Well it's certainly going to need to be tunable over 1 MHz, and in the range of about 3 MHz, that's not ideal; 500 kHz will give a sharper tuning; not a lot more, agreed, but it will help. As for determining what freq. that should actually be, the question of image responses arises. Obviously we do not want an IF and osc. injection freq. that gives us an image band that corresponds to broadcast bands which will have high-powered transmitters in them. 
At the same time, we do not want an IF that is that high that tuning becomes a problem. The calculation of possible IFs and osc. freqs. is just a matter of sitting down and doing a few sums, for which Excel is particularly handy.
That said, I've been thinking about your original idea of a receiver covering the three Amateur Bands, 7, 10 and 14 MHz, and came up with the nucleus of an idea. It doesn't use a harmonic generator as such, but does use two frequency multipliers and an additional mixer. (Ideally, that mixer really needs to be a double-balanced affair: easily accomplished with a twin triode or a 4-diode ring modulator. For this exercise, I'd go for the twin triode: greater output). The freq. multipliers and that mixer obviously need pre-set tuned L/C ccts. to select the required harmonic.
I think the attached drawing says the rest.
[attachment=4681]
Note: The 'R.F. pre-select' involves the use of a valve with tuned L/C ccts. in its grid and anode, so a twin-gang variable cap. is required here, in addition to another wafer on the 2-pole, 3-way switch , as shown. An R.F. gain control would be a useful addition, too. I'd also make the signal / osc. mixer another DBM too.
On that same theme, and with a tunable IF of 3.0 - 3.5 MHz, and previous to the above, I did come up with a scheme using two crystal oscs. at 3 and 4 MHz, and by the use of two mixers, produce osc. injection freqs. of 4, 7 and 11 MHz, (for RF input freqs. same as above). Unfortunately, upon an analysis of images, the 7.0 - 7.5 MHz signal input band has an image band of 1.0 to 0.5 MHz: the MW broadcast band! So that hit that idea on the head.
All-in-all, an interesting exercise though.
Al.
At the same time, we do not want an IF that is that high that tuning becomes a problem. The calculation of possible IFs and osc. freqs. is just a matter of sitting down and doing a few sums, for which Excel is particularly handy.That said, I've been thinking about your original idea of a receiver covering the three Amateur Bands, 7, 10 and 14 MHz, and came up with the nucleus of an idea. It doesn't use a harmonic generator as such, but does use two frequency multipliers and an additional mixer. (Ideally, that mixer really needs to be a double-balanced affair: easily accomplished with a twin triode or a 4-diode ring modulator. For this exercise, I'd go for the twin triode: greater output). The freq. multipliers and that mixer obviously need pre-set tuned L/C ccts. to select the required harmonic.
I think the attached drawing says the rest.
[attachment=4681]
Note: The 'R.F. pre-select' involves the use of a valve with tuned L/C ccts. in its grid and anode, so a twin-gang variable cap. is required here, in addition to another wafer on the 2-pole, 3-way switch , as shown. An R.F. gain control would be a useful addition, too. I'd also make the signal / osc. mixer another DBM too.
On that same theme, and with a tunable IF of 3.0 - 3.5 MHz, and previous to the above, I did come up with a scheme using two crystal oscs. at 3 and 4 MHz, and by the use of two mixers, produce osc. injection freqs. of 4, 7 and 11 MHz, (for RF input freqs. same as above). Unfortunately, upon an analysis of images, the 7.0 - 7.5 MHz signal input band has an image band of 1.0 to 0.5 MHz: the MW broadcast band! So that hit that idea on the head.
All-in-all, an interesting exercise though.
Al.
