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Hi, I faced an LC topology that I haven't seen before. (above pic)
Unlike a standard LC tank circuit, L is divided and connected to GND.
To understand it, I have studied many article about LC oscillators.
But, I was not able to find out any hint about this topology. :(
Could anyone help me analyze this circuit?

And I'm also curious about C1,C2(270 pF). I guess it is to block DC current.
Can they influence the capacitance of LC circuit?

Thanks in advance.

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  • \$\begingroup\$ The 270pF do indeed block DC but they are 270pF not say 1nF which gives a clue .They also have an impedance matching role so I would leave them at 270pF. The Osc is just a pushpull version of existing known popular osc circuits .Pushpull has advantages like even harmonic cancellation but with discrete transistors the doubling of parts count is deemed to be not worth it but with an IC the pushpull is practical. \$\endgroup\$
    – Autistic
    Mar 30 '17 at 19:25
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C1 and C2 are just DC block capacitors. L1, C3 and L2 is a pi network filter that provides just the right amount of phase shift to make the oscillater oscillate at the "desired" frequency. Basically it's a form of Hartley oscillator and relies on the oscillator output having some amount of internal resistance as well: -

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It could quite easily be re-configured to use a Colpitts confuguration of Pi network (two capacitors and one inductor): -

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So there is a phase shift of several tens of degrees brought about by the internal driver resistance and one of the inductors and this will be added to the phase shift of the 4.7 pF and the other inductor. The LC phase shift is nominally 90 degrees at perfect resonance but it will likely run slightly off resonance (a phase shift greater than 90 degrees) so that the total phase shift is 180 degrees. Therefore it is highly likely that the circuit internal to the oscillator is just an inverting stage.

C1 and C2 can change value but there will be a small change in oscillator frequency. However, given that the oscillator appears to be a VCO (hence the pins marked VCOA and VCOB) I don't think this will be a big deal.

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If you redraw the LC part of the circuit to connect both inductors L1 & L2 together where they are presently connected to ground, you will see this is a simple parallel LC tank. In this case there are two inductors in series, presumably with no mutual inductance since there is no indication in the schematic that both inductors are wound on the same core or in some way magnetically coupled.

The coupling capacitors C1 & C2 have a very large capacitance compared to the tank capacitor C3 and therefore have a very low impedance relative to the impedance of C3 at the resonant frequency of the LC tank. The amount of affect these coupling capacitors have on the "capacitance of LC circuit" (your words) depends on the composition and output impedance of the "OSC" block inside the SL1710. My guess is they have negligible effect on C3. Yes, they are there to prevent DC from flowing through the inductors - a condition that would detrimentally effect the AC characteristics and operation of the inductors L1 & L2.

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Split L's allow higher SRF and both add in series to parallel Tank cct . THe split ground connection lowers CM impedance to ground to reduce susceptibility of radiation noise ingress.

Alternate approach is shown in datasheet with varactor diode tuning added as well.

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