A Second SDR for the Desktop

soft66-in-box1 Figure 1: I liked the Soft66 Lite so much I decided I needed another for the desktop, in addition to the one purchased for the homemade tablet. For the desktop unit, a separate enclosure was needed

These tiny little things are addicting! The enclosure was outfitted with a couple switches to do bandswitch duty, since (unlike the homemade tablet) there will be no GPIO to apply for that function. A big, clunky SO-238 connector was attached to the box, to accept the PL-259 plug from the antenna coax. And of course, holes were made for the audio and power connectors.

soft66-in-box8f Figure 2: The various appendages have been added to the SDR board to make a handy desktop unit. The wires have yet to be soldered between the switches and the jumper headers. I ran out of female headers, so that’ll have to wait until the next BG Micro order. 🙂

desktop-sdr-with-cover1 Figure 3: The soft66 Lite powered SDR radio, with cover attached, doing service next to the desktop.

The diminutive little board might not match the sensitivity of the Omni, but for the kind of purposes I have in mind, I don’t think the difference amounts to very much. I suppose that I could insert a preamp into the coaxial feed. For now though, it’s working fine for what it is, and I can envision having it paired up with a little QRP transmitter of about the same compact size. Perhaps, both could be put inside the same box, if it were made a couple inches longer. The circuit board is about 2″ x 2″ wide (inches) and the box that I put the board into is 2.5″ x 4″. Including the lid, it’s two inches tall. I like the flanges on the little box, for attachment to (anything).

Next to the unit can be seen the Imedion powered battery pack, which can supply the SDR radio for weeks, and a 2W QRP transmitter for a day or so, key down, and probably a week intermittently. It’s the same battery pack that I use for backup on the homemade tablet, as I’ve described on https://programmingmiscellany.wordpress.com – all I have to do is adjust the pot on the converter from the tablet’s 5VDC to about 7.5VDC. Another thing I could do is bypass the LM7805 regulator, since all it’s doing is 7.5 -> 5VDC anyway.

Notice that I didn’t include the two extra crystals in the enclosure yet. Someone not familiar with SDR may wonder why multiple crystals are necessary. The reason is that the total bandwidth is limited by the sound card. A cheap sound card might give you 48KHz of bandwidth. A moderately priced one may provide 96KHz of bandwidth. A very expensive card could have 192KHz of bandwidth. By selecting from a small number of crystal frequencies, one can cover 48, 96, or 192KHz swaths of each band. In the case of the 192KHz card, you’d need only three crystals to cover the entirety of most HF ham bands (192 x 3 = 576 KHz, which is good enough to cover even fifteen meters (one of the wider bands)).

More problematic is the switching of the BPF (band pass filter). The Butterworth filter cuts the adjacent bands significantly, although I can hear signals in all three of them. I’m thinkng that a reed relay or five might do service here, but haven’t decided how to to it for sure. You can get some pretty tiny ones. Maybe the reeds would work for the crystals as well. The bandswitch on any old time tranceiver is always messy (and usually a troublespot).

The page at http://www.wb5rvz.com/sdr/hf_bpf/ illustrates the use of a pair of FST-3253 ICs to do the filter switching. This adds complexity and cost to my $20 setup, though. They’re only a dollar each, but I’d need the PC board and a few miscellaneous other parts. WB5RVZ is using the chips to switch BPF in an SDR radio, as I am. He may make a convert of me. The ARRL has a diode control-voltage switched BPF on the ARRL.org site, and that deserves consideration. I can see the circuit at wb5rvz combined with a BCD counter of some sort to provide BPS switching via a single momentary switch contact. It wouldn’t be that bad, I guess; I think I could fit a little piece of PCB into the existing box well enough for that.

I suppose that if I’d take the time to re-assess the whole switching arrangement, I’d homogenize the thing, and make it all play together. The “band switches” I used in anger, happened to be in the junk drawer, and kept my project minimal. Actually, a SPST switch must be added to the ones shown to provide the 40 meter band. They’re really counter intuitive to operate, so it’d be better to consolidate the BPS functionality with other related functionality, using an electronic switch (more complexity) for inuition’s sake. I may need to purchase the FST3253s. It seems to get more attractive, as I think about it longer. Someone on stackexchange suggested a CD4051B, but that would require a current driver on the output. The FST3253 already has that – and a very low output impedance.

Note that this all applies to the Soft66 Lite, because of its design. Other SDR radios may have different parameters for this. But, they’re not $20. The 192KHz card is a little pricey. If one looks on Amazon, they’re typically around $200. But, you say you’ve seen cheaper? You only think that because the BIG letters in most advertisements show the PLAYBACK sample rate, and not the CAPTURE one – which is what we need. The $50 cards may have a PLAYBACK of 192 KHz, but only 96KHz for capture.

On the Soft66 Lite, the band is selected with the jumper pins, by varying the multiplier (of the crystal fundamental). I have already installed switches for that (well, except for the 40 meter SPST). The switching scenario becomes much easier on the homemade tablet, where a GPIO connection is not only more convenient (no holes to drill for switches) – but facilitates the automatic scanning of bands from low-end to high-end, and the automatic switching of bands through the multiples. It would be pretty easy to make a HF band scanner of this, which is exactly what I intend to do. Stay tuned.

Basic band switching (other bands possible)

Band HD1 HD2 HD3
80 x x 2,3
40 x x 1,2
20 2,3 2,3 1,2
15 2,3 1,2 1,2
10 1,2 1,2 1,2

Note: x = no jumper.

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Note: the soft66 Lite hardware is a product that is sold on a Japanese website (http://zao.jp/radio) – and is not affiliated with this site or author in any way.

Imedion and PowerEx are trademarks of Maha Energy Corp.  This author and site has no affiliation with them.

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