I built a small two stage band pass filter that fits in the box with the SoftRock. Essentially I took the half wave input filter used in the receiver front end and duplicated it twice, including the shunt inductance of the input transformer. Combined with that already on the SoftRock itself, this provides a three section filter which is more than 100 dB down at the second harmonic of the local oscillator (and much more at the third harmonic). Performance of the filter was verified on a spectrum analyzer after construction. It is virtually identical to the modeled plot shown here. It is always good when real world results match the design model!
Some will argue that this filter is too simplistic. That may be true from a purist standpoint, but results have proved more than adequate. I wouldn’t call it a state of the art design. However it is very inexpensive and fits in a small space. Pass band loss is about 5 dB, mostly due to Q of the inductors. I wasn’t able to come up with a filter that looked better on reasonable size toroids without running into issues with overlapping turns and distributed capacitance causing problems. With any practical antenna (and preamp if needed) I don’t see this loss as a problem. Return loss isn’t great, but exceeds 10 dB across the pass band, which is approximately 420 to 520 kHz. In a receive application I see no problem with this. The input impedance of receivers is all over the place, some much worse than this one.
Live results are gratifying. Even with one of my 22 dB gain W1VD preamps in line, there was no sign of broadcast stations appearing in the useful range of the receiver. (Note: I don’t need the preamp with my receive antennas. I only used it as an acid test to evaluate filter performance, since it makes the signals we want to reject that much stronger.)
The receiver and filter fit nicely into a Hammond 1590BBK diecast box which gives it a nice look in a compact package. I probably didn’t need to use miniature coax (in this case RG-188) for the very short runs interconnecting the BNC input connector, filter board, and SoftRock. At this frequency and with this layout, the very small capacitance between unshielded wire leads would probably not cause harmful leakage around the filter. But I have a bunch of this stuff, so why not use it? Besides, the teflon cable is a joy to work with. The insulation on most of the small hookup wire I have melts easily while soldering. Not this stuff!
Building this receiver was a pleasant and worthwhile experience. I finally understand how simple SDRs work. Studying the schematic, reading the descriptions of individual stage function, building it and probing various locations in the receiver with an oscilloscope and spectrum analyzer was very enlightening. I learned more about filters. Perhaps most importantly, I am no longer afraid of projects involving surface mount components. Performance of the completed receiver is good enough that I can recommend it to anyone wanting a simple, inexpensive receiver for the 630 meter band or anything else in the 420 to 520 kHz range (with a sound card sampling at 192 kHz the receiver will tune 368 to 560 kHz, but the filter cuts off at 420 and 520 kHz). It outperforms my FT-2000 in every measurable way. If there is one caveat it would be that if one wished to use it for serious reception of QRSS or DFCW signals it either needs to be well insulated or have a crystal heater added. For everything else its frequency stability is excellent. To my knowledge there is no QRSS or DFCW activity on 630 meters at the present time.
Update 19 January 2018: Adding schematic for the two additional stages of front end filtering. Coils are the same as those in the Softrock Lite II 455 kit.