08 March 2014

Building a Sine Wave generator to allow calibration of the AD8307 Power Meter

Now that I had the -10dBm point established for the power meter the next step was to calibrate the power meter for other points using a sine wave generator in combination with a range of attenuators.

As a starting point I scaled the QRP Homebuilder sine wave generator http://www.qrp.pops.net/RF-workbench-5.asp from 10MHz to 11Mhz since I had an 11MHz crystal in my junk box. Thus I ended up with an oscillator having the following specs for the tank circuit.

L= 3.99uH, C = 39pF fixed plus a 2.7-10pf variable capacitor.

I was able to resonate the oscillator successfully, however the variable capacitor was turned down to 'almost' minimum setting. Reflecting the fact that the fixed capacitor is slightly too big. I decided to leave it as. According to the spec this generator should have the second harmonic down at around -29dBc. I am unable to measure this at this time so I take it on good faith.

A future experiment could be to build a low pass filter say with a cutoff frequency at around 12.5MHz. Inserting this filter between the sine wave generator and the power meter should not result in a measurable change of power reading. Another approach would be to build a pre-selector for the power meter and tuned to the second harmonic at 22MHz. This would be loosely coupled in order to ensure a high loaded Q. The insertion loss of this pre-selector can be measured using the power meter. Yes! 

The design includes a 19dB pad in the output circuit of the resonator. This to ensure a good 50Ohm output impedance. I built this pad first and measured the Return Loss using my MFJ259B as the RF source at 11MHz and terminating the pad using Terminator2. The result showed a satisfactory RL=29.3dB. See notes below for details.

Once I had built the sinewave generator I tweeked the emitter current according to the specs. I then connected the generator to the meter and adjusted the supply voltage until the reading exactly equaled the reference voltage of the -10dBm cmos square wave calibrator. 1565mV +/-1mV. All good!

I was now ready to calibrate the meter in full.

CMOS 10MHz Generator reference point

Sine Wave 11MHz generator reference point

Sine wave generator frequency 11.058MHz

Sine wave generator output wave form. Looking good!
Sine wave generator enclosure

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