Input power measurement (100k-100MHz) of rectifier using a directional coupler

Hi,

I am driving a half wave rectifier which has an input impedance >>50ohm. I can measure the output voltage and output power.

I would like to measure spectral power transfer/conversion efficiency (Pout/Pin) but I don't know the proper way of measuring the input power to my rectifier. How would you measure it ?

I am considering using a directional coupler but not sure how to do (I am new to RF circuits)

Potential problems:
1-My input signal will be Vpp 2-4V so 10-20dBm range. It will be mostly reflected due to the high input impedance of my rectifier (>10kohm). While the most of the power is reflected can I still accurately measure the small difference (10-100 uW) between the reflected and incident power (from RF power supply or arbitrary waveform generator).

2- Since the diode will conduct for a short part of the period, current will not look like a sine wave and there will be other frequency components. Will this create a problem? (Changing input impedance during a single cycle)

3- As I increase frequency, a capacitor parallel to my diode starts to conduct. So there will be a large instantaneous power, but this will be just charging and discharging the load capacitor and not consuming a lot of power. However my average consumed power will be still small around 10-100 uWatts. (based on my simulations)

4- https://www.minicircuits.com/pdfs/ZDC-10-1+.pdf I found this directional coupler, would you recommend to use this one or do you know better alternative covering my frequency range?

Thanks a lot,
Levent

You can use a cheaper and compact ( and accurate ) directional coupler from;
https://cdn.macom.com/datasheets/CH_CHS-137.pdf

If you hesitate from the generated harmonics coming from your rectifier circuit, use a BPF to eliminate the others.
As a remark, a calibration method will be necessary in your measurement system due to non-ideal coupling response,filter ripple etc.

You can add also a RF Power Detector from www.analog.com or www.linear.com as a complementary for your project.You will able to measure the power as DC value in dBm scale.
There are double detector versions too in order to compare incident and reflected wave powers.

BigBoss said:

You can use a cheaper and compact ( and accurate ) directional coupler from;
https://cdn.macom.com/datasheets/CH_CHS-137.pdf

If you hesitate from the generated harmonics coming from your rectifier circuit, use a BPF to eliminate the others.
As a remark, a calibration method will be necessary in your measurement system due to non-ideal coupling response,filter ripple etc.

You can add also a RF Power Detector from www.analog.com or www.linear.com as a complementary for your project.You will able to measure the power as DC value in dBm scale.
There are double detector versions too in order to compare incident and reflected wave powers.

Click to expand...

Thanks for the reply.

Lets say you have 10 pF connected to the directional coupler you suggested as load and driving it with 10 dBm, 10 MHz RF source. What would I measure from the RF Power detectors at point B and D (assuming input is connected to C and load is connected to A in the functional diagram mentioned in the datasheet).
I am confused about the effect of reactive power on these measurements. Looks like my system will suffer from parallel caps :bang:

Thanks a lot,
Levent

The directional coupler shown in post #1 can be used to measure either forward or reflected power, depending to the connection. To determine delivered load power, both quantities must be known. A bidirectional four-port coupler with forward and reflected power measurement channels is required to access it.

Nevertheless you can expect difficulties to determine a small consumed power amount exactly. You should also consider that part of the reflected power is contained in harmonics generated by the rectifier.

When connecting a reactive load or e.g. open or short, you get theoretically equal output at forward and reflected power coupler port. The phase angle between both tells you about the kind of load.

laygun said:

Thanks for the reply.

Lets say you have 10 pF connected to the directional coupler you suggested as load and driving it with 10 dBm, 10 MHz RF source. What would I measure from the RF Power detectors at point B and D (assuming input is connected to C and load is connected to A in the functional diagram mentioned in the datasheet).
I am confused about the effect of reactive power on these measurements. Looks like my system will suffer from parallel caps :bang:

Thanks a lot,
Levent

Click to expand...

Roughly...
At Point-D, you would expect to see "Reflected Power(dBm)-Coupling(dB)-Insertion Loss(dB)" and at Point-B, you will see "Input Power(dBm)-Coupling(dB)"

Don't worry about reactive part of the waves because Power Meters or Power Detectors will measure "Magnitude" of the RF signals in log scale ( dBm) so there won't be any reactive part.
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