N2ADR frontend TX measurements

After bringing up the transmitter section of my N2ADR frontend i measured some IMD and spectral purity data.
Quisk has a nice feature to create very clean measurement tones internally. This makes measurements very easy.
The amplifier after the TX DAC is a BGA6589 supplied with 80mA out of a 7.5V source.
When not driving the DAC with full power i see really good IMD figures. IMD3 around -45dB below the wanted signal.
With full drive the IMD3 decreases to around -20dB. (The figures were taken with 20dB attenuator between frontend and measurement receiver!)

IMD -6dB at 28MHz

N2ADR frontend IMD test with -6dB output at 28MHz


When looking to lower frequencies (lets say below 10MHz) and transmission of a sine carrier the spectral purity looks rather good. The next figure shows a 40M signal which gets around -110dB noise 100Hz off the transmission frequency.
7MHz carrier close

looking close to a 7MHz sine carrier


At the upper edge of the frequency range of the frontend the situation becomes significantly worse. The internal digital oscillator seems to create many spurs over a wide frequency range.
55MHz sin spectrum

spectrum of a 55MHz sine carrier


55MHz sine wide

wideband spectrum of 55MHz sine carrier


For low output frequencies the wideband spectrum looks rather promising except for the overtones. But the TX signal was completely unfiltered after the DAC. So proper lowpass filters for every band will be necessary anyway. Probably also the input of the MMIC was a bit overdriven. I did not check this so far.
80m overtones

80m Band sine carrier and overtones


Looking to the spectrum that would fall inside the amateur band during real world operation another effect appeared. Independant which frequency is used there are symmetric spurs left and right of the TX spectrum. Two at +-52kHz and two (with more noise) at +-61kHz. Currently it is not clear if these come from internal circuits of from the power supply. Probably both ?
3.6MHz sine carrier close by spurs

3.6MHz sine carrier close by spurs


Overall the performance is already rather good. Especially at the top end of the frequency range there is range for improvement which is necessary to make the frontend a good choice for usage with transverters for VHF/UHF contest operation.

more laser experiments

Last weekend i did some few daylight laser experiments during the V/U/SHF contest. Unfortunately the contact to Marcel, DK5WMA was not possible due to much fog on Saturday evening. Our receivers are still not capable to deal with daylight. But we work on that.

Laser experiments in JO60OM

Watch this video on YouTube.