Cannot get full power out of HMC7357


We am having trouble getting full gain and the maximum RF output power our HMC3573.    I am operating the device at 8447 MHz, and am biasing the device with Vdd = 8.1 VDC and a drain bias current of 1200 mA.   I am only able to get 27.1 dB of gain and +33.6 dB of RF power out of the device and I would like to get over 3 W out of the device, as is shown in the datasheet.   We have also purchased an evaluation board for the device and can not get full power out of that either.  

I have verified that the gates and drain pins are adequately decoupled and that the drain voltages do not droop when RF power is applied.   The device is mounted on a 30 mil RO4350C core with solid ground plane on the bottom that is then clamped to an aluminum block with thermal grease between them.   There appears to be adequate heat sinking occurring since the block heats up well and no loss of power is seen as the device and block heats.

While I am not seeing full power out of the device, when I increase the RF input power and record the output power and drain currents, I do get similar curves as shown in the Power Compression @ 8 GHz plot on page 5 of the datasheet.

Any ideas why we are unable to get full gain and power out of the device, especially the one mounted on the evaluation board that we purchased?   What else should I be checking?



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