G3VPX HF Transceiver - LC Filter design
Filters were designed using ELSIE which can downloaded free for filter up to 7 poles.- see www.TonneSoftware.com
Nearly all input and output terminations are 50 ohm. (All my module intercaonnections are designed for 50 ohm coax). You will note a range of different termination impedences for the bandpass filters. This is because 50 ohms design gives very small inductances and large capacitances which are both difficult to manage. The designed fiter termination impedances are chosen to allow matching to 50 ohms with an easily achieved turns ratio for a 50 ohm tap.
For example, some of the LO bandpass filter designs are for 312 ohms
312 = 50 x (25/4) ie an impedance ration for 25 : 4 which is achieved by an easy whole number turns ration of 5 : 2.
Sometimes other design impedances are used .... because, for example - if the coil has 8 turns then you can do a 5 : 2 turns ratio ... so you change the impence and the number of turns changes. It is sometime a little difficult and fiddly to find an impedance which gives a number of turns on the coil which will allow a achievable turns ratio to match the impedance!!
The coil formers used are 5mm diameter + ferrite shild + can (labelled Cambion). They were bought from Surplus Sales of Nebraska ( www.surplussales.com ). The actual formers are of glass fibre? and dont melt easily.
The LO BPF coils did not use the ferrite shields. The RF BPF coils did use the ferrite shields.
All filters were aligned on a spectrum analyser / tracking generator.
Links to my Elsie files are below. You should first save these and then load then into Elsie (clicking will just open them as text)
1st local oscillator BPFs
1.9MHz 3.7 and 7 Mhz 10 and 14 MHz 18 and 21 MHz 24 and 28 MHz
RF Bandpass filters (4 pole)
1.9MHz 3.7MHz 7 Mhz 10 MHz 14MHz 18 MHz 21 MHz 24 MHz 28 MHz
Cauer 76 Mhz LO LPF
45 MHz 2 Pole filter
45 MHz mathing to 800ohm 4 poles crystal filter (20kHz wide - from www.surplussales.com )