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schematic Here's a schematic of the VCXO oscillator section from a Siemens S55 mobile phone. AFC signal coming from the processor tunes the crystal via a varicap. There's also a thermistor going back to the processor for temperature compensation. It looks like a Colpitts oscillator, but I have no idea why there are two transistors. The upper transistor base is AC-coupled to ground, which hints a cascode configuration.

Why did they choose to use such two-transistor configuration? What is the role of R955 if AC is shorted to the ground anyway - or rather why is the upper transistor base polarization connected with the signal path? What do C958 and C952 do?

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  • \$\begingroup\$ Some kind of cascode oscillator I guess. Are these real service manual or original design schematics or is something that someone reverse-engineered? \$\endgroup\$
    – Fizz
    Dec 6, 2015 at 14:39
  • \$\begingroup\$ Pretty similar one in a Motorola patent: google.com/patents/US5621362 And another: google.com/patents/US5245298 \$\endgroup\$
    – Fizz
    Dec 6, 2015 at 15:00
  • \$\begingroup\$ @RespawnedFluff It's from the service manual. \$\endgroup\$
    – Michał B.
    Dec 6, 2015 at 17:01

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Invariably, using a cascode like this is to overcome the effects of the miller capacitor in the low transistor - it's collector is held at a fixed bias point by the emitter of the top transistor and, the top transistor's base is fed DC. The fact that it couples via R955 to the crystal is unimportant - it's important that the base of the top transistor is higher than the base of the bottom transistor or it won't be biased correctly.

C952 is in parallel with the varactor and it basically adds to the varactor capacitance thus making the range of the varactor a bit less than if C952 wasn't there.

C958 is there to make that whole varactor setup appear strongly capacitive - without C958 shunting R960 it probably wouldn't oscillate. Why R960 is there at all is the most puzzling but I expect it is to stop the AC waveform superimposed on the DC control becoming too distorted (as it passes through or close to zero volts) because of the potential rectifying effects of the varactor.

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