HP 5253B mod for tunable cavity filter
At the “Friedrichshafen Messe 2012” in the flea market I’ve found a HP 5253B frequency converter plugin at good price. The HP 5253B is a plugin for 5243L or 5245L, old 50 Mhz counters.
The converter is made around a very nice silver plated tunable cavity, 50-500 Mhz, with an harmonic generator for 10MHz and a mixer.
Converting this surplus in a good tunable passband filter is quite easy.
First of all I’ve removed all connecting cables around, the top pcb (video amplifier), the mixer (square box on the right) and the harmonic generator (square box on the left side).
I’ve made the links on two small pieces of copper clad, 40x40mm, to be inserted inside the cavity. The link consists in a circular loop of about 20-25mm diameter of silver plated copper wire. Then I’ve drilled an hole for the bnc connector on the front panel.
The two links are inserted using the slots on the left and right sides of the cavity, and connected with coaxial cables to the bncs on the front panel.
I’ve made some pictures with the cellphone (poor quality), the filter seems good.
The passband width is 1-2MHz @-3dB depending on center frequency, loss < 1,5dB over the entire band and return loss better of 24dB.
I will use it as a preselector for my sdr dongle.