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created: Jul. 2019. Revised: Sep. 2019
Introduction
Introduction
The Avago application note - AN 5380: Double balanced diode mixer using the HSMS-2822 - describes a diode ring mixer using a pair of 1:4 Guanella baluns (fig. 1). The shielded baluns are made by Sumida and have the following part no.: BM-7A.
Fig. 1 AN-5380's diode ring mixer using a pair of 1:4 Guanella baluns
Fig. 2: specifications of Sumida balun part no. BM-7A
The AN 5380 application note provided measured performances, but no simulated performance at all. The absence of simulated results was due to difficulty in modeling the said baluns. Although popular RF circuit simulators, such as Keysight ADS and National Instrument AWR, have a 1:n Guanella balun model (links to XFERTL1 in ADS and XFERTL1 in AWR), it lacked a centre-tap which is necessary for the IF port.
Fig. 3. Circuit simulators' 1:n Guanella balun model lacked a centre-tap, but this is required for the IF port
Material & method
Material & method
Substitute model for Guanella balun with centre-tap
To represent a 1:4 balun with its centre-tap, I substituted with a pair of 1:1 Guanella baluns, e.g. BALUN1 (ADS's BALUN1 & AWR's BALUN1) with the coils parallel connected at the 50 ohm side, and series connected at the 200 ohm side (fig. 4 bottom). It is justifiable to use two separate 1:1 baluns because the top and bottom inductor pairs don't share the same magnetic paths.
Extracting the balun model parameters
The physical balun was characterized by measuring the insertion loss of two baluns connected back to back as shown in fig. 4 [1]. Subsequently, the model parameters were obtained by curve-fitting to the measured result (fig. 5).
Fig. 4: Two baluns were connected back-to-back in order to measure their insertion loss (top). Circuit model of the back-to-back connected balun. The 1:4 Guanella balun was substituted with a pair of 1:1 Guanella baluns, Bal1 & Bal2, with the coils parallel connected at the 50 ohm side, and series connected at the 200 ohm side (bottom)
Fig. 5 Modeled (red trace) and measured (black) insertion loss of the back-to-back connected baluns
Ring mixer's equivalent circuit model
Due to the mixer being highly non-linear, its equivalent circuit model was simulated using harmonic balance (fig. 6a). A two-tone harmonic balance was used to predict conversion losses and LO-IF isolation. However for predicting LO-RF isolation, a large-signal s-parameter (LSSP) controller (a subset of harmonic balance) was used because this simulation does not require two sources.
Fig. 6a: Ring mixer's equivalent circuit model for simulating conversion losses and LO-IF isolation
Fig. 6b: Ring mixer's equivalent circuit model for simulating LO-RF isolation
The PIN diodes were modeled using SPICE parameters obtained from [2]. The package parasitics (C5-8 and L3-6) were from AN1124 [3].
Fig. 7: Model of HSMS-282x quad diodes
Results
Fig. 8: Modeled (blue dashes) and measured (solid black) conversion loss vs. LO power
Fig. 9: Modeled (blue dashes) and measured (solid black) conversion loss vs. RF frequency (high-side LO)
Fig. 10: Modeled (blue dashes) and measured (solid black) conversion loss vs. RF frequency (low-side LO)
Fig. 11: Modeled (blue dashes) and measured (solid black) conversion loss vs. IF frequency (high-side LO)
Fig. 12: Modeled (blue dashes) and measured (solid black) conversion loss vs. IF frequency (low-side LO)
Unfortunately, the simulated isolation for both LO-IF (fig. 13) and LO-RF (fig. 14) do not agree with the measurement. We hypothesize the model erroneously predicts better isolation because it does not account for mismatched diodes, and unbalanced RF paths.
Fig. 13: Modeled (blue dashes) and measured (solid black) LO-to-IF isolation vs. LO frequency (high-side LO)
Fig. 14: Modeled (blue dashes) and measured (solid black) LO-to-RF isolation vs. LO frequency (high-side LO)
Conclusion
Conclusion
An alternative balun model constructed from two 1:1 Guanella baluns can replicate the centre-tap which is absent from popular circuit simulators' 1:4 Guanella balun model. The proposed model allows the diode ring mixer with Guanella baluns to be simulated, possibly for the first time. The model is reasonably accurate in predicting conversion loss performances. However, it lacked accuracy for predicting isolation performances.
Reference
[1] A. A. Ferreira, Jr., J. A. J. Ribeiro, W. Pereira, “Evaluating Impedance Transformers With A VNA”, Microwaves & RF, Apr. 2009.
[2] http://www.hp.woodshot.com/hprfhelp/design/SPICE/schott.htm
[3] Application note AN1124, "Linear Models for Diode Surface Mount Packages", Avago.
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