Nick's video appears above.
First, let me say FB Nick. It is nice to see you making use of the AGC amp designed by Wes and Bob, using the board from Todd's Mostly DIY RF, using a mix of homebrew pads and Me-Squares from Rex, and finally the Franklin Oscillator that we spent so much time talking about on SolderSmoke.
But here are some comments:
-- I still don't think you need that RF amplifier in front of the mixer. And I suspect you would be better off without it. We did not use one in our high school 40 meter project, and never missed it. In fact, on one version of the high school receiver I even put in a simple 10k pot as an attenuator (no RF amp). Even up on 20 meters, I do not have an RF amplifier ahead of the diode ring mixer on either of the Mythbuster rigs I have built. Nick, maybe experiment a bit more and try the receiver just going from the BP filter into the mixer and see what happens.
Note that Wes W7ZOI DID NOT have an REF amp ahead of the diode ring mixer in his original 1968 40 meter Direct Conversion receiver (the one that launched the solid-state DC recevier revolution):
-- The Franklin oscillator is an interesting, but complicated circuit. The gimmick is, well, gimmicky. Here is the thing: You can achieve similar levels of stability using simple conventional, single transistor oscillators. We dispensed with the variable capacitors, and used PTO--style variable inductors. They worked fine. This Franklin oscillator still does seem to drift a bit, right? I would ground the board to the inside of the metal box.
I would also try putting all the stages on a single ground plane. This might help.
Thanks very much Bill! 73, Nick M0NTV
ReplyDeleteFranklin Oscillator:
ReplyDeleteFirst of all Kudos to Monty. Good work. I don't want any of these blog comments to take away from his work and passion.
While the Franklin oscillator offers potential advantages ( namely greater power gain from the 2 active devices + strong isolation of the LC tank circuit and therefore higher tank QL) -- its purported frequency stability in Ham circles elevated to a full-on urban legend by the late 1990's. Many claims seem based on misunderstanding, hearsay, hyperbole, expectation bias -- and not actual measurement.
Bill, I applaud the blog post where you investigated the history of this topology in amateur radio. Some use in microwave also occurred and an evolved variant using 2 FETs was awarded a patent in 1987 -- S. Kaltenecker, R. E. Stengel, R. T. Enderby and J. S. Irwin, Dual FET Oscillator, U.S. Patent
4,785,264 (November 1988).
My personal (and often very flawed + biased) hypothesis --- is that the purported Franklin stability myth started from this cited book:
F. Brown, ‘Stable LC Oscillators’, RF Design (March 1987); reprinted in G. A. Breed (Ed.), Oscillator
Design Handbook, Englewood: Cardiff Publishing (1991), pp. 76–80.
I borrowed + read the Brown book in the 1990s --- and sadly my old brain forgets things now and then. However, this book stated something to the effect that when carefully biased + adjusted for an optimum DC supply voltage, Franklin frequency stability with respect to DC supply voltage variations can be maximized -- and the Franklin may become even comparable to many crystal oscillators.
From my reading + understanding, they seemed to refer to immunity to DC supply voltage variations - nothing else. However, some people just latched onto this 1 statement ......"Franklin may become even comparable to many crystal oscillators"...... and repeated often enough, a myth arose.
A carefully regulated power supply may achieve similar results in many other oscillator topologies.
I suggest builders carefully regulate and properly filter all DC supplies used for oscillators as a 'first line' of action to minimize frequency/temperature drift.
Again -- kudos to Monty.
Thank you Bill !
Todd--VE7BPO--
Amendment - Sorry, I wrote Brown "book", instead of writing 'periodical'. Apologies--Todd
ReplyDeleteAn RF amp preceeding the mixer could be useful to 1) make up for the insertion loss imposed by Nick's bandpass filter (ZOI's antenna input is only single-tuned), and 2) to provide the mixer with a stable 50 ohm input--all the easier with a TIA amp. It may be that if Nick finds the amp useful, though, it would be near its lowest gain setting. Also, inasmuch as the amp itself could negatively-impact the overall dynamic range, it could be a good idea to use low-noise transistors--such as the BF106 (NF 2.1dB)--in place of the 3904s (NF 5dB).
ReplyDeleteI defer to E.H. Armstrong: "Men substitute words for realities, and then argue about the words".
ReplyDeleteM0NTV's receiver sounds great --has excellent DCR "presence". I didn't see a frequency drift issue in the video. Optionally, if one desires, replace the variable trimmer (not the best TCC) and the gimmick cap with fixed NP0? Also get all oscillator's RF nodes up onto an elevated single-sided PCB? Double-sided PCB can ruin frequency stability.
ReplyDeleteWe should think of the LC oscillator as an amplifier with feedback (pretty obvious). When it comes to frequency stability, topology does not matter as much as the design's tank coupling, component selection and construction. LC Oscillator design is much less about Colpitts/Hartley/Clapp (Topology) and more about Bode/Leeson/Barkhausen (Analysis). Temp stable components and construction are essential. I just use the LC Franklin because it works effortlessly for me. I have in my HB 2Meter FM synthesized transceiver. But I don't for XTAL oscillators. Use whatever works for you.
As to the RF preamp, the main purpose I see (at 40 meters) is providing reverse isolation to prevent tunable hum. If I don't see LO <<-50dBm at the antenna port, it occurs, especially with all the AC adaptors and appliances. As to the noise figure, at 7MHz, Nick's preamp is fine. Look at ITU-R P372, especially Figure 2. Plenty noisy at 7MHz!
Re-read this thread on my big screen PC. Yikes – to my horror – during my early post, I fumbled with my new cell phone spell checker and converted Nick’s call sign M0NTV into “Monty. “ My apologies to Nick and others.
ReplyDeleteNot impressed with AI-supported spell checking – it’s difficult enough to view Blogger comments on a cell phone when your an old far..... Best! Todd--VE7BPO--