Monday, February 3, 2014

Measuring Bridge : First Measurements

Here are some results of the initial phase measurements made.
Only the voltage divider has been adjusted by changing the capacitive dividing part.
I used 1.8Mhz as reference and the load was a 50 Ohm Dummy load.

Freq       Phase Error
1.8Mhz        0°
18.1Mhz      1°
24.9Mhz      1.65°
29.7Mhz      2.65°

These errors seem te be caused by the self inductance of the shunt resistors, and i was able to compensate this by adding some shunt capacitance to these shunt resistors.
At 30Mhz and using an estimated resistor inductance of 1nH yields an inductive impedance of 180mOhm. Since our shunt resitance is only 5 Ohm, this presents a large unwanted inductive part.

I added a variable capacitor on the current shunt side and adjusted it so that i had zero error at 29.7 Mhz and then made the next measurement.
The Magnitude adjustment was done likewise, i adjusted the trimmer on the voltage divider for the so that i had the same magnitude on 1.8 and 29.7 Mhz.

Then i did a measurement on all bands.

Measurement
  1.8Mhz 3.5Mhz 7Mhz 10Mhz 14Mhz 21Mhz 29.7Mhz
Phase Error 0 -0.48° -0.44° -0.29° -0.04° -0.22° -0.18°
Magnitude Error 0.02dB 0.04dB -0.16dB -0.2dB -0.17dB -0.11dB 0.03dB

As you can see from the above table, the remaining error is very acceptable and beyond expectation.


Here is an image from another angle showing the capacitive divider adjustment.


You can see the 3 small 1.5pF capacitors in series to create a total series capacitance of 0.5pF.
All used smd capactors are NP0//COG (Chip on glass)  These 1.5pF C's are rated for 1000V so they can be used even for high power levels.