Although the QRSS-community seems not to be as technical as it used to be, some thoughts about the matter from my side.
Some OM, again I am writing about novice/foundation/newcomer-lis, may not be allowed the lower band edge. But still, most activity takes place at those spots.
I asked myself, if I could find crystals to suite both needs. You will find some combis for one or the other option on this blog.
Meanwhile, I believe that 4.000MHz is the ideal I.F. for QRSS. Here's what can be done (more or less easily):
The trick here, use a CB transmit (overtone) crystal for 27.015MHz (5T) and operate it a its fundamental, i.e. 9.005MHz. A division by 2 (flip flop) will end up at 4.5025MHz. A VXO at 9MHz may be pullable by a few kHz, hence, we may be able to cover a substantial portion of the present 600m hamradio band.
Should a future allocation be somewhat higher, there are many other CB-TX-XTALS available.
Should a future allocation be somewhat lower, there are many CB-RX-XTALS available.
Essentially the same as above... the crystal being a 12T. For those who are not aware, there is maritime navigational (and weather) information transmitted on 600m, to be precise, 518kHz (international frequency) in FEC.
NAVTEX also knows a local frequency, which is 490kHz. This frequency is reached with a 39R (26.940MHz) xtal.
500kHz can easily be generated from a 4MHz signal by dividing the latter by 8 (ripple counter). Running a 4MHz Pierce oscillator, the generated frequency will be above the 4MHz series frequency. Assume we generated a frequency of 4001kHz, 1/8 would be 500.125kHz, resulting in a mixed QRG of 3500.875kHz (TX).
For RX, a tweaked (fine tuned) L.O. can be used as B.F.O. to provide a reasonable beat for reception.
That would be the lower band edge solution... further comments here... however, there are better options!
Again the lower band edge, however, this is subtractive, therefore, temperature drifts will not add up but rather cancel (or at least reduce another).
This QRG is open to novice/foundation/newcomer-license holders! Temperature drifts will not add up but rather cancel (or at least reduce another). The frequency is at the upper edge of the 40m data segment, I believe, it is an ideal playground for testing all sorts of modes.
The classical 30m QRSS frequency is in close range. A local oscillator will have to generate a frequency of 6.139Mhz, which is reachable by either pulling of penning of a 6.144MHz standard crystal.
This is a no-brain-er! Just run a 10.0MHz LO.
This is a no-brain-er having improved temperature behavior... subtractive...
Here, the local oscillator would be sub-harmonic. 7.040MHz is just one example of many possibilities opened by crystals available for the 40m ham-radio band.
This again is a no-brain-er having improved temperature behavior... subtractive...
This is a no-brain-er...
Please feel free to add some ideas as a comment!