problem with prescaler IC (MC12022SLB)

Hi there.
I'm setting up a prescaler IC and I'm using MC12022SLB. Everything seems to be working very well. But when the input pin of the IC is not connected, there is an output wave around (3-8 MHz) on the oscilloscope. I dont know where this frequency is coming from? Any clue?
I really appreciate your help.

Assuming you have the supply lines decoupled and your construction is good at hight frequencies, it will just be 'normal' instability of the input amplifier when it's input is floating. Do you have the input coupling capacitors fitted and the 50 Ohm resitor across the input when you see these frequencies?

Brian.

yeah I used the coupling caps and the 50ohm resistor according to its datasheet. Now that I observe the output signal closely I can see that the 8Mhz signal is always there, being added to the output signal. Just when the input signal is connected to ground the signal is gone.

I have not used that particular device before but I have used the MC12034 which is similar. They can be sensitive down to just a few mV so keeping them stable can be difficult. You will probably find that when a signal within it's design range is applied at the input, it will swamp the oscillation and drive the counter chain and amplifier out of it's linear region so it is less susceptible to noise.

Have you tried changing the division modulus to see if the frequency changes?

It was a long time ago but I think the last time I needed a 1.1GHz prescaler I used the Fujitsu MB501 rather than Motorola devices.

Brian.

a lot of prescalars are not unconditionally stable if there is no input signal. Read the data sheet carefully for solutions.