Can AD8346 be operated at frequencies below 800 MHz?

Question: FAQ: Can AD8346 be operated at frequencies below 800 MHz? Answer: For starters, there is only one good reason to try and use the AD8346 at frequencies below its specified 800 MHz minimum. And that reason is the fact that the device has relatively low supply current (45 mA) and operates from supply voltages as low as 2.7 V. One alternative idea would be to use AD8345 which is essentially a down-banded version of AD8346. It is pin-compatible with AD8346 and is specified to operate from 140 MHz to 1 GHz. It does consume more current than AD8346 (65 mA vs 45 mA) but also operates with supply voltages down to 2.7V. If power consumption is not an issue, there are a number of newer devices, ADL5370, ADL5375, ADL5385 and ADL5386 which cover various portions of the sub-800 MHz band and some of which operate all the way down to 50 MHz. But let's come back to the original question. The only real issue with operating the AD8346 below its specified minimum frequency of 800 MHz is that the sideband suppression will start to degrade. As you can see from the plot, Sideband Suppression appears to be tending upwards as you go below 900 MHz. But it's starting out from a very good place (-46 dBc at 900 MHz). So you can operate the device below 800 MHz, especially if you have the capability to the skew the I and Q inputs from your DAC to compensate for the degrading sideband suppression. If you can live with sideband suppression in the mid-30s, I think that it will be fine out of the box (i.e. uncompensated). It is hard to say what the lowest frequency of operation really is as we have not characterized the device much below 800 MHz

Temperature Drift of IQ Modulator LO Leakage after Calibration at 25 degC

Question: In the attached plot (labeled Figure 10) from the ADL5375 IQ Modulator Datasheet, LO Leakage degrades significantly vs. temperature at higher frequencies after compensation at 25 degC. What is going on here? Answer: It is important to understand that this plot shows the change in LO Leakage over temperature after compensating the LO Leakage at 25 degC. So as you can see from the plot, at all frequencies, the LO Leakage at 25 degC looks good. This is simply because the LO Leakage is being compensated at each of those frequencies (i.e. each frequency point has different calibration coefficients). What happens next is that we change the temperature while holding the 25 degC calibration coefficients constant (the calibration “coefficients” are simply offset levels on the I and Q inputs). So why does the post-calibration LO Leakage degrade over temperature so much above 2.5 GHz? To understand this we need to understand the LO Leakage mechanism a bit better. LO Leakage is generally assumed to be caused by internal offset voltages on the I and Q inputs. This is why we apply compensating offset voltages to these inputs to reduce the LO Leakage. However, there is also an LO Leakage path between the LO input port to the RF Output (see attached image). So the net LO Leakage at the output is the vector sum of the signals leaking through these multiple paths. The LO Leakage path from the LO Input to the RF Output has some important characteristics. 1. As the frequency changes, the amount of leakage through this path will change with the leakage level tending to increase with increasing frequency. 2. As the temperature changes, the amount of leakage through this path will change. The attached plot (labeled Figure 9), shows how uncompensated LO Leakage changes with frequency and temperature. The increase in LO Leakage with frequency is consistent with the above explanation. If we compensate LO Leakage at, say, 1 GHz, we are compensating for LO Leakage that is mostly caused by I and Q offsets with LO-to-RF Leakage playing a less significant role. When we cycle over temperature, the 25 degC compensation tends to hold up well and the LO Leakage at +85 degC and -40 degC does not change by much (see plot labelled Figure 10). At 4 GHz, things look different (referring again to the plot labelled Figure 10) . At this frequency, the initial (un-compensated) LO Leakage is dominated by LO-to-RF leakage and I/Q offset based LO Leakage is less significant. Even though most of the LO Leakage is not coming from the I and Q input offsets, we can still use these inputs to compensate the overall LO Leakage. So regardless of where the original LO Leakage comes from, we can always compensate it by applying compensating offset voltages to the I and Q inputs. However, when we change temperature (or frequency) without re-compensating, the leakage of the LO-to-RF path changes and this causes the overall LO Leakage to change (usually it degrades). So the benefit of compensating at 25 degC quickly disappears, particularly at the highest frequencies where LO-to-RF leakage dominates. You can read more about the topic of IQ Modulator Error Correction in AN-1039, Correcting Imperfections in IQ Modulators to Improve RF Signal Fidelity

AD8345 Pout vs. Vinbb Transfer Function

Question: Why does is the transfer function of the AD8345 (SSB Pout vs. Baseband Differential Input Voltage) not a straight line? Shouldn't the gain of the device be constant vs. input level. Also as the baseband input level doubles from 0.5Vpp to 1Vpp, the output level increases by 7 dB instead of the expected 6 dB. What is going on? Answer: The SSP Pout line is curved because of the logarithmic scale on the right. If you were to change the horizontal scale to logarithmic volts, you would see a straight line until the onset of compression (aternatively if you were to make the vertical scale on the right, linear, it would also yield a straight line). When we plot this data on our newer datasheets, we use a logarithmic voltage scale on the horizontal axis (see example plot below). This, I think, is more useful because it is much easier to identify the device's ouput compression point. You note that as the baseband drive level doubles from 0.5 Vpp to 1Vpp, the output power increases by 7 dB instead of the expected 6 dB. I agree that this is unexpected; it should indeed be 6 dB. I can only assume that it is a slight plotting error. Note that in Figure 16 of the AD8345's Rev. B datasheet, the scale for SSB Ouput Power is off by approximately 3 dB.