I constructed three variants of this mixer. Number one was made using Mini-Circuits broadband 1:4 transformers type T4-1. The transformers are indeed very broadbanded but also quite expensive. Number two was made with broadband transformers using ten trifilar windings on Amidon FT37-43 (mu =850) cores. Number three was made using eleven trifilar windings on low-cost small balun type cores with mu=750 (type 11133203). I do not know who makes these cores (anyone?).
Of these mixers, the conversion loss and IP3 was determined by Cor, PA0CHN.
He used two crystal oscillators at frequencies of 3.730 and 3.780 MHz.
These two signals were amplified, attenuated, combined, attenuated and amplified
to a level of 0dBm. We determined that this setup had internally generated
IP3 products of better than 75dB below the base frequencie levels. The output of the combiner
was connected to the RF input of the mixer. A signal generator with a frequency
of 12 MHz and output level of 0dBm was connected to the LO input of the
mixer. The IF mixer output was connected to a spectrum-analyser.
The conversion-loss was determined by measuring the difference in signal
level of the RF input an IF output at 12-3.730 MHz. IP3-out was determined by
taking half the difference in signal level
of RF input and IP3 products at the IF output. (This way of determining IP3
depends on having an RF input level of 0dBm, see
ARRL handbook).
IP3-in can than be calculated as the sum of IP3-out and conversion-loss.
Results were:
mixer transf. IP3-in conversion-loss --------------------------------------------------------- 1 T4-1 +41 dBm 6 dB 2 FT37-43 +43 dBm 6 dB 3 11133203 +44 dBm 6 dB
