Sunday, August 16, 2015

Homebrew dummy load

I have created a simple 50 Ω dummy load to test transmitters. I also added a simple RF diode detector so I can measure the peak voltage, and calculate the power.


The dummy load consists of eight 100 Ω resistors rated at 2 W so the load should handle 16 W, at least for short periods. I constructed the dummy load using a combination of ugly construction and Manhattan style, by gluing pieces of PCB  (as isolation pads) on top of a ground plane PCB.  Then I soldered the components directly on the copper without drilling holes.

RF Probe

The RF probe part consist of a simple 1N4148 diode and a 0.01 uF ceramic capacitor. I only had a 50 V capacitor in my junk box, but it should be sufficient given that this is a 16 W dummy load and 16 W translates to 40 V peak.

I added female banana connectors, which connects to a multimeter. The power can be calculated by Ohms law by subtracting the forward voltage on the diode from the measured voltage, then multiply by 0.707 (to get RMS), then square the result and finally divide by 50 Ω. Some homebrewers add a voltage divider to their RF probes using a 4M7 resistor to get RMS voltage directly. I did not bother since I am sure the input impedances on my multimeters varies.

The calculations might seem a bit cumbersome, but I might print out a small lookup table and glue it to the box to have some ballpark figures. However, this is not a precision instrument. The forward voltage on the diode varies with load and can be somewhere between 0.4 and 0.7 V. I simply use 0.5 V in my measurements.

So far, the dummy load has been very convenient when testing my Softrock RXTX and its low pass filter,

Further reading

1. Jeelabs, Forward voltage drop on a diode
2. NXP, 1N4148 data sheet
3. N5ESE, Classic RF Probe

Thursday, August 13, 2015

Lowpass filter for 30m

I just finished a 30m lowpass filter for the Softrock Ensemble RXTX tranceiver.

I used the schematic provided by WB5RVZ

L200 and L201 is wound to about 0.9uH, whereas C200 and C202 is 100pF and C201 is 330pF. Everything is soldered to a PCB (ugly construction) and I added two switches and a coax to enable on and off switching of the filter.

Ugly construction...

I tested the filter using a 25 MHz function generator and my Rigol 1052 scope. The filter was connected to a dummy load. The first two pictures below show 1 Vptp at 10 MHz with the filter off and on respectively. The two pictures thereafter show 1 Vptp at 20 MHz (on and off).

10MHz lowpass filter switched off

10MHz lowpass filter switched on

20MHz lowpass filter switched off

20MHz lowpass filter switched on

As a three pole filter it should have a roll off at 18db/octave. This seems about right when I compared 12 MHz and 24 MHz (9.3V and 0.9V ptp respectively). I measured the -3dB point to be around 14.7 MHz. The purpose of the filter is to reduce the the 2nd harmonic when transmitting in the 30m band (10.1 MHz). The second harmonic is down about 13dB.

Function generator on burst mode, Oscilloscope on FFT, center frequency is 10.5MHz and filter is off

Same as above, but lowpass filter is switched on

I also looked at the FFT on the Rigol scope while using burst mode on the function generator. In the pictures above, the center frequency is 10.5MHz and the grid is 12.5MHz in the X axis and 10dB in the Y axis. Here it seems like the filter contributes to 20dB attenuation at 23MHz which is about right.  I have not measured the second harmonic while using the Softrock, since I have no real spectrum analyzer. However, as the filter is working, I guess I can legally transmit on 30m. 73!