Design Article
Extending useable frequency span of 1:1 wideband transformers used for distortion measurements
Michael Steffes/Sr. Applications Manager/High Speed Signal Path, Intersil Corp.
12/6/2012 2:11 PM EST
Baluns
The two bold rows are baluns that will be considered here. A clue for a path to higher passband frequencies can be found in comparing the characterization impedance. Increasing the source and load impedance will shift the passband up in frequency while moving it down will shift the passband down. While the ADT1-1WT seems remarkable at 800Mhz upper F-3dB, that is specified using 75Ω. Operating in a 50Ω environment, its passband frequencies will shift down. A simple Spice modeling technique allows a very close fit to actual response (ref. 1). Comparing the simulated response for the 2- focus devices gives the curves of figure 2, where the ADT1-1WT is modeled at its 75Ω specification but simulated here at 50Ω source and load.
Click on image to enlarge.
Shifting the response flatness region to a higher frequency
Taking this source and load impedance up should extend the flat response region to a higher frequency. This cannot be pushed too far as the interwinding capacitance will come in at lower frequencies for higher source Z. However, setting out to operate the transformer with a 100Ω source and 100Ω load, while still presenting 50Ω final output source impedance for a spectrum analyzer input, will give the example simulation circuit of figure 3 using exact resistor values (ref. 2).
Click on image to enlarge.
Comparing the 2- predicted response shapes for a 50Ω vs. 100Ω test, using a source value of 2 in the 50? case, and the 6.84 shown above for the 100Ω case, gives the response comparisons of fig. 4. As expected, the predicted F-3dB passband has shifted up 2X where now the upper 0.5dB cutoff is a more useable 380Mhz.
Click on image to enlarge.
Figure 4. ADT1-1WT response comparison for 50Ω vs. 100Ω source/load conditions.
Using off the shelf wideband baluns is a common way to go back and forth between single ended and differential signal paths. Using the approach presented here has yielded very solid intercept measurements through 300MHz for a very wideband, low power, FDA like the ISL55210.
For full article click to pdf here.
The two bold rows are baluns that will be considered here. A clue for a path to higher passband frequencies can be found in comparing the characterization impedance. Increasing the source and load impedance will shift the passband up in frequency while moving it down will shift the passband down. While the ADT1-1WT seems remarkable at 800Mhz upper F-3dB, that is specified using 75Ω. Operating in a 50Ω environment, its passband frequencies will shift down. A simple Spice modeling technique allows a very close fit to actual response (ref. 1). Comparing the simulated response for the 2- focus devices gives the curves of figure 2, where the ADT1-1WT is modeled at its 75Ω specification but simulated here at 50Ω source and load.
Click on image to enlarge.
Figure 2. Nominal 50? source and load simulation for 2 transformers.
This
is from the input of the transformer to the 50Ω load so the 6dB
matching loss from the source is neglected. The red Macom device curve
exactly matches its 50Ω specified F-3dB range of Table 1 (400kHz
--> 500Mhz) while the green ADT1-1WT has a bit more range on the low
end, but is now very similar on the high end moving to a 50Ω simulated
(vs 75Ω specified) source and load impedance. This simple model does
not include insertion loss so a 0dB midband gain is shown. Both
transformers have fallen below 0.5dB flatness at < 200Mhz whereas it
would be desirable to use this balun output approach for FDA OIP3
measurements through 300MHz. Shifting the response flatness region to a higher frequency
Taking this source and load impedance up should extend the flat response region to a higher frequency. This cannot be pushed too far as the interwinding capacitance will come in at lower frequencies for higher source Z. However, setting out to operate the transformer with a 100Ω source and 100Ω load, while still presenting 50Ω final output source impedance for a spectrum analyzer input, will give the example simulation circuit of figure 3 using exact resistor values (ref. 2).
Click on image to enlarge.
Figure 3. Modified balun interface with 100Ω source and load.
The
Rm element is not strictly required to achieve a stepped up impedance
looking out to the transformer secondary and is set to 10MΩ here to take
it out. The Rm element is required to move the balun passband down in
frequency using lower impedances but still getting to 50Ω output
impedance. The circuit of fig. 3 puts 100Ω source and load on
each side of the transformer while still presenting a 50Ω source to the
50Ω cable that is usually used to move the signal to the spectrum
analyzer. That element is Rt in fig. 3. To emulate the 50Ω
specification condition for the 1:1 transformer we would set both Ro
elements to 25Ω, the R1 = 0, and R2 = ∞. The V1 source emulates the FDA
output and is set to a 6.84 value to normalize out the 16.7dB insertion
loss to Rt for simulation. R4 simply provides a DC operating point and
would not be used when the real source is present. Comparing the 2- predicted response shapes for a 50Ω vs. 100Ω test, using a source value of 2 in the 50? case, and the 6.84 shown above for the 100Ω case, gives the response comparisons of fig. 4. As expected, the predicted F-3dB passband has shifted up 2X where now the upper 0.5dB cutoff is a more useable 380Mhz.
Click on image to enlarge.
Figure 4. ADT1-1WT response comparison for 50Ω vs. 100Ω source/load conditions.
Using off the shelf wideband baluns is a common way to go back and forth between single ended and differential signal paths. Using the approach presented here has yielded very solid intercept measurements through 300MHz for a very wideband, low power, FDA like the ISL55210.
For full article click to pdf here.
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