IQ controls - different formula from datasheet

Question

Good afternoon, 
 
 From the datasheet at hmc631 (v00.1007) , the formulas 
 Vi = Vmi + G/Gmax * cos(theta) 
 Vq = Vmq + G/Gmax * sin(theta) 
 are the reference to be used to set Vi and Vq, once Vim and Vqm are known, or for a rough first sweep, assumed to be 1.5V. 
 
 However, I have several HMC631 installed on a PCB, where the relationship appears to be different: I get the highest gains for [Vi = 0.5V, Vq = 1.5V] (phase = 180) but not for [Vi = 2.5V, Vq = 1.5V], there is a difference of around 6dB between the two values. 
 Moreover, sweeping all pairs of values between 0.5V and 2.5V (even those that do not come from the formulas), the maximum span I can get is ~11dB, not the 40dB advertised in the datasheet. Below an example: 
 
 Data on the left appears to be more regular, but I did not want to get too close to the absolute maximum rating, moreover the DAC I am using can't even go negative. 
 
 I am generating the signal using a NanoVNA-F V2, and with the same device I am also measuring the output. 
 
 Any ideas what may be going on?

Thanks!

enash · Accepted Answer

I don't follow the plot you pasted in but I suspect that the issue is related to offset voltages inside the part.

In order to maximize the range of the device you need to find the (externally applied) I and Q voltages that give you the lowest possible output. I proposed the procedure below in response to a similar post.

The I and Q input levels are 0.5 to 2.5 and are referenced to these Vmi and Vmq voltage which are both 1.5V. So if you set Vmi and Vmq each to 2.5V, you will get a phase of 45 degrees. If you set Vmi and Vmq to 0.5, you get a phase of -135 deg. The datasheet does not talk about the part having any offset errors but I'm sure that it does. My guess is that it is these offset errors that are causing phase errors at low gain settings. So you have to find the voltage that you apply to the I and Q inputs that will results in no signal at the output, that is, you want to set your output level to zero. Here is how to do that calibration.

Apply a fixed RF input power level.
Set the Q voltage to 1.5V (that is effectively 0V)
Next Adjust the I voltage from 0.5 to 2.5. As the voltage approaches 1.5V, the output will drop. At the point at which the output level hits its trough (minimum), stop adjusting the I voltage and hold it at whatever level yielded the mimimum output level.
Now adjust the Q voltage from 0.5V to 2.5V. Again you will see that at some voltage close to 1.5V, the signal level will hit a (lower) minimum level.
So if the I and Q voltages that yielded the two minimum levels were, say, 1.555 and 1.444, then those become your "zero points". The get a phase of 45 deg, you would add a fixed voltage to both levels.

enash · Answer

If you are not going to use one of the inputs, I would tie it to the mid point voltage. If you are only using the I or the Q control, you are exercising the part in only one of its two dimensions. So if the Q voltage is 0, then all you can to is adjust the gain at a fixed angle of 0 degrees if the I voltage is positive. If you make the I voltage negative (relative to the mid point), now you get to adjust the gain with the signal at -180 degrees. This is not very useful if you are trying to make a vector modulator or a phase shifter.

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IQ controls - different formula from datasheet

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