Filter Modulator ver2

From BATC Wiki
Jump to: navigation, search

Board information - version 2

PCBs supplied after 28 February 2017 are Version 2 and include the modifications to improve spectral performance.

F-M V2 Top.jpgF-M V2 U-side.jpg

Notes on the pictured example: Q01 not fitted (out-of-stock at the time), L50 replaced by hand-wound inductor (original lost in carpet) and R50A bridged to allow 5V input on J51.

General notes: Do not fit L10 or C11; they are not required if using a linear regulator. R33, R34 and R45 are not fitted as they are not required. C54A (100 pF) is shown as optional for high frequency decoupling on the circuit diagram, and there is unlabelled space for it next to C54 on the PCB; it is not required in this version. J51 and R50A can optionally be replaced with an LED and current limiting resistor.

The Version 2 the parts list and schematics are available for download:

Although the board includes the options to fit a switching voltage regulator (an R78E5), the regulator introduces spurs at -50dB on the output spectrum , so is not the recommended option.

The latest version of the parts list (13 Sep 2017) includes new part numbers for previously out-of-stock inductors.

DO NOT FIT L10 or cut the track beneath it as it does not suppress the spurs and introduces voltage drop, which upsets the modulator balance.

Modifications to Improve Constellations on BATC Boards before Serial 400

BATC Shop Filter-Modulator Boards with serial numbers prior to 400 and home-constructed boards using the parts lists and schematics above have the digital IQ filters for 125 KS, 250 KS, 333 KS and 500 KS set slightly narrower than the optimum. This was not apparent during development and testing on the Sharp and Eardtek tuners, but shows as a lower MER and a distinctive "4 constellations of 4" pattern when using the Serit tuner.

4of4.jpg

The IQ filter bandwidth can be increased to improve the apparent MER and constellation by changing the resistors R24A, R24B, R24C and R24D. The changes required are:

680K in parallel with R24A (97.6K) [or replace R24A with 86.6K Digi-Key 311-86.6KFRTR-ND]
390K in parallel with R24B (48.7K) [or replace R24B with 43.2K Digi-Key 311-43.2KFRCT-ND]
270K in parallel with R24C (36.5K) [or replace R24C with 32.4K Digi-Key 311-32.4KFRCT-ND]
180K in parallel with R24D (24.3K) [or replace R24D with 21.5K Digi-Key 311-21.5KFRCT-ND]

R24A - D are on the underside of the board and the parallel modifications can be made using normal leaded resistors.

This modification does not materially change the transmitted spectrum - it just makes the signal slighly wider and closer to the DVB-S specification.

Note that this modification is not required for BATC Shop ready-built Boards with serial numbers above 400 as they were manufactured with the revised component values.