ANOTHER DIRECT CONVERSION HF SSB TRANSCEIVER
|PLL VCO||DDS VFO|
|Tayloe Mixer/Detector||Exciter & Receiver Audio|
The PLL and VCO Board
In order to provide the two RF signals phase shifted by 90 degs for driving the Tayloe Sampling Detector and Modulator a Local Oscillator of four times that frequency is needed. The VCO board provides the stable Local Oscillator injection. The 56 to 64 MHz square waves generated by the VCO is used as the LO for the 14 MHz band. The divided by 2 signal, 28 to32 MHz is used for the 7 MHz band while the divided by 4 signal, 14 to 16 MHz is used for the 3.5 MHz band. I had to abandon the use of a DDS VFO as the Local Oscillar because the unit generated numerous spurs especially on the higher bands and nothing I tried could get rid of them. For stability I phase locked the VCO to the DDS VFO that operated at a much lower frequency. This gave good spur-free results.
I tried a few VCO circuits and PLL configurations with varying results. My final configuration was a VCO using logic devices running at 56 to 64 MHz. This was built around a 74AC86, Exclusive-Or Gates configured as inverters as that was the only usable high speed device I had in the junk box. The 56 MHz clocks a 74AC74 D-Flip/flop which divides by two in one half to put out 28 MHz and again divides by 2 in the other half to provide 14 MHz. These three signals provide the 4 times the operating frequency signals for deriving the quadrature LO signal for the 80, 40 and 20 meters bands; all square waves (See fig. 6). I used 74HC4066 analogue switch to select the desired LO frequency.
The 14 to 16 MHz VCO signal is also sent to the PLL board where a 74AC193 up/down counter configured as a divide by 10 counter, provides frequencies ranging from 1.4 to 1.6 MHz. On that board the DDS VFO signal which ranges from 1.4 to 1.6 MHz is processed by a wave squaring circuit and used as a reference frequency to lock the divided down VCO signal using a 74HC74 phase/frequency detector. The output from the phase detector drives a Charge Pump and Loop Filter circuit. The control voltage is fed back to the varicap diodes on the VCO.
A TL082 op amp configured as a comparator monitors the phase lock voltage and lights a green LED if the voltage is between +2.2 and +8.2 volts indicating that the VCO is operating in a safe lock condition. However, the VCO is tuned so that 14 - 16 MHz is at the upper end of the lock range ensuring the operation of the varicap diodes in a region of their highest Q thus providing the best possible spectral purity for the VCO signal (See fig. 7).
The Internet provided an invaluable source of ideas. The pages of QRP 2001 and those of a project "Tri-band Receiver" done by Jean Taeymans prompted my ideas of using a high frequency phase-locked VCO. Also, ideas from a DDS VFO/PLL project by PA0KLT on the Web Pages of PA2OHH were very useful. I use a DDS VFO which operates at a relatively low frequency as the stable reference in the PLL. Also Hans Summers approach in his huff and puff Franklin VFO for an 80 meter HF Receiver also using a Tayloe detector, prompted me to go the way of using logic devices for the VFO there-by generating square waves which were needed for the RF quadrature generator.