PFR-3 Group 2 calibration

This is the fun part of the build because it involves actually seeing the rig come to life, so to speak. The instructions call for some concentration, and a little experience if you want to get things spot on. It also calls for some specialised gear, although it is pointed out that this is not absolutely necessary. Of course, the specialist gear here is a frequency counter and an oscilloscope, neither of which I have. Fortunately, the instructions also assume most builders of this rig would not have them, so alternative methods of carrying out the alignment and calibration process are outlined.

I wanted to have mine calibrated as accurately as possible, so called on the assistance of Ray VK4ZW and Wallace VK4CBW. Ray has loads of knowledge and experience, so he knows what he is looking (or listening) for. He also has a good frequency counter. Wallace volunteered his oscilloscope, which Ray knew how to use.

The first thing we tackled was the DDS frequency calibration. This was done at 10,000 MHz and we found no need to make any adjustments here. The frequency counter reading was just about spot on.

Ray VK4ZW connecting up the frequency counter.

Next was the IF offset adjustment and setting the BFO trimmer. This was a little more tricky. We turned on the oscilloscope and connected the probe to Pin 7 of U3 and had to fiddle with the dials of the scope to get the trace nice and solid on the screen. Once that was achieved, it was just a matter of clicking the tuning switch of the PFR-3, watching the signal on the scope until the amplitude rolled off sharply, and counting the number of clicks until this happened. Then I needed to halve that number and click back up the other way to arrive at the centre of the pass band of the filter. Easy.

A good, solid signal seen on the scope.

Adjusting the BFO was the tricky part, not least because it required listening to the beat note and the side tone in a pair of headphones. That was tricky enough for an inexperienced builder like me, but it was also challenging because the trimmer used to make the adjustment is mounted under the board. The danger here was more one of having power leads come adrift through moving the board around while getting the screwdriver to the trimmer. The aim was to get the two tones to merge.

Cal o On the LED display in preperation for the IF offset and BFO trimmer set.

Once that was done, the settings were stored by clicking the MENU switch. Beautiful.

Next is to assemble the transmitter section.


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