PM5324 oscillator

A recent question in a CircuitsOnline thread (link here), raised my interest in the oscillator section of this RF sweep generator. The oscillator section is around the (ECO) emitter coupled oscillator topology. This topology will oscillate at almost any frequency with almost any LC combination and is not critical with transistor types (of course using LF transistors won’t work at UHF..). The disadvantage is that the current through the common emitters needs to be carefully adjusted. if not the output won’t look like a sine wave output and there will be many harmonics (though this could be useful at sometimes). Also, because the transistors will saturate, the upper frequency is limited.

Emitter-couple- oscillator (ECO)

Not so, the PM5324 oscillator!

The actual oscillator is build around Q301 and Q302. C527 is the feedback capacitor. R510 and R515 are the bias resistors and do not play a role in the actual oscillation. The LC section is connected of the top end of R519

The Automatic Level Control (ALC) section is build around D404-D407, Q305 and Q306. Note the completely symmetrical layout. Q305 and Q306 form a difference amplifier with a current output. If potmeter R512 with R511 is set to 5k1 just like R521), the section around the Q305 and Q306 is in balance and there will be a current flowing into the collector of Q305, this will get the oscillator going. Q303 and Q304 act as impedance buffer from the oscillator section to the output (of C533) and of the ALC of C532. The oscillator signal is rectified by D406 and D407. This DC voltage on the basis of Q306 will increase as well, causing it to conduct more. Q305 will be forced to conduct less, however. Thus the collector current of Q305 will decrease and hence the amplitude of the oscillator.
If the position of R312 is slightly altered there will be slight more positive bias on the basis on Q305 than on the basis on Q306 and a higher oscillator voltage is needed to get the bridge into balance again. R312 can thus be used to set the output level of the oscillator.

Note that because of the way the oscillator is powered the parallel LC section in Q301 oscillator can be connected to ground. This is beneficial in the construction and will help to diminish the hand-effect as well.

Now if there is thermal tracking between D404/D405 and D406/D407 and the transistors in the difference amplifier Q305/Q306, the ALC section is to some extend insensitive to changes in ambient temperature. Which is a nice property for a test oscillator!

Need little oscillator, huh?

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