Thanks for the comments Al, just to verify that the RF grid tuning and the mixer grid tuning are ganged (did that the other day) the two coils that the ganged capacitor tunes (RF grid & Mixer grid) are not identical but they were all I had for test purposes, I managed to lash up a slug and trimmer on one of them in order to get them to track, they track quite well. The tunable regen detector capacitor however is not ganged to anything and is not designed to be. The proceedure at present is to set my sig. gen. to a frequency 3 Mhz above the lowest signal frequency I wish to tune (10 Mhz for 7 Mhz) peak up the ganged RF/Mixer capacitor for max noise, there will be two peaks Fo plus signal & Fo minus signal so therefore the Fo minus peak is selected, the regen det tunes from 3.0 Mhz down to 2.5 Mhz therefore by tuning the regen det from 3.0 down to 2.5 Mhz the signal frequencies are tuned from 7.0 Mhz up to 7.5 Mhz the apparent frequency reversal being due to subtractive product of the mixing process, if the sig. gen was set to the lower difference frequency of 4.5 Mhz then by tuning the regen det from 3.0 Mhz down to 2.5 Mhz the signal frequecies of 7.5 Mhz down to 7.0 Mhz are being tuned due to the addative product of the mixing process, moving from the 7 Mhz amature band to the 14 Mhz amature band is quick and easy.
I know that Al and others will already know how all this mixing stuff works, I have wrote some of the above for the benifit of any newcomers to the hobby, working out the mixing process with a fixed IF freqency is fairly straight forward but some times when using a tunable IF (which is what the tunable regen det is) it sometimes can get a bit confusing.
As for the next phase I have not yet decided, I would still like to try the harmonic generator idea for the local osc. I have been looking at the RA17 harmonic generator circuit again I am pretty sure that the 0 to 32 Mhz LPF between the harmonic generator and the mixer it feeds does not have any bearing on the harmonic generation or selection of the harmonics itself only insofar as to suppress harmonics above 32 Mhz which of course could play havoc with the 1st VFO mixing process in that receiver, if any one knows any different could you please let me know.
In order to mess about with this Harmonic generator idea I will have to order up some stuff so it might be a while before I can try it.
Another word of note, I have tried tunable IF's from around 10 mhz down to 2 mhz the best one so far, taking into account the consistancy of regeneration across any tuned IF coupled with virtually zero pulling from non to full regeneration, direct signal breakthrough etc seems to be a tunable IF of 2.5 to 3.0 Mhz so that means tuning 500 Khz chunks of the spectrum at a time, getting it to tune 1 Mhz chunks is possible but some compromises have to be made as is always the case with these things, I have not discounted 1 Mhz in the final design it all depends if I can get the harmonic generator off the ground as setting the band set in 1 Mhz increments would be easier than 500 Khz so far as band setting and dial/scale calibration is concerned.
If I have time I will post yet another updated circuit showing things as they are at the moment, no doubt there will be many more revisions! The final design is more likely to employ B9A/B7G valves rather than Octals so various valve voltage parameters wiil no ndoubt have to be changed but this is not a problem in itself.
Lawrence.
I know that Al and others will already know how all this mixing stuff works, I have wrote some of the above for the benifit of any newcomers to the hobby, working out the mixing process with a fixed IF freqency is fairly straight forward but some times when using a tunable IF (which is what the tunable regen det is) it sometimes can get a bit confusing.
As for the next phase I have not yet decided, I would still like to try the harmonic generator idea for the local osc. I have been looking at the RA17 harmonic generator circuit again I am pretty sure that the 0 to 32 Mhz LPF between the harmonic generator and the mixer it feeds does not have any bearing on the harmonic generation or selection of the harmonics itself only insofar as to suppress harmonics above 32 Mhz which of course could play havoc with the 1st VFO mixing process in that receiver, if any one knows any different could you please let me know.
In order to mess about with this Harmonic generator idea I will have to order up some stuff so it might be a while before I can try it.
Another word of note, I have tried tunable IF's from around 10 mhz down to 2 mhz the best one so far, taking into account the consistancy of regeneration across any tuned IF coupled with virtually zero pulling from non to full regeneration, direct signal breakthrough etc seems to be a tunable IF of 2.5 to 3.0 Mhz so that means tuning 500 Khz chunks of the spectrum at a time, getting it to tune 1 Mhz chunks is possible but some compromises have to be made as is always the case with these things, I have not discounted 1 Mhz in the final design it all depends if I can get the harmonic generator off the ground as setting the band set in 1 Mhz increments would be easier than 500 Khz so far as band setting and dial/scale calibration is concerned.
If I have time I will post yet another updated circuit showing things as they are at the moment, no doubt there will be many more revisions! The final design is more likely to employ B9A/B7G valves rather than Octals so various valve voltage parameters wiil no ndoubt have to be changed but this is not a problem in itself.
Lawrence.
