# Input impedance

I'm trying to figure out what the input impedance of the ADF4113 is.

Looking a figure 15, I'm not sure how to read that.

In my setup input is on RFinA and RFinB if connected to GND via a 100pF capacitor (HF to GND).

If the Bias generator is a current source it is HF open, and then the impedance would be 1k ohms.

But the figure says there is 1.6V at the generator out, which made me think it probably works more as a voltage source, and therefore is HF short, which makes the impedance 500 ohms.

So to figure this out, I tried to measure the impedance, by connecting a generator with a known source resistor, and from that my guess is 1k ohms.

But could some one help me on how get the impedance from the datasheet?

• @Jacob - I've added a link to the datasheet, so that people know what you're talking about. Not sure if your figure 15 references this document, though. Apr 11, 2011 at 8:14
• @steven, Ah thanks. :-) You are right, I should have done that. Apr 11, 2011 at 10:40

Figure 5 has S-parameters for ADF4113 RF input. You can calculate input impedance from these.

$R=\dfrac{Z_0(1-\rm{MAG}^2)}{1+\rm{MAG}^2-2\cdot\rm{MAG}\cdot\cos(\frac{\rm{ANG}}{360}\cdot 2\pi)}$

$X=\dfrac{2\cdot\rm{MAG}\cdot\sin(\frac{\rm{ANG}}{360}\cdot 2\pi)\cdot 50}{1+\rm{MAG}^2-2\cdot\rm{MAG}\cdot\cos(\frac{\rm{ANG}}{360}\cdot 2\pi)}$

$Z = \sqrt{R^2 + X^2}$

Where $Z_0 = 50$

I hope I latexed the formulas correctly, just in case they are from this page.

• Hi. Thanks for your help. Okay thats how you do it! ....I got some reading to do. ;-) Apr 11, 2011 at 10:10
• @Jaroslav I was trying to shrink the equations a bit and started to wonder: Do $M$ and $\varphi$ relate somehow to the S-parameters?
– jpc
Apr 11, 2011 at 10:17
• @jpc please don't, it become completely nonobvious how to calculate the values. Apr 11, 2011 at 10:21
• @jpc MAG and ANG are the values from the datasheet, e.g. MAGS11 and ANGS11. Apr 11, 2011 at 10:24
• @jpc actually I think the convention is to leave the multiply sign out altogether, but I am not sure it would help readability ... Apr 11, 2011 at 12:20