G4HUP
DFS Data
Back to Documentation page
DFS Schema DFS Filters
DFS Filter Data Page
This page gives a brief explanation of the purpose of each of the filters, 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 unsheilded coils from the Toko and Coilcraft ranges, typically the S18 (obsolescent) MC117, MC120, and Unicoil 10 types. A top coupling capacitor location is provided on the PCB, but is unnecessary when inductive coupling is used. An un-masked land is available on the top side of the PCB to allow creation of a screen around the filter - this may be necessary if you also use an inductively coupled filter after the VHF mixer (F7). Some improvement in the filter shape can also be obtained by using a small piece of PCB material as a screen between the two coils and introducing a very small top-coupling C (0.4pF or less) as C206 (F1) or C603 (F7). 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 link coupled tuned circuits based on re-wound Toko 10K 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 is a single stage tuned circuit for the LF multiplier, based on a Toko 10k series inductor (re-wound). 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 tuned stage on the collector of the post mixer amplifier, followed by a two stage, top-coupled, LC filter. The tuned amplifier allows a multiplication of the mixer output frequency to be made, if necessary, and the two following tuned circuits allow this to be filtered to a very clean input for the second mixer. Filter F7 (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. If you are using an inductively coupled filter in this location, the top-coupling capacitor is not required - but note the comments under F1. Filter F8 (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 F9 (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.
- diode - multiplication factor of diode multiplier stage
- Div1 and Div 2 - ratio of respective dividers
- MF Mult, LF Mult - multiplication factor required on divider 1and divider 2 outputs respectively
- Div2 i/p - source of input for second divider - eg Div 1= div 2 fed from div 1 output (same freq as the multiplier/LPF) or 10 means it is fed directly from the input source etc.
- Mix 1 and Mix 2 - identifies LSF (subtractive product) or USF (additive product) of mixer
- Mix 1 Mult - optionally specifies multiplier ratio applied to output of mixer 1
DFS Filter Data top The frequencies for each filter are shown in the table below, alongside the output frequency. This information is based on the Schema's shown in the previous section. Clicking on a filter frequency will open a window showing a set of component values known to work in this position. Where more than one filter design option has been identified for a particular instance, these will be separately listed in the table that opens
Page created 26 Oct 2007
Page last updated 13 Apr 2009
