21-06-2012, 12:32 AM
I think that that approach is heading for problems. I haven't done the full sum & product evaluations, so I'll argue my case qualitatively.
At the anode of the mixer, before the resultant spectrum gets to the BPF, you will have products of the 1 MHz tuned spectrum (R.F. tuning) and all the harmonics from the 1 MHz oscillator (your 'comb'). Therefore, the BPF will need to be tunable to reject all the unwanted products. To produce a BPF that is tunable will require several tuned circuits, all ganged to the R.F. and mixer grid tuning capacitor: sounds like at least a 4-gang tuning capacitor will be required - maybe more. And then there is the little issue of the tracking: the tuning of the BPF will have different Cmax and Cmin ratios to that of the R.F. grid tuning and the mixer grid tuning.
In conclusion, I reckon that your best approach is to arrange for a 'clean' L.O. injection signal at the required freq. (according to the R.F. & mixer tuning freq.) and feed that into the mixer. It may be possible to arrange for a 3rd. section of a 3-gang tuning capacitor to perform the necessary filtering to produce just the required nth. harmonic of the 1 MHz master osc. source. The simplest route, of course, is to build a stable wide-range tunable osc. that covers the required freq. range for the L.O injection and simply select the required osc. freq. prior to tuning over the required signal range - in effect, what you have been doing by using a sig. gen. as the L.O. injection source! Of course, that moves well away from the H.G. approach that you prefer, so presumably that's a no-no.
Further that that, at the moment, I can't think of any other suggestions. What is quite evident is that your requirement to tune the whole R.F. range from 2 - 29 MHz (or something like that) does make the design difficult. Now if you were aiming at only the Amateur Radio bands (or any other specific R.F. freq. ranges) then the design becomes a lot easier. Whatever, I'll have a further think about your general freq. coverage requirement and see if I get any bright ideas, but I'm getting a sneaky feeling that if anything relatively simple can be done for general coverage, then Racal would have done it, instead of the design they produced for the RA-17! (Or Plessey with the PR155, and various other manufacturers and their designs, which are nearly all synthesizer or pseudo - synthesizer based).
We'll talk again. In the meanwhile . . . . keep at it!
Al.
At the anode of the mixer, before the resultant spectrum gets to the BPF, you will have products of the 1 MHz tuned spectrum (R.F. tuning) and all the harmonics from the 1 MHz oscillator (your 'comb'). Therefore, the BPF will need to be tunable to reject all the unwanted products. To produce a BPF that is tunable will require several tuned circuits, all ganged to the R.F. and mixer grid tuning capacitor: sounds like at least a 4-gang tuning capacitor will be required - maybe more. And then there is the little issue of the tracking: the tuning of the BPF will have different Cmax and Cmin ratios to that of the R.F. grid tuning and the mixer grid tuning.
In conclusion, I reckon that your best approach is to arrange for a 'clean' L.O. injection signal at the required freq. (according to the R.F. & mixer tuning freq.) and feed that into the mixer. It may be possible to arrange for a 3rd. section of a 3-gang tuning capacitor to perform the necessary filtering to produce just the required nth. harmonic of the 1 MHz master osc. source. The simplest route, of course, is to build a stable wide-range tunable osc. that covers the required freq. range for the L.O injection and simply select the required osc. freq. prior to tuning over the required signal range - in effect, what you have been doing by using a sig. gen. as the L.O. injection source! Of course, that moves well away from the H.G. approach that you prefer, so presumably that's a no-no.
Further that that, at the moment, I can't think of any other suggestions. What is quite evident is that your requirement to tune the whole R.F. range from 2 - 29 MHz (or something like that) does make the design difficult. Now if you were aiming at only the Amateur Radio bands (or any other specific R.F. freq. ranges) then the design becomes a lot easier. Whatever, I'll have a further think about your general freq. coverage requirement and see if I get any bright ideas, but I'm getting a sneaky feeling that if anything relatively simple can be done for general coverage, then Racal would have done it, instead of the design they produced for the RA-17! (Or Plessey with the PR155, and various other manufacturers and their designs, which are nearly all synthesizer or pseudo - synthesizer based).
We'll talk again. In the meanwhile . . . . keep at it!
Al.
