I am trying to find the associated input impedance to the SA605 mixer RF inputs. I have found several App notes associated with the Chip that discusses the design. On the datasheet they present the following:

My question is how do I use this information to find the associatedenter image description here impedance at 88.1MHz?


Use the 50MHz and the 100MHz data. Note the capacitive portion remains at 2.5pF. Then linearly interpolate between 50 and 100MHz for the Resistance.

The resistance drops ~300 ohms per 10Mhz, as frequency increases. So just add 300 to 3100, or 3,400 ohms at 88 or 90Mhz.


can we use the Smith Chart for this? No. The center of the Smith Chart is (usually) normalized to 50 ohms. Note the plot does not cross thru the Center; in fact, the curve is a looping arc in the lower half of the chart ---- a classic capacitive curve.

At high frequencies, the Z is 25 ohms. Given the Z of 1pF at 1Ghz is -j159 ohms, we'd expect -j80 ohms at 2GHz; the chart (and the left table) shows 21ohms, or about 4pF.

At low frequencies, the curve is in right portion of the chart ---- the classic high impedance region, and a small capacitor is excellent component to be high impedance at low frequencies. The curve is very close to the border of the chart, so there is little resolution of the actual Z, except the Z is "high". And the right table gives values: 3,000 or 4,000 ohms.

  • \$\begingroup\$ are doing this by inception or are you using the smith chart? \$\endgroup\$ – learnmore Mar 9 '18 at 3:53
  • \$\begingroup\$ @learnmore, don't you see the table of values? Are you familiar with mathematical concept of interpolation? :D \$\endgroup\$ – Ale..chenski Mar 9 '18 at 6:10

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.