Diode Transformer Double Balanced
This Diode Transformer Double Balanced mixer is also referred to as a ring diode mixer or a diode product detector. The double balanced configuration results in isolation between all three ports. The theory of these mixers is covered in the book "Radio Receiver Design" by Kevin McClaning and Tom Vito.
An inherent advantage of this mixer is broad bandwidth, limited primarily by the available bandwidth of the transformers. When wound as multifilar transmission line transformers, bandwidths of more than two decades are possible. In this way, one general purpose design may serve many applications. For information on the physical realization of these transformers you may refer to the book "Transmission Line Transformers" by Jerry Sevick.
This mixer exhibits a conversion loss of about 7 dB. In the normal form with a quad of diodes the nominal LO drive is about +7 dBm. These mixers exhibit good intermod performance. Multiple diodes may be used in series in each branch of the ring to increase the intermod performance. This requires higher LO drive levels. Typical high level and very high level mixer drive levels are +13 and +23 dBm. The typical output 3rd order intermodulation intercept point is roughly equal to the LO drive level.
These mixers work well when terminated in 50 ohms. In fact, for best intermod performance, the IF port in particular should be terminated with an excellent match to 50 ohms at all frequencies. Rather than terminating the output with an IF filter that is reactive at the RF and LO frequencies, for best intermod performance this mixer is terminated with a broadband amplifier with good return loss, or with a diplexer that passes the IF and terminates the RF and LO frequencies.
The default model is the built-in non-linear diode model. This mixer often uses hot-carrier diodes, but high-speed silicon diodes may be used as well.
- Compute inductances for transformers (modelled as mutually coupled coils).
Freq_XFMR = FreqRF
if ( FreqLO < FreqRF) then Freq_XFMR = FreqLO
Lpri = (1e9 * 6 * Zo) / [ 2 * p * Freq_XFMR * 1e6 ] nH
Lsec1 = Lpri / 2 nH
Lsec2 = Lsec1 nH
K = 0.99