VintagePC

05-08-2014, 02:17 PM

Okay folks... here's something I'm hoping some of the seasoned folks here might be able to help with. It's gonna be a wild ride in trying to understand oscillators and some theory, so hold on to your hats! :)

:scratch2::scratch2::scratch2:

Specifically, I'm looking at the implementation of the 6AF4 oscillator on Pg. 4 of this SAMS: http://www.earlytelevision.org/pdf/Blonder-Tongue-99-Sams-259-3.pdf

Here's what I've been able to puzzle out, and I'd like to know if I'm correct in my understanding of the below points (or not). The upper half of the circuit can be ignored since it has no bearing on the LO frequency - if you haven't read the rest of the sams it's just a diode superhet mixer.

1. L9 and L8 - I suspect these function as RF chokes to keep the B+ clean and the RF from back feeding. No impact on frequency.

2. The oscillator design: I suspect it is a colpitts type oscillator based on the similarity to some tube reference designs I have found - but I'm getting confused any time I try to relate components in the SAMS to a reference design (e.g. on Wikipedia, http://en.wikipedia.org/wiki/Colpitts_oscillator) so that I can attempt to calculate frequencies (or component values, based on a known frequency.. which is my ultimate goal, see below.)

3. C7 and C6 are decoupling caps which serve to block the B+ from the chassis, as the other end of the tuning inductor (L3) is attached to the chassis. Since they're in the grid-anode circuit I suspect they still affect frequency by 1/C= 1/C7 + 1/C6 + 1/Ct where Ct is the capacitance of the tuning mechanism, C is the total capacitance of the LC tank circuit.

4. The item labelled as "gimmick" adds some minute capacitance (Ct?) in parallel with the tuning coil to tweak the frequency. If this Ct is several orders of magnitude smaller than C7 or C6, (pF vs µF?) it would then dominate those terms in the inverse equation above... and C7/C6 would therefore be negligible in determining frequency?

4a. Similarly, the output "tickler" is off the cathode of the tube.. Colpitts reference designs have the output from anode to cathode though... which is why I'm not entirely sure of (2).

How am I doing so far? I don't have a significant background in EE but it's a hobby and I do understand most of the basics and am trying to build. ;)

Where I'm ultimately interested in going is determining what modifications to the circuit are necessary to lower the frequency range of the LO. As designed it would span UHF 14-83, minus Ch5 (for superheterodyne downmixing) or 394 - 808 MHz.

Target frequency range is cable 14-83 minus ch5, or 44-500MHz.

However... baby steps... I'm hoping to break it down in a way that is logical for my thought process and then work up to the point where I understand it well enough to attempt that. :)

Thanks in advance, and I'm looking forward to the discussion and the fascinating tidbits people usually have to offer around here!

:scratch2::scratch2::scratch2:

Specifically, I'm looking at the implementation of the 6AF4 oscillator on Pg. 4 of this SAMS: http://www.earlytelevision.org/pdf/Blonder-Tongue-99-Sams-259-3.pdf

Here's what I've been able to puzzle out, and I'd like to know if I'm correct in my understanding of the below points (or not). The upper half of the circuit can be ignored since it has no bearing on the LO frequency - if you haven't read the rest of the sams it's just a diode superhet mixer.

1. L9 and L8 - I suspect these function as RF chokes to keep the B+ clean and the RF from back feeding. No impact on frequency.

2. The oscillator design: I suspect it is a colpitts type oscillator based on the similarity to some tube reference designs I have found - but I'm getting confused any time I try to relate components in the SAMS to a reference design (e.g. on Wikipedia, http://en.wikipedia.org/wiki/Colpitts_oscillator) so that I can attempt to calculate frequencies (or component values, based on a known frequency.. which is my ultimate goal, see below.)

3. C7 and C6 are decoupling caps which serve to block the B+ from the chassis, as the other end of the tuning inductor (L3) is attached to the chassis. Since they're in the grid-anode circuit I suspect they still affect frequency by 1/C= 1/C7 + 1/C6 + 1/Ct where Ct is the capacitance of the tuning mechanism, C is the total capacitance of the LC tank circuit.

4. The item labelled as "gimmick" adds some minute capacitance (Ct?) in parallel with the tuning coil to tweak the frequency. If this Ct is several orders of magnitude smaller than C7 or C6, (pF vs µF?) it would then dominate those terms in the inverse equation above... and C7/C6 would therefore be negligible in determining frequency?

4a. Similarly, the output "tickler" is off the cathode of the tube.. Colpitts reference designs have the output from anode to cathode though... which is why I'm not entirely sure of (2).

How am I doing so far? I don't have a significant background in EE but it's a hobby and I do understand most of the basics and am trying to build. ;)

Where I'm ultimately interested in going is determining what modifications to the circuit are necessary to lower the frequency range of the LO. As designed it would span UHF 14-83, minus Ch5 (for superheterodyne downmixing) or 394 - 808 MHz.

Target frequency range is cable 14-83 minus ch5, or 44-500MHz.

However... baby steps... I'm hoping to break it down in a way that is logical for my thought process and then work up to the point where I understand it well enough to attempt that. :)

Thanks in advance, and I'm looking forward to the discussion and the fascinating tidbits people usually have to offer around here!