Sunday, 2 February 2014

DVB-T dongles and old modems

Ive ordered a DVB-T dongle to try and use as an SDR receiver instead of the PCTV one, which doesnt seem to want to work for anything. Having read various reviews, and a couple of articles in Radcom, ive opted for one based on the RTL2832U decoder IC, and the R820T tuner. This combo seems to be touted as having a frequency range a bit lower than the E4000, but is easier to get. Im not too bothered about coverage above about 1.5GHz anyway, so the extra coverage in the lower spectrum is preferred,

 It might take a while to get here from Hong Kong though. I'll probably replace the plastic case with a metal box as suggested in Radcom, along with the power line filtering and a proper coax connector. I have a few small die cast aluminium boxes spare, or it might be another Altoids job! In conjunction with the free download SDRSharp software, Im hoping to put this to use monitoring the UHF military airband.

Various parts are on order for building another digimodes interface. The central part of these is the isolation transformer. Unfortunately, these new are rather expensive and hard to come by. Luckily, what are cheap these days are 56k PCI modem cards, which have on them, along with a few other useful parts, 1:1 600 ohm isolation transformers!

The photo shows a typical example, the transformer can clearly be seen. There is also a DPDT relay, some electrolytics, and a little buzzer! plus various SMT bits which are generally not worth the effort, but sometimes amongst those are SMT LM386 audio amps. The big difficulty is removing the transformers without breaking the very delicate wiring. Cutting the board away is usually the best idea until only easily unsoldered bits are left.

The various jack sockets and potentiometers I need have also been ordered from China. I know people will say to 'buy British' but when I can get ten parts from China with free shipping for the same price as one or two from a UK supplier, knowing full well that the UK suppliers parts are made in China, then why should I be out of pocket?

Tests of the Alinco DX-70 and the digimode interfaces level control last night showed that the same overdrive power reduction occurs on both high and low power settings, so it is almost certainly an excess audio level problem. Monitoring 80m last night showed a rather full band of PSK signals, but I was generally too tired to make any QSOs.

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