Wednesday, 19 August 2015

Network Analyser


I recently bought a few (3) ADF4351 development boards from China and am
currently writing some software to control them. Out of the box you can use software
written by Analog Devices to evaluate them.

So far I have used one of the boards to provide a 10 Ghz beacon, there are reasonably
strong harmonics of the signal up to 12 Ghz.

It is important to terminate the unused output port otherwise the boards oscillate around
1.2 GHz.

I plan to use a couple of the boards combined with a receive mixer and a soundcard
as a simple form of network analyser. It will be possible to use the fundamental signal
and the 3rd harmonic to give me something useful up to 12 Ghz. The sound card will
do an FFT to measure the receive signal, it will be able to measure both the
fundamental and the harmonic.

Mini circuits do a nice 6 GHz mixer at a reasonable price which is fairly flat across the
entire range and has an IF port with a DC - 1 GHz response making it ideal for
low frequencies.

For frequencies below 35 MHz it will be possible to use a 3rd fixed oscillator and mixer.

Obviously because of the harmonics of these oscillators without filtering they will
not be much use for characterising active devices, for than I plan to use a technique
similar to that employed by Keysight in their early network analysers. They use a
filtered 3.8 GHz oscillator which they mix with a 3 - 6 GHz YIG oscillator and
low pass the result. Of course I now need to find a source of suitable YIGs.
As usual I missed the real bargains by a few days. There was someone selling exactly
the YIGs I need a couple of weeks ago at a very cheap price, all 20 of them sold in hours!

The software (and bodge board for the SPI interface) will track 2 of these synth
boards an IF apart. The IF will be in the KHz range and I plan to use a 4 channel
24 bit sound card for the receiver.

I also have a copy of Joel Dunsmore's book entitled "Handbook of Microwave
Component Measurements" which is the reference work for those using / designing
Vector Network Analysers.

I am collecting parts from eBay when they are available so I am not sure how long this
will take to do.

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