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Factors that affect the conversion loss of a double balanced mixer:
-Diode insertion loss
-Transformer core losses (conversion loss will deteriorate at the high and low frequency extremes due to transformer roll-off)
-Conversion loss is a function of the carrier drive level
-Spurious product generation
Conversion loss can be improved somewhat by providing a short circuit impedance at the output port for the undesired sideband
If I can add:
Another importent parameter is the Isolation of the mixer. Now you can play with the diode Vb or can use two or more in series or parallel. This way you can optimize for the best C.L and isolation.
Improve the matching at IF and FR ports in order to get better C.L
Find the correct Balouns to get wide and low C.L
There is a hard fundamental limitation. The sum of the powers of the output signals cannot be greater than the input signal. The absolute minimum loss is with just the sum and difference outputs (which are equal power) which produces a 3 dB loss. When the diodes are saturated there are many more output products and so the one you want will be about 6 dB down from the input.
many thx guys 4 ur replies, but i'm really wondering, coz all of other parameters are almost perfect in my case (IF & LO return loss, and isolation as well) except 4 the conversion loss and at only a certain frequency band, do u think it might be a problem with the LO matching network ?
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