30-04-2012, 07:42 PM
All this horrible weather has prompted me to do a bit more experimenting with my birds nest regenerodyne, I have been working on the tunable regen stage, originally I used an old SW coil that was originally designed to tune to my design center freqency (3 mhz) at the top end of the tuning capacitor setting (High L/C Ratio) it worked ok in reception terms but tended to drift a bit and pull when tuning the RF stage and/or going in and out of regen.
I decided to go for a low L/C Ratio so looked for another spare coil, I eventually found one that tuned to around 5.5 mhz with the tuning capacitor I have fully engaged, I decided to screw this frequency down by using a couple of larger value fixed capacitors, in the end the coil ended up with two 500pf micas across it (.001 uf), this gave me a center frequency of 2.75 mhz which was near enough to the initial 3 mhz idea and a nice low L/C Ratio this greatly improved the drift characteristics and when receiving SSB transmissions there was no discernable pull when going in and out of regen and only the slightest amount of pull when tuning the RF stage.
The tuning of this latest arrangement was done with the tuning capacitor I have been using all along in series with a padder.
Because I don't have a reduction drive at the moment I padded the tuning capacitor to give a 100 khz spead, the final design will have a 500 khz spread through a suitable reduction drive. Interesting looking these old coils I have, most of them were originally in the anode circuit of an RF amp, the tuned secondary going to the grid of a mixer, the primary winding in inductance terms is very large compared to the secondary, in fact they are wave wound and look similar to MW or LW coils, the reason for this as far as I can acertain is that the large inductance combined with its own self capacitance and stray circuit capacitance resonates just below the lowest frequency of the secondary tuned circuit thus compensating for the lower gain due to the secondary having a low L/C Ratio when tuned to the low end of its band.
In the above arrangement the regen winding has to be quite closely coupled to the tuned winding to get it to go into full oscillation (a must for strong SSB signals) across the designed 500 khz spread.
I have successfully tried different RF coils and can receive SSB on all bands tried, 3.5 mhz through to 18 mhz, I have not tried Top Band yet as there is normally nothing on there in my location but being a low band (1.7 mhz) I cannot forsee any problems should I choose to do so.
The advantage of using a first converter followed by a tunable regen came into it's own when trying the different ranges as the regen action and the tuning rate remain the same which ever band you are listening to.
I also did some checks for image rejection, I adjusted the center tuning frequency of the regen so that when the local osc frequency (my sig gen) was set to be above the regen frequency and tuning the 7 mhz amature band the image frequency (approx 5.5 mhz higher) corresponded to a very strong commercial broadcast up around 12 to 13 mhz, there was no discernable breakthrough the 5.5 mhz spacing between the amature band and the commercial station doing its stuff, the other advantage of this system on that particular band is that to change reception from the amature frequencies to the commercial frequencies on the higher image just needs the RF tuning to be retuned and peaked, you can obviously go the other way and set the local osc below the desired frequency to be received then tune the RF down again to see what's down there.
I was going to try a tapped cathode circuit for the regen as well but I will leave that for the moment.
Hope any regen builders find some of this usefull.
Lawrence.
I decided to go for a low L/C Ratio so looked for another spare coil, I eventually found one that tuned to around 5.5 mhz with the tuning capacitor I have fully engaged, I decided to screw this frequency down by using a couple of larger value fixed capacitors, in the end the coil ended up with two 500pf micas across it (.001 uf), this gave me a center frequency of 2.75 mhz which was near enough to the initial 3 mhz idea and a nice low L/C Ratio this greatly improved the drift characteristics and when receiving SSB transmissions there was no discernable pull when going in and out of regen and only the slightest amount of pull when tuning the RF stage.
The tuning of this latest arrangement was done with the tuning capacitor I have been using all along in series with a padder.
Because I don't have a reduction drive at the moment I padded the tuning capacitor to give a 100 khz spead, the final design will have a 500 khz spread through a suitable reduction drive. Interesting looking these old coils I have, most of them were originally in the anode circuit of an RF amp, the tuned secondary going to the grid of a mixer, the primary winding in inductance terms is very large compared to the secondary, in fact they are wave wound and look similar to MW or LW coils, the reason for this as far as I can acertain is that the large inductance combined with its own self capacitance and stray circuit capacitance resonates just below the lowest frequency of the secondary tuned circuit thus compensating for the lower gain due to the secondary having a low L/C Ratio when tuned to the low end of its band.
In the above arrangement the regen winding has to be quite closely coupled to the tuned winding to get it to go into full oscillation (a must for strong SSB signals) across the designed 500 khz spread.
I have successfully tried different RF coils and can receive SSB on all bands tried, 3.5 mhz through to 18 mhz, I have not tried Top Band yet as there is normally nothing on there in my location but being a low band (1.7 mhz) I cannot forsee any problems should I choose to do so.
The advantage of using a first converter followed by a tunable regen came into it's own when trying the different ranges as the regen action and the tuning rate remain the same which ever band you are listening to.
I also did some checks for image rejection, I adjusted the center tuning frequency of the regen so that when the local osc frequency (my sig gen) was set to be above the regen frequency and tuning the 7 mhz amature band the image frequency (approx 5.5 mhz higher) corresponded to a very strong commercial broadcast up around 12 to 13 mhz, there was no discernable breakthrough the 5.5 mhz spacing between the amature band and the commercial station doing its stuff, the other advantage of this system on that particular band is that to change reception from the amature frequencies to the commercial frequencies on the higher image just needs the RF tuning to be retuned and peaked, you can obviously go the other way and set the local osc below the desired frequency to be received then tune the RF down again to see what's down there.
I was going to try a tapped cathode circuit for the regen as well but I will leave that for the moment.
Hope any regen builders find some of this usefull.
Lawrence.
