Oscillator thoughts

I'm still noodling around with the Neophyte, which is still oscillating terribly off frequency; about 1.2 MHz low from the 40 meter band target for this design.

Today, I tried to adjust the 42IF123 primary coil slug to see how "adjustable" the oscillator might be. It will adjust the oscillator output frequency but not by very much. Here are the results.

T2 slug maximum counter-clockwise (all the way "up"):

• Top end: 6,801.2 kHz
• Bottom end: 6,559.0 kHz

T2 slug maximum clockwise (all the way "down"):

• Top end: 4,993.2 kHz
• Bottom end: 4,822 kHz

A surprisingly large tuning range; about 2.0 MHz of tuning or about 1.8 MHz of shift, although the slug is entirely too coarse to be useful for tuning. As you can see, I still won't be able to get this on the 40 meter band as built.

That got me to thinking about the oscillator. I've poked around the components and construction layout; everything is as it should be. So one wonders: If the device is built properly, could the problem be in the design? Hubris, arrogance, pride, take your pick, but it's an honest question. 

As 40 meter direct conversion receiver, an ideal oscillator should percolate in that band and range across the entire band; 7,000 kHz to 7,300 kHz (I did say "ideal").  The original design from the QST article shows the standard Colpitt's oscillator layout (the amplifier transistor is internal to the NE602 chip and is omitted from my drawing).

WA3RNC's original Neophyte oscillator

Since a Colpitt's oscillator operator frequency is determined by a parallel LC tank circuit, bear with me while I render the complex WA3RNC tank circuit to its reduced form. Note that C12, a 4-365 pF air variable capacitor is set to a mid-range 180 pF for the purposes of this reduction. First, we'll reduce C10, C11, and C12:

Which is then added to the transformer's internal 47 pF capacitor. Simultaneously, I reduce C7, C8, and C9 on the right side of the transformer to yield the following:

Since capacitors in parallel are a simple sum, resulting in the equivalent parallel RC tank as:

Final equivalent tank circuit for WA3RNC's oscillator

Using the equation for parallel resonance, I get a resonant frequency of about 5,700 kHz. Strange, that's just about where my Neophyte operating frequency lies. Is it at all possible that the values for the oscillator NP0 capacitors from the QST magazine article is wrong? I have a spare 68 pF capacitor on the table, so if I add that in series between the C10 and C12, that raises the tank resonance to 7,360 kHz, which may be close enough for me to adjust the span using the transformer tuning slug so that I can cover 7,000 to 7,300 kHz.

Someone out there tell me if I'm flying too high for an unpressurized cabin, or if I'm on the right track, please! Comments?