Hi,
I am using an AD HMC505 VCO (spec. 6.8-7.4 GHz) as the VCO in a PLL (ADF4108) to generate a LO tone.
The desired tone frequency is at 7.5 GHz. While this is out of the spec'ced operating range for the VCO, the tuning curves and output power in the datasheet indicate that the device is able to operate at this tone with an output power of ~11 dBm with a tuning voltage of 7V. Originally the HMC532 VCO was spec'ced but unavailable to purchase so the HMC505 was used in place.
Problem Statement
In a test of the VCO (unlocked / PLL disabled), the required tuning voltage to output 7.5 GHz was found to be 9.5V and the output power was ~1 dBm, or ~7 dB lower than minimum from the datasheet.
With the PLL enabled, the ADF4108 is pulling Vtune of the HMC505 to the rail limit of 11V, which results in a tone at ~7.6 GHz. The output power is ~ -2dBm. This results in too low power at the RFin of the ADF4108 and it does not lock.
I am concerned by the low output power at the HMC505 output.
Existing Debugging
The measurements are taken at J18 using a Keysight FieldFox SA and conformable coax with ~ -1dB of loss which I have accounted for in the above power measurements.
I have validated the SA using a signal source to check that the measured power is valid.
The power to the VCO is good (3V0) but it is drawing only 73 mA of current, compared to the nominal 80mA at a 3V3 supply.
There is no short from the RF output to GND (in case of a short under the package). Can you provide any advice on how to proceed - whether this is likely a faulty IC or other potential issues.
Any advice on additional troubleshooting appreciated.
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Kind regards, Jonathan
