### Q

I have some questions on AD8368. 1, in figure 39, why the Power in value isless than -20dbm, the EVM will be getting worse? 2, Table 6, how to decide the

value of C20,C2 and C4? Since they are all DECL capacitors, how to calculate

each capacitor value? 3, How to calculate the value of C23?

### A

To your questions on the AD8368: 1, Among other things, EVM is a function ofSignal to Noise Ratio and will degrade at low input powers. One way to look at

it, is SNR = Pr/(NoB), where Pr is the received power of the system, No is the

noise causing the error in EVM and B is the bandwidth of the signal. Assuming

noise always stays the same, if the received power decreases, the SNR decrease

and the noise, No , seemingly contributes more to the EVM, since EVM is

basically the measure of how much the original vector is in error. 2, Here is

an un-published revision of Table 5 in the datasheet: 380MHz C20=56pF C2=56pF

C4=33pF; 240MHz C20=150pF C2=150pF C4=68pF; 140MHz C20=470pF C2=5.6pF C4=68pF;

70MHz C20=1nF C2=5.6pF C4=1nF. 2, To set C23, you need to know what your high

pass corner frequency needs to be on the output offset control loop. An

equation on page 12 in the datasheet will correlate frequency to CHPFL (C23)

value. C23 = 10nF is a good all around cap value, and should serve you just

fine in most situations. From my experiments on the AD8368, not getting these

capacitor values exactly correct should change the performance all that much.

Start with 10 nF on C23 and whatever frequency you’re operating at, pick the

appropriate values for C20, C2 and C4.