On Saturday 21-2-26, Ross VK2ARD and Paul VK2AMT met up at the clubrooms. The purpose was for Ross to show how to tune cavity filters. The filters were quarter wavelength for 70cm. Cavity duplexers allow the use of a single antenna by providing sufficient isolation between the repeater’s transmitter and receiver. The cavities are resonant tuned circuits. We used the club’s Rigol DSA815 spectrum analyser.
The analyser allows you to select frequencies (called markers) on each cavity. These indicate the centre of the filter’s operation. Both Bandpass and Notch filters can be set up, or both filters in the one cavity. There are adjustments provided on each cavity to allow the adjustments, eg frequency, to be made.
The image above shows a Bandpass Filter, the left peak on the curve. This is centered on the marker frequency. The dip on the right is a Notch Filter. The Notch is intended so supress the repeater’s own transmitter from affecting the repeater’s receiver. Normally the peak on the right would be deeper, but it’s enough to demonstrate the basic parts of the tuning process.
The image above shows a Bandpass Filter at about 434.750 MHz, the jagged parts of the curve indicate noise. One important part of the process is to minimise Insertion Loss. This page is a very basic description of part of the tuning process. Personally, I found it extremely interesting. Many thanks to Ross for taking the time to demonstrate this. I encourage other club members to avail themselves of any opportunity to have this explained. It does de-mystify the cavity filters used in our repeaters. By the way, the club’s Rigol Spectrum Analyser is a beauty!
de Paul VK2AMT
More fun with the Rigol Spectrum Analyser
On Saturday March 7th Ross and Paul met up once again at the clubrooms for some more experience in tuning cavity filters with our Rigol Analyser. The picture below shows Ross starting to input a Marker for the centre frequency of the filter required. Once this is done, you can see the waveform change as the cavity filters are adjusted to produce a Bandpass Filter or a Notch Filter.
