The AM Forum

Many years ago, i bought a Avantest R4131 spectrum analyser with a bad input mixer. Had it repaired and used the analyser many years, but always liked to have a tracking generator to sweep filters, antennas etc. But these are quire rare and expensive and I could not afford one, so I did build one. I used the programm PUFF to design the amps, mixer and filters. After moving to Spain in 1996, I had it stored, and now I found it back and fired it up.
Unbelievable, after all the abuse of storing, transport, humidity, dirt  and more, it still worked perfect!!!
It mixes the filtered LO with an oscillator at the IF frequency up to 4 GHz. The resulting LO+IF is filtered in a ceramic puck filter with 2 resonators and amplified.  Than I amplify a sample of the sweep generator of the analyser that runs also up to 4 GHz in a 3 stage wide band amplifier and mix it with the LO+IF. The result is a frequency equal to the analyser input frequency. That is filtered with a puck notch filter and a low pass filter and amplified in a wide band amplifier. All semiconductors are GaAs FETs. I once did destroy an output FET and replaced it, that is clearly visible at the pic (not such a nice job, I damaged the PCB when de-soldering). But it works and the output is +10dBm over the full bandwidth.
The output runs from zero to 3 GHz quite straight within approx 1 dB (except a slow decrease of 2 dB due to the coax cable at the output) without any dips.
So I can sweep all my filters again!!. Fun ;D
I also made an extension to measure 24 GHz at my analyser with a few PLL generators mixers and a PLL stabilzed gunn oscillator. Also found that back, so one of the coming weeks I might try that thing as well in future, but there is very much oxidation in the micro strip circuits So I am not to hopeful.

Nico,

Interesting. Nicely done.

I didn't see dielectric resonator pucks being used when I was working in the microwave industry, some 20 years ago. Filters were always made with microstrip and didn't require much tuning. So, your puck filters are something new to me (you made me do some reading). Perhaps they're easier to implement in do-it-yourself construction? Less precision needed? Tuning appears to be with metal foil attached to the top surface. Very crafty.


Don

Hi Don
You find a puck ceramic resonator in almost all older sat down converters. They are only approx 3 mm diameter for 10 GHz. There  tuning is done by a screw approaching the top of the puck.
I tune them indeed with some aluminium foil on top. The electric field is vertical, the magnetic field horizontal around the puck. So you can couple two just by putting them close like coaxial resonators. For lower frequencies the form is coaxial, not a puck form. So a tube where the inner and outer surface are silver plated and the ceramic is the dielectricum. When the inner conductor and the outer conductor are connected at one side you have a 1/4 wave resonator as found in all cellphones.