G1HBE

Better than Nothing.

All those numbers in such a small box...

August 2015.  I've been messing about with SDR's for a few years, starting with a cheap DTV stick and progressing first to a Funcube Dongle Pro Plus and now the new kid on the block the SDRPlay.  It's great - roughly the same level of performance as the FCDPP but with the ability to display up to 8 MHz of spectrum at once. It uses a 12-bit ADC which gives a reasonable dynamic range (72dB in theory). But as with most of the budget SDR's, the front end filtering is a bit skimpy and this can lead to things popping up where they shouldn't, especially bolow 60 MHz where there is an additional up-conversion.

I decided to do something about this and set about re-boxing the Play's PCB in a nice case together with a filter board. The good thing about building your own front-end filters is that you can filter the bits you are interested in and leave the others alone. I have an interest in LF, MW, Topband (160m) and the lower HF bands, so I paid most attention to these.

I had noticed that Topband  and especially 80m got a bit of a bashing from strong MW signals (and to be fair we do have some mightily strong ones around here), so I built a couple of filters to suit. I won't say I designed them, because I just put the requirements into my old favourite RF Sim99 software and out popped the designs. The 80m band is catered for by the HP filter, which covers 2 to 40 MHz. This is pretty wide as filters go, but it cuts off fairly quickly below 2MHz so it keeps out the MW signals.

Topband got a top-coupled BPF all to itself, and LF was treated to a 0 - 500 KHz LPF.  VHF (above about 120 MHz) is introduced directly to the input of the SDR board via a dedicated antenna socket and a very simple diplexer.

Below is the assembled 'sandwich' of boards. The SDRPlay PCB is nestling between the bottom board which is just used as a screen and the filter board above.

 

 

 

 

 

Look at that - proper coils and everything! Anyway, I needed some kind of box, so I fished out one of my last 'rally specials' that I'd been saving for just such a project. Below is the back panel of the box to which (after some very careful measuring, drilling and filing)  the board assembly is fitted.

 

 

 

Selection of the filters is done directly by a miniature 5-way switch.  I did consider using relays, but there weren't enough of them in the junk box. There's a 'bypass' position that gets rid of all the extra filtering and just uses the SDRPlay's filters.

 

 And here below is the finished article. It's an SDR with a knob on the front that selects the filters. There's red LED too, which is very reassuring.

 

 

 

And just to show how 80m was cleaned up by the extra filter, here are two shots. On top we have 80m when the band was dead. Most of those spikes are products from MW. To be fair to the SDRPlay, I could make them go away by attenuating the input or reducing the RF gain manually (AGC was on for these shots), but this tends to spoil the SNR. The bottom shot was taken just a few seconds later but with the filter in. 

 

 

 

 

I'm quite impressed with the SDRPlay. The support is good too - there's a website that has help pages on how to install the various bits you need and there's a forum where you can natter with other users. 

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