayloe Mixer/Detector||Exciter & Receiver Audio|
I have been through several VFOs starting with drifters. Then I tried the huff and puff stabilization (hardware control) followed by the PIC microcontroller versions, the latter giving very good results. I then migrated to phase-lock-loops and finally DDS VFOs. They all have their advantages and disadvantages. I like the DDS VFO the best of all in spite of its misgivings. I still need to clean up the ones I built as they are not shielded (just put in a box without top or bottom cover). I will also need to try some broadband bypassing to get rid of the numerous spurs they generate.
The DDS VFO I finally ended up using for this project was one I had originally built to provide frequencies from 0 to 16 MHz. It is a kit I had bought from W8DIZ, creator of the MPIG transceiver projects. I only used the circuit board with the +5 volt regulator and the already soldered AD9853 DDS chip, its clock and the output low-pass filter ( DDS board). I used my own controller on a separate board mounted at the top.
The AD9835 DDS chip is clocked by a 50 MHz crystal oscillator module which can generate a useful range of frequencies up to about 16 MHz. However, I just utilize the 1 to 2 MHz part of the spectrum. The RF output passes through a low-pass filter which provides a nice sinusoidal wave that can be amplified by the following linear amplifier to about +10 dBM. The 16F84 PIC microcontroller outputs data on its I/O ports A and B to the SCLOCK, SDATA and FSYNC lines that controls the DDS chip and also to the 16 character LCD to display the operating frequency using eight digits together with the frequency step by which the operating frequency is incremented or decremented when the shaft encoder is rotated. Input control data to the 16F84 is via these same ports. There are three momentary make switches and a shaft encoder connected to these ports. Toggling one switch will change the steps upwards from 10 Hz to 100 Hz to 1000 Hz to 10000 Hz while another would change the steps downward from 10000 Hz to 1000 Hz to 100 Hz to 10 Hz. The third would change the band sequentially from 3.5 MHz to 14 MHz and over again. A 4 MHz crystal clocks the 16F84 (See fig.8 ).
The controller software sends code to the DDS chip to generate frequencies from 1.0 to 2.0 MHz. However 1.4 to 1.6 MHz is the range that applies to the 3 ham bands. The 56 to 64 MHz generated by the VCO is applied to a divide by 40 counter to produce 1.4 to 1. 6 MHz which is phase locked to the 1.4 to 1.6 MHz reference from the DDS VFO.