IQ Filter Modifications

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Introduction

Development and testing of the Portsdown Filter-Modulator board was initially conducted using a Sharp tuner. The Sharp tuner measures MER in a different (less-critical) way to the Serit tuners that are now in general use. The reported MERs using a Serit tuner, particularly at low symbol rates, are lower than might be expected. The cause is that the IQ filters (low-pass, switched capacitor) were set to be slightly too narrow, and this can be corrected by decreasing 4 resistors on the filter-modulator board.

Applicability

This modification is only required on home-constructed boards, and on manufactured boards with serial numbers below 400. The latest batch of built boards (serials 400+) already have the modification incorporated.

Modifications Required

The modifications to widen the IQ filter bandwidths (only required at 500KS and below) just involves 4 parallel resistors or replacements:

680K in parallel with R24A (98.0K) [or replace R24A with 86.52K Digi-Key 311-86.6KFRTR-ND]
390K in parallel with R24B (49.0K) [or replace R24B with 43.26K Digi-Key 311-43.2KFRCT-ND]
270K in parallel with R24C (36.8K) [or replace R24C with 32.5K Digi-Key 311-32.4KFRCT-ND]
180K in parallel with R24D (24.5K) [or replace R24D with 21.6K Digi-Key 311-21.5KFRCT-ND]

R24A through D are on the underside of the board and you can use resistors with leads across them in parallel. Some of the resistors specified above have recently become difficult to purchase, if so, try other physical sizes.

Effect on the Transmitted Signal

The following measurements were made using MiniTioune V0.6d software.

SR MER Sharp MER Serit pre-mod MER Serit post-mod
125 24 18 24
250 24 19 24
333 24 19 24
500 24 20 24

The effect of a 19 dB MER transmitted signal against a 24 dB MER transmitted signal will be to add extra phase inaccuracies on noisy paths, but in testing it has been very difficult to measure any difference in weak signal performance. It is estimated that the penalty in difficulty in receiving the signal is in the order of 1 dB.

G8GKQ 17 Sep 18