Every builder, I suppose, learns in their own way — I believe that making and measuring real circuit’s yields dividends par none. Hats off to the SPICE kings — simulation might be the next best thing for those who lack the instruments needed to measure data and make comparisons — and for getting starting values for an experiment. To me, however — real versus simulated life including electronics just feels better.
Above — Jupiter Block Diagram
20.1 MHz VFO
With some oscillator details gleaned from Jim Sky
, my target receive frequency = 20.1 MHz.
Jim shared that we’ll want to avoid WWV 20 MHz and also need to occasionally steer around any nearby carriers.
So I set out to make a frequency agile oscillator and struggled, sputtered and got beat up in the process. Old Murphy, stupid mistakes, bad parts and the-like tangled up nearly every LO experiment. As a result I didn’t build the best possible LO due to fatigue and frustration — but at least I felt reinvigorated to explore mixers.
My LO strategy involved mixing a 16.93 MHz xtal oscillator ( xtal Q = ~100K ) with a 3.121 to 3.216 MHz Hartley L-C VFO. I planned to use the EMRFD Figure 4.24 method to extract low noise + distortion from the xtal oscillator and mix it with the VFO signal in a Gilbert cell mixer like the NE612.
A reader sent me 4 NE612s last year — it turns out all of them were fried. Sadly, I didn’t suspect these mixers until much in-situ debugging —- wasting parts + time. I didn’t want to wait until fresh NE612 mixers arrived and set out to make a homebrew mixer on the bench.
First I’ll show the L-C VFO:
Above — VFO schematic with a buffer giving a 50 Ω output impedance The output power measured -8.7 to -10 dBm across its tuning range. With practice, it’s fairly easy to make a temperature stable VFO in the 1-3.5 MHz range. I employed light resonator coupling and perhaps overkill DC filtering + output buffering. While testing, I could not pull the VFO frequency with downstream manipulation despite trying hard to do so.