I'm working on a homebrewed receiver and am looking at using the
LT5512
as the mixer. I'm not sure of how to arrange an impedance match and
unbalance to balance conversion over a wide range. Perhaps people
can comment on the following so I can see if I'm on the right track.

The LT5512 datasheet says:

Frequency Impedance
10MHz 18.2 + j0.14
240MHz 18.1 + j2.8
450MHz 18.1 + j5.2

One simple approach seems to be:


50 ohm input - ----------- -----------|---------- IN+
) ( ) ( ___
) ( ) (- ground --- 6 pF LT5512
) ( ) ( |
ground - ----------- -----------|---------- IN-
3:1 1:1
Coilcraft Coilcraft
WBC3-1TL WBC1-1TL

The 3:1 transformer converts the unbalanced 50 ohm input to 16.67
ohms.
The 1:1 center tap transformer converts the unbalanced 16.67 ohms to a
balanced 16.67 ohms. The center tap also supplies a DC ground for
biasing the LT5512 inputs. The 6 pF capactor reactance is:

Frequency Reactance
10MHz 2652.58
240MHz 110.52
450MHz 58.95

placing it in parallel with the LT5512 inputs results in:

Frequency Impedance
10MHz 1/(1/(18.2 + j0.14) + 1/2652.58) = 18.21
240MHz 1/(1/(18.1 + j2.8) + 1/110.52) = 17.58
450MHz 1/(1/(18.1 + j5.2) + 1/58.95) = 16.7

Questions:

1) How off base is my simple analysis?

2) Is there a better way to do this?

3) How significant is the 1.6 ohm mismatch at 10 MHz?

-- John
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