Thursday 9 August 2012
455KHz filter from 455KHz ceramic resonators
Anyone pondering a 455KHz filter for their nice shiny yaesuicomkenwood rig... well I have played about with building a 455KHz filter using cheap ceramic resonators just like crystals in a ladder filter.
The idea is certainly is not new. G3VA, LA8AK, VK5BR and G3JIR wrote about it a while back. TT covered it numerous times the last one I have was from May 2003. There was a in interesting test rig diagram showing how to measure the series resonant frequency of ceramic resonators.
On one side you have a signal generator and a digital frequency counter, feed the signal into a resistance network, to which is connected one side of the ceramic resonator, then an identical resistance network feeds the output into a an RF mV meter. An Oscilloscope monitors across the ceramic resonator. At the series resonance frequency the signal will change.
I also used a couple of the options from http://w7zoi.net/xtal_methods/xtal_methods.html
Anyway starting with the batch of resonators I have I measured a handful until I found some that were about the same value.
The why is because I am about to do a trip and neither my FT817 or FT897 has a SSB filter in either rig. How difficult can it be to make one instead of parting with cash? (I started to write this before I had a little issue with the FT817!)
So I have a few CSB455E (yellow) 455KHz ceramic oscillators and thought I'd make up a simple ladder to see what is possible. These are the results following the methods laid out here http://users.tpg.com.au/users/ldbutler/LadderFilter.htm using a ceramic resonator instead of a crystal.
If I use the G3JIR values (source Radcom TT) in all sort of programs seemed to ring true.
Series frequency 442kHz
Parallel frequency 456kHz
Series Capacitance c18pF
Parallel Capacitance c280pF
Series frequency 443kHz
Parallel frequency 457kHz
Series Capacitance c17pF
Parallel Capacitance c276pF
R = 10.3 Ohms
Ls = 7.92 mH
Circuits and images of the filters:
2 units (aka 2 pole)
4 units (aka 4 pole)
6 units (aka 6 pole)
Images of the outputs:
I also tried these options http://lea.hamradio.si/~s57nan/ham_radio/xtl_fltr/xtl_fltr.html
The math gets a bit fun if you want to calculate what is going on but I go started off doing the plug it in and measure method and I used a spectrum analyser and a VNA.
The ceramic resonators I picked up from ebay at the princely sum of 6.99USD for 50 of them including postage costs.
The capacitors were from the junk box.
Probably too late for this trip so will do it when I return but I will make up a small board with SMT caps to mirror the commercial Collins filters and fit into the rig.
Posted by M1KTA at 08:20