Flexible VXO-PLL Oscillator

DL6ZB, Rolf Heine (ex DL6MBI)

PLL/DDS frequency generation systems are complex and require sophisticated programming and measurement. On the other hand, many homebrewers don't want to use microprocessor-controlled PLL circuits or DDS systems, yet they don't want to do without rock-solid frequency generation either. Here is a simple, low-cost, but very effective solution.

This VXO-PLL currently works on 40 m for direct-conversion receivers and simple transmitters, but it can also be used for other frequencies and IF systems. The output frequency depends on the divider ratio (74HC161) and the VCO frequency range. I use this VXO-PLL in my Type 002 direct-conversion QRP transceiver project. The tuning element is a simple variable capacitor; a varicap diode is also suitable.

How Does It Work?

Example:

• VCO frequency range = 7.0–7.2 MHz
• 74HC161 division factor = 10
• VXO crystal frequency = 10.000 MHz
• CO crystal frequency = 10.700 MHz

Theory of operation:

The VXO output is mixed with the CO frequency (10.7 MHz) of the SA612's internal crystal oscillator. The SA612 output frequency (f1 − f2) is approximately 700 kHz.

Now suppose the VXO is tuned down by a few kHz (by up to about 25 kHz). The SA612's output frequency then rises, because the difference between the CO frequency and the VXO frequency increases.

Example: CO frequency = 10.7 MHz, VXO frequency = 9.985 MHz.

f(ref) is then 10.700 − 9.985 = 0.715 MHz = 715 kHz. Multiplied by the 74HC161 division factor of 10, this sets the VCO output frequency to 7.150 MHz.

Why 7.150 MHz?

The VCO output frequency is divided by 10 in the 74HC161 divider, so the divided VCO frequency is in the range of 700–725 kHz. The 74HC4046 phase/frequency comparator compares the 74HC161 divider output with the SA612's output (after signal shaping). The output of the 74HC4046 is low-pass filtered by the TL071 VCO loop filter. The DC output of the loop filter drives the BB201 varactor diode. This VCO design was chosen because it is broadband, low-noise, simple, low-cost, and reliable.

This VXO-PLL principle can be used for any frequency by adjusting the divider and/or crystal frequencies. It is very flexible. For 20–10 m output, use a divider with division ratios higher than 16.

Division factor vs. output frequency: