This circuit comes from the Progressive Communications Receiver that is in most of the recent ARRL Handbooks and the article "A Progressive Communications Receiver", by Wes Hayward and John Lawson, QST, November 1981, Page 11.
C4 and C5 are adjusted in value so that C6, of almost any value, can be used in this VFO. The formula for calculating the proper values is on page 21 in the November 1981 of QST. Working without the formula, C4 in series with C6 will effectively lower C6's value and shorten its range. The parallel capacitor C5 will also shorten the range of C6, and lower the low end frequency of the VFO. Use C3 to make major changes in the frequency of the VFO. The values for this receiver are the following:
L1 - 20 Turns, Tap - 5 Turns from ground end
Trim - 8 pf
C3 - 56 pf
C4 - 47 pf
C5 - 220 pf
C6 - 365 pf
The values given above are for a VFO that will work with the 3.457 MHz crystal filter: 40 meters - 10.547 MHz and up, 20 meters - 10.453MHz and up.
Crystal Filter Bandwidth for the formula to determine the proper VFO frequency with a given crystal filter frequency.
This VFO is probably the most stable FET circuit out there, but I have found that the active device you use can make a big difference in the stability of the VFO. With the mosfet (Gates 1 & 2 tied together), the
VFO drifted considerably on warm up and was very sensitive to temperature variations. When a 2N5486 was used, there was very little warm-up drift and stability during minor temperature variations was acceptable.
The output is fed through a 10 pf capacitor to the input of a mosfet RF amplifier.
This circuit can be used at frequencies between 2.5 to 10 MHz.
VFO's With the NE602
A stable VFO is one of those homebrew projects that always seems to give the most trouble. It's my opinion that the NE602 may have been overlooked as one of the most stable VFO oscillators around. In the Signetics application note "Applying the Oscillator of the NE602 in Low Power Mixer Applications", the NE602 is described as a chip with three subsystems, a Gilbert cell mixer, a buffered emitter follower oscillator, and RF current and voltage regulation.
Please note the third system - "RF Current and Voltage Regulation". The bias is temperature compensated for RF and current variations.
I have found the NE602 oscillator to be rock solid from the instant it is turned on. The vfo circuit is very easy to design for any frequency (see NE602 Front End) and uses fewer parts than the FET VFO.
The downside is that the NE602 is more expensive than a single FET, and is more susceptible to frequencies floating on the 12 Volt line, which requires careful bypassing and maybe some shielding. Using a 7805 voltage regulator does a great job of isolating the NE602 from the 12 Volt line.
The combination of a 7805 voltage regulator and the NE602 makes a very stable VFO with a minimum of effort. The extra $5 is worth the expense.
The NE602 VFO can be substituted for the FET VFO with no other circuit changes. The two RF amplifiers are more than adequate to bring the signal level to the 5 to 6 volts Peak-to-Peak that the Single Balanced Mosfet Mixer requires.
To use the NE602 as an oscillator only , I quote from the article "NE602 Primer", by Joseph J. Carr, Elector Electronics USA, January 1992, Page 25 (picture is Fig. 10 on page 25):
"The NE602 is usually thought to be a receiver or frequency convertor, but it can also be used as an oscillator or signal generator. Normally, the LO signal and the RF signal are suppressed in the output. Figure 10 shows a generic circuit that will
allow the LO signal to appear at the output (no RF or IF signal appears). In this circuit, one RF input (pin 2) is bypassed to ground for RF, while the other input (pin 1) is grounded for d.c. through a 10k resistance."
The Ultimate VFO
Check out the article "A Drift-Free VFO", by Jacob Makhinson, QST, December 1996, Page 32. Subtitled "Build this clean, quiet, stable VFO", his design claims very low phase noise and fits in with receivers that are low noise and have high dynamic range. I would imagine a normal VFO would work with his frequency stability circuit if you don't need absolute silence, and old CD's would work great for his opto-interupter. The circuit is inexpensive to build. I haven't built one yet so can't make any comments at this time.