Thanks Al for your interest, I wish I could put what's in my head into text via the keyboard like what you do,
I have always had a problem with this ever since I can remember so thanks for that.
I need now to explain the main reasons why I initially chose the IF frequency and the IF bandwidth.
The IF center frequency of 2.75 Mhz was chosen as the best all round frequency taking into consideration what is required from the design.
As you will know it's all a trade off between image ratio and frequency stability when choosing the frequency of a tunable IF. From the outset I planned a double tuned RF stage to allow me to reduce the IF for image ratio purposes, the other benefit of a lower IF is that it makes the tunable regen det a lot easier to design and build when taking in to account such things as stability/drift/smoothness of regeneration etc.
The frequency selected has also made it easer to design the regen det tuning coil for a lower than normal L/C ratio whilst still maintaning it's ability to go in and out of regeneration across the whole of the tuned circuits bandwidth, the low L/C ratio coupled with a lower than usual grid leak capacitor value masks the effect of G1 interelectrode capacitances when the regen is brought into oscillation and beyond, this results in an almost drift free oscillation as the regen is advanced, a prime requirement for SSB/CW tuning and listening, the other benefit is that by increasing the amplitude of the oscillation even further the effects of strong signal overload are much reduced all this can now be done with almost no drift.
Reason for initial choice of a 500Khz tunable bandwidth:
When tuning all my old comms receivers the one that stands out for me is the tuning mechanism used in the National HRO, for simplicity and resetability it takes some beating considering how long ago it was designed, the HRO when fitted with it's banspread coils tunes approx 500 Khz on a reduction drive of 20 to 1 (10 turns) this gives a scale increment of 1 Khz, this system feels right when operating it even when using the xtal single signal tuning the turns to frequency ratio feels about right.
My initial idea was to use an old HRO drive mech. if possible.
However none of this is set in stone yet, a 1 Mhz bandwidth could still be employed but a compomise has to be made, in order to obtain full regeneration across a 1 Mhz bandwidth while maintaining a low L/C ratio the ratio of the cathode tap to main winding has to be reduced, this tends to lead to a sharper ( less linear) regen action characteristic rather than a softer one (more linear) a way to reduce this is to alter the pentodes operating characteristic by reducing the anode load however this results in less overall AF gain in that stage but this could be compensated for with a better final selection of af amp/output valve (higher gain) another way would be to use an reverse log pot for the regen control.
The Harmonic Generator:
I have a feeling that the harmonic generator/amp is where it will succeed or fail it will depend on how much drive I can get out of the Xtal osc and how selective the tuned anode tank is in the harmonic generator/amp and wether or not it will supply a minimum of 200 mv mixer injection on all the selected harmonics while rejecting the unwanted harmonics down to a level that's insufficient to drive the mixer, bit of a tall order but you never know. I will start off experimenting with a 1 Mhz xtal or possibly use the sig. gen. output and overdrive it.
I managed to tap into the 500 Khz xtal calibrator in the B40D this afternoon and via a lashup tuned circuit I managed to extract the 20th harmonic (10 Mhz) and succesfully tuned the 40 meter amature band I have no idea what the final voltage of this harmonic was but the volume control was only one point down as compared to using the 200 mv from my sig. gen. I tried for the 34th harmonic (17 Mhz) to cover the 20 meter band but no joy, wether that's down to the xtal osc or my harmonic extraction lashup I am not sure....probably a combination of both. It also worked on RAF and Shannon Volmet on 5.4/5.5 Mhz. There were some funny hetrodyning issues but not had chance to look into that yet.
Lawrence.
I have always had a problem with this ever since I can remember so thanks for that.
I need now to explain the main reasons why I initially chose the IF frequency and the IF bandwidth.
The IF center frequency of 2.75 Mhz was chosen as the best all round frequency taking into consideration what is required from the design.
As you will know it's all a trade off between image ratio and frequency stability when choosing the frequency of a tunable IF. From the outset I planned a double tuned RF stage to allow me to reduce the IF for image ratio purposes, the other benefit of a lower IF is that it makes the tunable regen det a lot easier to design and build when taking in to account such things as stability/drift/smoothness of regeneration etc.
The frequency selected has also made it easer to design the regen det tuning coil for a lower than normal L/C ratio whilst still maintaning it's ability to go in and out of regeneration across the whole of the tuned circuits bandwidth, the low L/C ratio coupled with a lower than usual grid leak capacitor value masks the effect of G1 interelectrode capacitances when the regen is brought into oscillation and beyond, this results in an almost drift free oscillation as the regen is advanced, a prime requirement for SSB/CW tuning and listening, the other benefit is that by increasing the amplitude of the oscillation even further the effects of strong signal overload are much reduced all this can now be done with almost no drift.
Reason for initial choice of a 500Khz tunable bandwidth:
When tuning all my old comms receivers the one that stands out for me is the tuning mechanism used in the National HRO, for simplicity and resetability it takes some beating considering how long ago it was designed, the HRO when fitted with it's banspread coils tunes approx 500 Khz on a reduction drive of 20 to 1 (10 turns) this gives a scale increment of 1 Khz, this system feels right when operating it even when using the xtal single signal tuning the turns to frequency ratio feels about right.
My initial idea was to use an old HRO drive mech. if possible.
However none of this is set in stone yet, a 1 Mhz bandwidth could still be employed but a compomise has to be made, in order to obtain full regeneration across a 1 Mhz bandwidth while maintaining a low L/C ratio the ratio of the cathode tap to main winding has to be reduced, this tends to lead to a sharper ( less linear) regen action characteristic rather than a softer one (more linear) a way to reduce this is to alter the pentodes operating characteristic by reducing the anode load however this results in less overall AF gain in that stage but this could be compensated for with a better final selection of af amp/output valve (higher gain) another way would be to use an reverse log pot for the regen control.
The Harmonic Generator:
I have a feeling that the harmonic generator/amp is where it will succeed or fail it will depend on how much drive I can get out of the Xtal osc and how selective the tuned anode tank is in the harmonic generator/amp and wether or not it will supply a minimum of 200 mv mixer injection on all the selected harmonics while rejecting the unwanted harmonics down to a level that's insufficient to drive the mixer, bit of a tall order but you never know. I will start off experimenting with a 1 Mhz xtal or possibly use the sig. gen. output and overdrive it.
I managed to tap into the 500 Khz xtal calibrator in the B40D this afternoon and via a lashup tuned circuit I managed to extract the 20th harmonic (10 Mhz) and succesfully tuned the 40 meter amature band I have no idea what the final voltage of this harmonic was but the volume control was only one point down as compared to using the 200 mv from my sig. gen. I tried for the 34th harmonic (17 Mhz) to cover the 20 meter band but no joy, wether that's down to the xtal osc or my harmonic extraction lashup I am not sure....probably a combination of both. It also worked on RAF and Shannon Volmet on 5.4/5.5 Mhz. There were some funny hetrodyning issues but not had chance to look into that yet.
Lawrence.
