Measuring Commercial and Homebrew Injection Transformers

Having a Bode100 on a test loan I tried to make a full test run on all injection transformers I had at hand.

The following units were available:

  • Omicron B-WIT100 wideband 1:1 transformer designated as 1Hz to 10MHz with no dB specs (“usable” range).
  • My NVT-1 injection transformer, also 1:1, designated as 40Hz to 7MHz (-3dB).
  • A homebrew 40dB attenuating injection transformer, from 60Hz to 200kHz, with a max. phase difference of 5°
  • A homebrew 52dB attenuating injection transformer, from 70Hz to 400kHz, with a max. phase difference of 5°.

The measurement setup was:

  • Bode 100 VNA from 5Hz to 10MHz in S parameter mode, all ports 50Ohms.
  • 0dBm power level
  • 10Hz IF bandwidth
  • Attenuators set at 20dB for transmission, 10dB for reflection.
  • User cal for gain and impedance

OK, the B-WIT100 is first:

BodeBWit100PictureOutside

MeasurungInjectionTransformersBode100Vna_5Hz_10MHz_50Ohms_0dBm_10HzIF_B-WIT100

3dB ranges from 15.59Hz to 7.7MHz. Phase is 33 and -43 degrees off at the 3dB points.

The NVT-1 is next:

InjectionTransformerPhotoNVT1Homebrew

MeasurungInjectionTransformersBode100Vna_5Hz_10MHz_50Ohms_0dBm_10HzIF_NVT-1.PNG

3dB ranges from 22.41Hz to 7.33MHz. Phase is off by 40 and -43 degrees at the 3dB points. This is very close to the original, I would say, especially when you take into account that core material has tolerances in the 10s of percents.

The next transformers are combinations between an input matching attenuator, a transformer and an output resistor. This was done for two reasons:

  • my circuits needed very small injection levels, so attuation was welcome and standard ARBs or other tools could be used without external attenuators
  • bandwidth and matching improves.

First, we have a 40dB model:

InjectionTransformerPhoto40dBHomebrew.png

MeasurungInjectionTransformersBode100Vna_5Hz_10MHz_50Ohms_0dBm_10HzIF_40dBModel.PNG

The lower 3dB values is below 5Hz. The upper 3dB point is at 1.63MHz, with phase error below 32° and -39.° at the high end. The spec of 5° phase error from 60Hz to 200kHz seems to be correct. Last not least the 52dB model:

InjectionTransformerPhoto52dBHomebrew.png

MeasurungInjectionTransformersBode100Vna_5Hz_10MHz_50Ohms_0dBm_10HzIF_52dBModel

Bandwidth is from 5Hz to 4.35MHz (-3dB values). Phase error at the corners is 42 and -38 degrees. 5° phase error is available from 70Hz to 400kHz.

 

Conclusions

The Bode 100 VNA is a useful tool to characterize commercial and homebrew injection transformers. The values obtained are close the the Keysight E5061B-3L5.

It is absolutely OK to use homebrew injection transformers. The NVT-1 comes very close to the commercial B-WIT100, and with combinations of attenuators/matching networks plus transformers you can reach 3dB bandwidths from 5Hz to 4.35MHz (-3dB) with a parts value of a few €.

 

 

 

 

 

 

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