My 137 kHZ page

Here are my VLF experiences up to now:

At present I'm working around a 137 kHz VXO.

It uses a single BJT (2N2222) that's act as oscillator-mixer; in past I saw it on the web called  "wide range" VXO on SPRAT written article by G3ESP.

Schematic is here (click to enlarge)

When  2 different crystals are connected to a normal quartz oscillator, it probably oscillates at same time at two frequencies...

I had begun with two 4.4 MHz Xtals and I obtained accetable, but not reliable results:

Then I found  in my junk box, a couple of CB XTALS, I use them  in the fundamental mode, so the fundamental frequencies Df is just 135 - 137 kHz, I shift a little the second XTAL, in order to make agile the VXO, then I filtered the output VLF signal and so now I have a quite good 135-137 kHz oscillator.

                              Manhattan style construction                         Measurements                        On the right side the oscillator-VXO

                                                                                                                                                on the left side the amplifier/buffer and output filter

On April 15th I added an amplifier-buffer stage after the XTAL oscillator-mixer, and made measurements again: here are the results

                                                Spectrum is good, 2nd harmonic over 30 dB down                FT817 used to  send to PC the detected signal

                                        3rd harmonic is higher as the RF signal is a square wave

         I zoomed the Spectran waterfall in order to check the Hz drift of the VXO                                 This a simple QRSS Spectran set

In April 18th I made a new test, in order to understand if is possible to use a double xtal as in the super VXO circuits, and see if the frequency shift Df is higher, the new schematic is this:

as you can see just added a second crystal in parallel to the X2 at 27120 kHz. The result is that with just one xtal, the Df is 3.6 kHz (about 2.6 %); zith two xtals Df is 5.8 kHz (about 4.2 %). The frequency drift if very low, as with a single xtal, and Spectran measurement is here:

Results seem quite good. investigations will go ahead...

Update 2007 April 18th