Saturday, January 22, 2011

The JUMA-RX1 a DDS Controlled Grabber Receiver

Yep, this time it's Finnish, guys. JUMA, I guess that is short for JUha (OH2NLT) and MAtti (OH7SV), sells some nice DDS kits. I found them when looking for 136kHz and 500kHz transmitters.

JUMA also offers shortwave kits. I figure, the RX1 kit makes a very nice grabber receiver, covering the 2.2km, 600m, 160m, 80m and 40m bands by means of a DDS VFO.
The rest of the receiver is old skool direct conversion, so using there is no side-band rejection. Not a great deal for 3500800Hz or 7000800Hz, since, not much signal is to be expected below our bands.
The QRSS range in the 2.2km, 600m or 160m bands are at frequencies where the other, i.e. lower, side-band can be occupied. Filtering for those bands will be a necessity. For the 160m band one may consider building a crystal front-end filter for the QRSS frequency. For 2.2km and 600m, this would certainly not do. Not all is lost for LF and MF, since preferred aerials (magnetic loops and frames) are ideally very narrow-band and will, if tuned right, help to at least reduce the lower side-band.
Additionally, filters can be build for the 80m color burst frequency, 40m WSPR and for any other frequency for which crystals are available.

The RX1 kit is all SMD, this could be seen as a hinder by some builders. Personally, I slowly get used to the tiny parts. With a proper PCB holder, a special SMT soldering iron and respective 0.5mm solder SMT is not much harder than regular through hole electronics. SMT has even got advantages, e.g. no excess leads need to be trimmed.

The DDS and the housing alone would justify the expenses of the kit, the DC-RX is essentially for free. Moreover, all mechanical bits and pieces are supplied.
With JUMA even offering the source-code of the firmware for download, which is even written in C, I figure one could easily modify the hardware to a superhet (e.g. 455kHz IF) and program an offset into the firmware.
A possible mod, in my view, could be to have the receiver PCB operated at 455kHz, with a decent IF-filter in place of the 40m low-pass. The DDS-VFO will, in such a scenario, serve an additional front-end, whatever it will be...

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