Re: LF: Oscillator Stabilty for QRSS?

[G0MRF@aol.com]

In a message dated 8/13/02 7:54:19 PM GMT Daylight Time, jsm@gulfislands.com writes:


> Perhaps someone who has already 'been there' can offer their advice. 
> I would like to start building the exciter stages for what will be my 136KHz tx (hopefully we will have access to the band within the year!). The DJ1ZB exciter (modified by DF3LP) is the circuit that I am questioning. It mixes two xtal controlled 4MHz oscillators, 136 KHz apart, to extract the 136KHz signal. Can I build two VFO's, somehwere around 1MHz and set them 136KHz apart, that will  be stable enough to handle QRSS work? I am worried about drift but also thinking that they may be stable enough if kept in the 1 - 2MHz region. As well, they should (if identical) drift by the same amount and thus be self-correcting? Is this a reasonable avenue to pursue or should I just break down and buy a couple of xtals??
>  
> Steve / VE7SL  Mayne Island, BC


Hi Steve

The 300 W transmitter detailed on my web site uses 2 crystals at 8.000 and 8.275 to generate 136kHz.   I selected 8 MHz to get the VXO portion to cover most of the 136kHz allocation. I think your 1MHz solution would have less drift, but would also only cover a very small portion of the band. Good enough for QRSS only.

The real problem will be frequency setting accuracy. It is very difficult to set a VXO to within 100mHz and for keeping a sked overnight, where you may wish to leave the TX running and wait for your phone to ring, a DDS would be a better long term choice.

The crystals will work, but I suspect after a while you'll start searching for a DDS.

73

David  G0MRF        www.g0mrf.freeserve.co.uk/300w.htm