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Saturday, 23 March 2013

50 Ohm SMD Dummy Load Prototype Construction

The PCBs for my first go at a SMD 50 ohm dummy load arrived the other day followed by the 2200 ohm SMD resistors.

For those that don't know what a dummy load is, it is basically a resistor that you put on the output of a radio frequency source, such as a transmitter, while testing or experimenting, so that you can do things like:

1) avoid transmitting rubbish while testing and doing R&D with the circuit it is attached to, or

2) do power measurements

The reason it is 50 ohms is because most amateur radio equipment with unbalanced outputs, such as PL-259, BNC or N connectors, is designed to work into loads that look like a 50 ohm resistance to the equipment.

Why 50 ohms, and not 75 ohms like a TV antenna?

The 50 ohms comes from the practicality that a 50 ohm characteristic impedance coaxial line is a good compromise between power handling and loss.

A lower impedance (around 30 ohms) would improve power handling, and a slightly higher impedance (around 70 ohms) would reduce losses, and 50 ohms was chosen as a reasonable compromise.

Why do it with SMD parts?

The smaller an RF circuit is, the less prone to parasitic inductance and capacitance effects which will make it deviate from ideal behaviour, in this case, behaving like a 50 ohm resistor.

Why 44 resistors?

More resistors equals more power handling ability, and forty four 2200 ohm resistors in parallel equals 50 ohms, and a nominal 11 watts power handling capacity.

My quick and dirty way to solder SMD parts uses a hot plate and a jig to lift the PCB off the hot plate at just the right time.

You can also see http://www.ahars.com.au/htm/hb_reflowsoldering.html for more details.

We start with solder paste, an applicator, in this case a needle, and some tweezers, the PCB, the SMD parts, and good lighting:

We apply solder paste to the pads. This took about 5 minutes. One half was done with discrete blobs on pads, the other half with long smears along the pads for comparison. Smears are quicker:

We apply the SMD parts with tweezers. Another 5-10 minutes. Older amateurs on beta blockers and not using energy drinks might be quicker:

We put the PCB on the reflow jig, which is in contact with the hotplate. We turn on the hotplate, and for this hotplate, I know it has to go for 250 seconds, then turn off the hotplate, then lift the jig off the hotplate at 280 seconds, starting at room temperature:

At around 180 seconds, most of the solder paste has melted, going from grainy grey to a shiny silver appearance:

At 250 seconds it is turned off, and at 280 seconds, the jig is gently lifted and chocked with whatever is nearby, and the hotplate can be moved out of the way:

After the PCB has cooled, any residual solder balls can be removed with a cotton bud. As can be seen, a long line of solder paste worked out much the same as carefully placed dollops on each pad:

Before adding the BNC connector, we check that it comes to 50 ohms with the multimeter, to check that there were no solder bridges from the reflowing. We then add the BNC connector:

We want the dummy load to behave like a 50 ohm resistor, to enable us to see how our equipment we connect to the dummy load is behaving. To know if it will behave like a 50 ohm dummy load, we need to test it.

A crude but effective test of the dummy load's behaviour at different frequencies is to use the impedance bridge of the kit built VK5JST antenna analyser.

An antenna analyser is, after all, a piece of equipment designed to see if an antenna behaves like an ideal 50 ohm load for our transmitter... so testing a dummy load with it is not too heretical, just don't tell Jim, VK5JST...

We find an SWR of 1.03 at 50MHz or so. This is within the error margins of the analyser's impedance bridge at these frequencies, and the 50 ohm load appears to be behaving like a fairly ideal 50 ohm load:

Then, at 30 MHz or so, the SWR is about 1.02:

And at 25MHz and below, the analyser thinks it is a fairly pure 50 ohm load with no reactive components at all, giving an SWR of 1.00:

The next step is to get some proper testing done with a vector network analyser or similar gear, for more accurate performance assessment, but preliminary indications are quite encouraging.

The optional through hole power measurement components (Capacitor, diode and resistor) can be added later if desired.

So, SMD reflow soldering needn't be slow, painful, or expensive. Within half an hour of starting to apply solder paste, the BNC was on and I was testing the dummy load.

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