28-05-2012, 11:50 PM
Thanks Al, originally my idea was for the receiver to cover the whole of the shortwave spectrum up to 30 Mhz, hence the idea of the harmonic generator (HG) for osc stability reasons.
I could go conventional VFO on the osc theme but going much above 15 Mhz I feel stability would be a problem so far as homebrew is concerned, mixing a seperate lower frequency VFO with a selection of xtals for the osc injection would be a reasonable solution it could be done with two valve envelopes.
I think I will go for the HG idea first, I just have to give it a go...
I cannot see that the LPF that the RA17 uses needs to be employed in this case. So far as I can ascertain the RA17 requires all of the harmonics from the HG up to the 32nd to be presented to the harmonic mixer to mix with the output of the ist vfo iin order to produce the 37.5 Mhz needed to feed the second mixer, for obvious reasons any harmonics over the 32nd need to be filtered out in order to reduce the unwanted higher harmonics reaching the harmonic mixer, thus the LPF is employed.
In the case of the regenrodyne HG idea the same range of harmonics are required (up to 32 Mhz) if the receiver is going to cover the shortwave spectrum but we don't need them all at the same time to feed the mixer as is the case with the RA17, we just need one harmonic, that being the one that is selected to set the Mhz range, the plan being to select the wanted harmonic to feed the mixer by way of a tuned tank circuit in the anode of the HG/Amp, this is the reason why I don't think the LPF will be needed as any harmonic above or below the selected one will be rejected by the tuned tank circuit.
Lawrence.
I could go conventional VFO on the osc theme but going much above 15 Mhz I feel stability would be a problem so far as homebrew is concerned, mixing a seperate lower frequency VFO with a selection of xtals for the osc injection would be a reasonable solution it could be done with two valve envelopes.
I think I will go for the HG idea first, I just have to give it a go...
I cannot see that the LPF that the RA17 uses needs to be employed in this case. So far as I can ascertain the RA17 requires all of the harmonics from the HG up to the 32nd to be presented to the harmonic mixer to mix with the output of the ist vfo iin order to produce the 37.5 Mhz needed to feed the second mixer, for obvious reasons any harmonics over the 32nd need to be filtered out in order to reduce the unwanted higher harmonics reaching the harmonic mixer, thus the LPF is employed.
In the case of the regenrodyne HG idea the same range of harmonics are required (up to 32 Mhz) if the receiver is going to cover the shortwave spectrum but we don't need them all at the same time to feed the mixer as is the case with the RA17, we just need one harmonic, that being the one that is selected to set the Mhz range, the plan being to select the wanted harmonic to feed the mixer by way of a tuned tank circuit in the anode of the HG/Amp, this is the reason why I don't think the LPF will be needed as any harmonic above or below the selected one will be rejected by the tuned tank circuit.
Lawrence.
