Dear LF Group,
I don't claim any great originality for this, but I have modified a couple
of the SoftRock Lite v6.2 kits by KB9YIG for use on 136k and 500k - the
details are attached.
I changed the local oscillators to 4MHz for 500k and 1MHz for 136k, with the
"x8" jumper set. This gives nominal LO frequencies of 500kHz and 125kHz
respectively, putting the output frequency within the range of a sound card.
At 4MHz, the existing SoftRock LO circuit just about works, but it didn't on
1MHz. I made some component value changes to the oscillator and buffer to
get a reasonable waveform on each band, which are detailed in the
attachment. 4MHz crystals are easy to get - I had several 1MHz xtals in my
junk box, but these seem to be harder to buy now than they once were. An
alternative for 136k would be to use a 1MHz DIL oscillator module, feeding
the flip-flop directly. Another possibility is a 500kHz ceramic resonator,
but the stability is not as good as a crystal.
The original input filter could be re-designed for the LF/MF range, but the
components wouldn't fit on the tiny board, and the rejection of broadcast
band signals would not really be good enough. So I designed off-board input
filters with higher rejection of signals at harmonics of the LO frequency.
Like the original design, these require no adjustment and use fixed value
components - values are given in the attachment. I used 1/2-watt resistor
sixed axial-leaded chokes on 500kHz, which were fine. Similar components had
insufficient Q at 136k, but the larger radial-leaded chokes wound on a
ferrite bobbin were adequate..The original tuned mixer input balun
transformer is replaced with an un-tuned ferrite-cored transformer.
I have used the two SoftRocks with I2PHD's "Winrad" software, and DL4YHF's
SpecLab, both with excellent results. The Rocky software does not like the
PCs I have available. The sensitivity is not quite as good as my other
receivers, but since I normally use a preamp with various receiving loops,
this is not important. The sensitivity should be adequate when used with a
TX type antenna. It is important to set up the amplitude and phase balance
carefully, especially on 136kHz, where there is a lot of Loran noise in the
image sideband.
Let me know if you have any problems viewing the attached schematic,
Cheers, Jim Moritz
73 de M0BMU
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