G4HUP
DFS Data - Issue 2
Back to Issue 2 Documentation page
This page gives a brief explanation of the purpose of each of the filters used on the Issue 2 PCB, and links to filter data for specific frequency implementations.
As more information becomes available, this page and the tables will be updated - eventually you should be able to click to find filter and component values that will work in almost any implementation of the DFS! Filter Functions Filter F1 (Sheet 2) - two pole LC filter to select the wanted harmonic from the comb produced by the diode multiplier. The PCB implementation allows the use of shielded or unshielded coils from the Toko and Coilcraft ranges, typically the S18 (obsolescent) MC117, MC120, and Unicoil 10 types. It is strongly recommended that this filter is screened around the outside and between the inductors. An un-masked land is available on the top side of the PCB to allow creation of a screen when unscreened coils are used. Do not use a magnetic screen material - eg tin-plate. Filter F2 (Sheet 9) - This is the filter option in the MF multiplier path (selected by links SJ901/2) and allows you to select the appropriate multiple of the MF signal from the divider. Currently this is implemented as top coupled tuned circuits based on Toko 7K series coils Filter F3 (Sheet 9) - This is the LPF option, selected by links SJ901/2, which can be used when the frequency out of the divider requires no further multiplication. It is implemented as a three stage LC LPF in 0805 size SMD components. Filter 4 (Sheet 3) - This identical in concept to the F2 filter. The multiplier is selected via links SJ301/2. Filter F5 (Sheet 3) - This is the corresponding LPF to Filter 4, selected via links SJ301/2 - again, it is intended for use when the output of the second divider is the wanted frequency, and requires no further multiplication. Filter F6 (Sheet 5) - F6 is the post mixer filter for the MF mixer, M501. It is implemented as a two stage BPF following the mixer. The output of the filter is capacitively tapped into the base of TR501, which can be used as an amplifier or multilpier. Filter F7 (Sheet 5) - F7 is the output filter for the MF post mixer amplifier/multiplier, and allows this stage to be used as a mutliplier. Filter F8 (Sheet 6) - A two stage, top coupled, LC filter to select the wanted output of the VHF mixer, M601. As with filter F1, the PCB implementation allows for the use of either shielded or unshielded coils from the Toko and Coilcraft ranges, and a PCB land is available for you to construct a screen around this stage, if needed. See the comments under F1. Filter F9 (Sheet 7) - This is the crystal filter, using 5th overtone crystals, cut for the wanted output frequency. These are room temperature, series resonant crystals, with no special requirements. The filter is adjusted for maximum output and minimum levels of unwanted signals using the three trimmer capacitors. Some improvements in balance of the low level spurii can be achieved by careful fine adjustment of the previous stages during this final tune-up. The values for the capacitors and inductors in the filter are known to be effective over the range from 90 to 120MHz, and should need no alteration. Filter F10 (Sheet 7) - Output LPF to reduce levels of second and higher harmonics. It is implemented as a three stage LC filter in 0805 size SMD components - no tuning is possible. This filter is essential to the output purity of the DFS, and must not be omitted. Harmonic products of the HF component mix with the low frequency signal giving side frequencies spaced around the second and higher harmonics - from f2 upwards the amplitude increases to f4, where the side frequency is equal in amplitude to the harmonic, and is only about -25dBc - higher products diminish in amplitude. The LPF reduces the level of f2 to better than -45dBc (from -15dBc) and reduces all other to better than -60dBc - ie a similar level to other spurii.The Schema listed here show how the input frequency can be multiplied and didvded to produce specific output frequencies. It also shows which mixing product (LSF or USF) should be chosen from the mixers to achieve the wanted result.
Page created 19 May 2008
Page last updated 19 Mar 2010