Hello.I have a VCO with +4dbm output at 50 ohm load. What is the min and max input sensitivity of RF IN1 pin 15 OF TSA5511 and what is its impedance?I am thinking of using a resistor attenuator of -5db to feed pin 15 .
In the past when i was feeding pin 15 with -7dbm it wouldnt lock,it needed -6dbm at least to lock.

Download the manufacturer datasheet, it should be all there.
But as far as I know, the input sensitivity is not the same for all frequencies and has a minimum and maximum level for safe operation. They usually express it in millivolts not dBm.

In the datasheet says input sensitivity minimum 12mV ,maximum 300mV.Does that mean that for a TSA5511 can have minimum sensitivity 12mv and another TSA5511 can have a minimum sensitivity of 200mV?

The datasheet doesn't say input sensitivity, it says input level. Hence the guaranteed minimum sensitivity is 12 mV and maximum input level is 300 mV. The typical but not guaranteed sensitivity, which will be much lower than 12 mV, probably 1-2 mV, is shown in the curve in Fig. 6.

Just for interest's sake, I tried varying the level into a 5511. At anything over about 4mV, it would lock reliably. It didn't seem to matter if I gave it 500mV - it still worked OK, but the waveform looked clipped somewhat, suggesting that there's some sort of crude diode clipping protection on the input (probably Schottky Diodes, at a guess). I couldn't find a convenient way of measuring its input impedance, but it would appear to be in the region of a few 10s of kΩ - so it shouldn't present any kind of significant load, unless you're trying to connect some very high impedance / low output exotic oscillator.....

Considering the age of this device, it's a marvel of the chip-designer's art. There are a few things that I don't like about it - particularly the strange way that some of the configuration is done - but it's an excellent chip in most respects.

However - I still prefer my discrete CMOS approach, because I can tailor it specifically to function, just by changing a few components, and it's generally cheaper than the PIC & PLL combination.