Flexible VXO-PLL Oscillator
DL6ZB, Rolf Heine (ex DL6MBI)
frequency generation systems are complex and need sophisticated
programming and measurement. On the other hand a lot of
home-brewers don't want to use microprocessor driven PLL circuits
systems, although they don't want to miss rock stable frequency
Here is a simple, low-cost but very effective solution for this.
Example of a 7.0-7.2 MHz VXO-PLL. Variable capacitor (approx. 350pF) isn't shown. It is
connected to the front side connector (see capacitor symbol).
actually works on 40m for direct conversion receivers and simple
transmitters, although it is usable for other frequencies and IF
output frequency depends on the divider ratio (74HC161) and the VCO
range. I use this VXO-PLL for my direct conversion QRP
transceiver Type 002 project. The tuning element is a simple variable capacitor. A varicap diode is also suitable as a tuning element for this.
How does ist work?
Theory of operation:
- VCO frequency range = 7.0-7.2 MHz
- 74HC161 dividing factor = 10
- VXO crystal frequency = 10.000 MHz
- CO crystal frequency = 10.700 MHz
The VXO output
is mixed with the CO frequency (10.7 MHz) of the SA612s internal
crystal oscillator. The SA612 output frequency (f1-f2) is approximately
700 kHz (+/-).
Now suppose the VXO will be tuned down a few kHz (to about 25 kHz
max.), then the SA612s output frequency rises because the
difference between the CO frequency and the VXO-frequency spreads.
Example: CO frequency = 10.7 MHz. VXO frequency = 9.985 MHz
f(ref) is then: 10.700 - 9.985 = 715 kHz. The
VCO output frequency is multiplied by the 74161 division factor=10. The
output frequency now is tuned to 7.150 MHz.
Why 7.150 MHz ?
The VCO output frequency is divided by 10 by the 74HC161 divider. The
divided VCO output frequency at the 74HC161 divider is now in the
tuning range of 700-725 kHz. The 74HC161 divider output will now be
phase/frequency compared with the SA612s output (after signal forming)
by the 74HC4046 phase/frequency comparator. The output of the 74HC4046
is low-pass filtered through the TL071 VCO loop filter. The loop filter
DC output drives the BB201 varactor diode. This VCO design was choosen 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. Use any
divider with division ratios higher than 16 for 20-10m output.
Divider Factors vs. Output Frequency:
(Click picture to download JPG file for higher resolution)