Oscillators using crystals, aka XO, are supposed to be a reliable source of signal having a stable frequency. OK, compared to VFOs (Variable Frequency Oscillators), using inductors and capacitors, that is safe to say, however, there are some remarkably stable VFO designs out there...
The whole game of combining crystal frequencies, at least for A1A or F1A purposes is, to avoid the potential drift a regular VFO would possibly suffer from. Well, amateur radio, as we all know, is not a channelized game however. So, what are those crystals and combinations of xtal-qrg good for when it comes down to usability?
Well, first of all, there are some frequencies of major interested, such as the QRP callings QRGs. A rig covering a single frequency could still be very useful, depending on the frequency and its use. Just remember the good old times, everyone was using a color burst crystal (3.579MHz) and had a great share of fun.
A crystal oscillator can provide so much more than just a single frequency. And this is why:
a crystal is functioning as some sort of L-C (inductor-capacitor) resonant circuit (check out the internet for more info). Such circuits can be influenced by additional reactance, e.g. a capacitor and/or inductor in series, which will bend the resonant frequency either up or down.
Using the right amount of (variable) inductance and/or (variable) capacitance, a crystal's resonance can be pulled by a certain percentage, which can be quite a bit depending on the frequency.
This frequency pull can further be enhanced by using two or more crystals in parallel. Now we are talking "super VXO" (please check the internet, there is some very good documentation available from Japan).
What's the benefit? Assume we are going back to the example for the 40m band, used in the xtal-combi-post. Assume we pull the 8.867MHz oscillator by +/- 5kHz (which is no problem on that frequency at all), the resulting mix with a 1.843MHz frequency would allow for a range of 7.019 to 7.029Mhz, representing a very useful portion of the 40m CW range. With a super-VXO, this range could be from approx. 7.000 to 7.040Mhz.
This is what crystal combinations are all about. A stable VXO converting into a "cheap" I.F., and we got us a
cheap receiver having a crystal CW-filter.
BTW, this is what this entry was all about.