I myself had some difficulties to build a longwave VFO solution from scratch. After experimenting with 5-6 MHz VFOs and frequency dividers, I came accross a nice synthesizer kit. With a frequency range between 1 Hz (!) and 54 MHz (!) it is a real overkill for the LF band, but the features contained in it are excellent for what we are doing. The resolution is 1 Hz, the accuracy, once calibrated is in the same range and long time stability is excellent. The synthesizer is either available as a kit (see below) or as a build and assembled unit. It makes use of an integrated DDS circuit named AD9850.

There are a couple of information sources available for the AD9850.

The Analog Devices AD9850 is a CMOS, 125 MHz complete direct digital  synthesiser on a single chip. It uses a single 5 V or 3.3 V supply, and with a 125 MHz clock, can generate a low-distortion sine wave or a square wave, from under 1 Hz up to about 60 MHz, with a tuning resolution of 0.0291 Hz. Full data on the device is available from ADI's web site:

 
Off course, this device was never created for low-frequency, however, I got hold of a kit that includes a controller and a display, it has one Hertz resolution and ten memory channels and so it is ideal to drive a longwave-PA. It even has a build in correction factor for frequency aacuracy, so once you found out how many ppm you are away from the qrg where the device should be, you can program that permanently into the memory. This allows a freqency stability and repeatability of around 1 Hz.

There are two devices available, one with a frequency range between 1 Hz and 16 MHz (kit 139.39 US$, assembled 189.95 US$) and the other one from1 Hz to 54 Hz (kit 169.95, assembled 219.95 US$). Not cheap, however, I took the 54 MHz version and use the device also as a very accurate test-transmitter. The company delivering these devices is

S & S Engineering
14102 Brown Rd.
Smithsburg, MD 21783

E-Mail to:  N3SAD@AOL.COM

Their webside can be found at