Friday, January 28, 2011

4MHz - the Magic QRSS I.F.

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):

600m 4.5025MHz-4.000MHz=(27.015/6)MHz-4.000MHz=502.5kHz
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.

NAVTEX 4.5175MHz-4.000MHz=(27.105/6)MHz-4.000MHz=517.5kHz
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.

80m 4.000MHz-500kHz=3.500Mhz
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.

40m 4.000MHz+3.000MHz=7.000MHz
That would be the lower band edge solution... further comments here... however, there are better options!

40m 11.000MHz-4.000MHz=7.000MHz
Again the lower band edge, however, this is subtractive, therefore, temperature drifts will not add up but rather cancel (or at least reduce another).

40m 11.059MHz-4.000MHz=7.059MHz
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.

30m 4.000MHz+6.144MHz=10.144MHz
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.

20m 4.000MHz+10.000MHz=14.000MHz
This is a no-brain-er! Just run a 10.0MHz LO.

20m 18.000MHz-4.000MHz=14.000MHz
This is a no-brain-er having improved temperature behavior... subtractive...

17m 4.000MHz+14.080MHz=4.000MHz+2x7.040MHz=18.080MHz
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.

15m 25.000MHz-4.000MHz=21.000MHz
This again is a no-brain-er having improved temperature behavior... subtractive...

10m 4.000MHz+24.000MHz=28.000MHz
This is a no-brain-er...

10m 4.000MHz+24.000MHz=4.000MHz+2x12.000MHz=28.000MHz

Please feel free to add some ideas as a comment!

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