08 March 2014

Establishing the -10dBm point for the AD8307 Power Meter.

I built the CMOS 10Mhz calibrator exactly according to the design published on the QRP Homebuilder website RF workshop page 5 found here http://www.qrp.pops.net/RF-workbench-5.asp under Part 2. This uses an MX045 CMOS clock chip with an interesting specification which can be found here http://www.qrp.pops.net/images/2012/RFWB5/cmos-clock-datasheet.pdf

The QRP Homebuilder webpage gives a full description of the use of the calibrator. Interestingly this is a square wave generator which I understand results in a better more accurate result than a sine wave. I assume that this is due to the huge bandwidth (500Mhz) of the AD8307. Thus a square wave will generate harmonic energy all the way upto 500Mhz which is 'captured' by the AD8307 and summed to a power number. The generator is calibrated to a 50mV DC source and was easily done.

Note that this calibrator cannot be used with attenuators to calibrate any other power level than the -10dBm level. For that task a sine wave generator must be used (makes sense).

At this point I changed the input resistor on the Power meter to a 1% tolerance 52.3Ohm unit from the original 51Ohm unit. Interestingly the measured -10dB point only changed from 1527mV to 1527.3mV.

The wave form below is not exactly a squarewave reflecting the limited 20MHz bandwidth of the scope.

ZS6RSH Build of the CMOS 10MHz calibrator

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