2022-08-29 13:28 Status: #addto Tags: [Phase shifter](Phase%20shifter.md) # Balun Used to transform differential symmetric blocks to a single-ended block.  ## Where are they used?  [^st] ## Autotransformer Uses three taps on a single winding.  ## Matching Impedances  [^1] ## Balun Description A balun is any three port device with a matched input and differential outputs. Ideal S-params are: $S_{12}=-S_{13}, S_{21}=-S_{31}, S_{11}=-\infty$ ## Theory - Outputs aren't necessarily matched - Can be used bidirectionally, but be careful of differential side matching - Output impedance may be different to input - No constraint on $S_{23}$ so outputs may not be isolated (could mean different return loss for differential/common mode signals) - Equal power outputs but 180 degree phase difference ## ADS Simulation Have 4 goals: - 3dB for S21 and S31 - Phasebalance - phase(S(2,1))-phase(S(3,1)) - make a measeqn = 180 or -180 deg - Amplitude balance (measeqn) - dB(S(2,1))-dB(S(3,1)) = 0 Then optimise. There will be some variation across the bandwidth. [^3] [^4] ## Uses ### Symmetrical Power Amplifier  [^5] ## Problems with Using Single Ended S-parameters Typically a balun is not used as a three port device, but a two port device where one port has two modes. Better to use mixed mode S-Parameters. ## Importance of Isolation Simple test: expect it to just look like a cable.    ## Top three mistakes in balun design ### Length Matching Have to make sure everything is phase matched! Differential length will result in a 'phase walk off'. Traces, cables, connectors and other interconnects.  ### Trace Isolation Two differential traces close by, the isolation between neighbouring traces is important. ### Lack of Isolation low differential port isolation = high differential mode RL. Bad for SE2D. ## Marchand Balun  ## Spiral Balun 2-20GHz Interesting parallel design.  [^6] ## Spiral Layout in more depth  Several degrees of freedom: - Size of spiral (each section approximately lambda/4 at centre frequency) - Impedance of primary (high Z since thin track) - Impedance of secondary (wider means lower Z) - Spacing between tracks [^7] ## Simulations ### Mixed Mode S-Parameters  [^8]  [^9] Find the Single ended s params. Then convert to differential -  **S21ds = (1/√2) • (S21ss - S31ss)**  **S21cs = (1/√2) • (S21ss + S31ss )**  **CMRR = 20 • log(S21ds / S21cs )**  This gives you the common-mode rejection ratio of the balun.  Imbalance = -S21ss/S31ss [^10] 1:1 (50 Ω single ended to 50 Ω differential/25 Ω single ended) transformer and which one is a 1:2 (50 Ω single ended to 100 Ω differential/50 Ω single  ended) transformer [^11] --- # References [^1]: [Balun - Wikipedia](https://en.wikipedia.org/wiki/Balun) [^st]: [Baluns - STMicroelectronics](https://www.st.com/en/wireless-connectivity/baluns.html) [^3]: [Tutorial 8: Balun Design and Simulation - YouTube](https://www.youtube.com/watch?v=oJTt0pLRjP4) [^4]: [Test, measure balun, and compare antenna baluns or balums (w8ji.com)](https://www.w8ji.com/balun_test.htm) [^5]: [Symmetrical Power Amplifier and Balun consideration - YouTube](https://www.youtube.com/watch?v=CZTZ0bU_tDY) [^6]: https://www.youtube.com/watch?v=Af_lfTczBXY [^7]: https://youtu.be/rPTjrR7D9j0?t=185 [^8]: [Mixed Mode Scattering Parameters: What Are They and How Do I Use Them? – Marki Microwave RF & Microwave](https://www.markimicrowave.com/blog/mixed-mode-scattering-parameters-what-are-they-and-how-do-i-use-them/) [^9]: [Mixed-Mode conversion (mit.edu)](https://space.mit.edu/RADIO/CST_online/IdEM/idem/dataops_Mixed_Mode_Conversion.htm) [10]: [Microwaves101 | Mixed-Mode S-Parameters](https://www.microwaves101.com/encyclopedias/mixed-mode-S-parameters) [^11]: [BAL vs. BALH and Microwave Black Magic – Marki Microwave RF & Microwave](https://www.markimicrowave.com/blog/bal-vs-balh-and-microwave-black-magic/)