How the series feedback of DRO works?

You should give a reference where the circuit is described as "series feedback".

I think applying the term for this circuit needs at least further explanation. Feedback in an oscillator must be positive feedback, it's not obvious at first side how a passive two-pole can achieve positive feedback.

It does in fact, if connected to a respective negative real impedance. Negative impedance are usually analyzed as an oscillator topology of it's own, in contrast to positive feedback circuits. The negative impedance of the transistor base port is created by parasitic elements like package inductance in combination with the fT pole.

Its from Microwave engineering by David M Pozar.I would attach an implementation photo of such a resonator.You can see that there is no connection from the output to the input.

There isn't any feedback in that circuit schematic.There should be a transmission line at the emitter.

And why would there be a transmission line at the emitter?

I'm not sure if the photo corresponds exactly to the schematic in post #1, there seems to be an additional (emitter connected?) transmision line.

The post#1 schematic suggests an oscillator based on negative impedance rather than external feedback.

I agree, the above schematic/picture is not series feedback DRO type.

DRO with the resonator in series configuration would be something like this:
http://jaunty-electronics.com/blog/w...llator_DRO.gif

That is called parallel feedback,not series.You can google that.

I notice that the circuit topology in post #1 is designated "series feedback" in literature, e.g. http://www.hfoscillators.com/apskhan.../1987_DROs.pdf

It often uses a source connected transmission line or series inductor. But as already said, parasitic elements can be sufficient to fulfill the oscillation condition.

I don't think that harping on about terms will bring you far. There are different feedback terminologies used in different application ranges, e.g. the more descriptive series/shunt terminology in analog circuit design. RF circuits are however dealing with ports and impedances or S-parameters. In this application range, "series feedback" apparently refers to a topology where in- and output signal of the feedback path are using the same port.

I notice that your initial post is asking "why it's called ...?" I believe, the more interesting question is "how does it oscillate?". If you understand why, you don't necessarily need to answer the first one unless you are writing a text book or preparing a lecture.

For me, negative impedance is the intuitive way to understand the circuit operation.

The DR coupled with microstrip line resonates and the transistor acts as amplifier.There I also had some doubts because I designed the output matching section for oscillation (not for stablization as in amplifier.)And yes discussing the terminology wouldn't lead anywhere.

I think the link posted by FvM explain very well all the theory of the series feedback DRO.
From what I know, the output impedance matching is more important (and necessary) in series feedback DRO than in parallel type.