Hi Stefan,
Thank you for sharing!
You wrote:
...
The RX has a 50 Ohm input and uses a SBL-3 DBM. The circuit just uses a
single BF862 amplification stage (about 20 dB gain), the rest is passive.
LO frequency is 125 kHz which is achieved from down dividing 4 MHz (xtal)
by a CMOS4060. So the IF is at 12 kHz for a 137 kHz signal. 12 kHz can be
sampled by the soundcard and so it can be processed by SpecLab. In SpecLab,
further band limiting, noise blanking, a auto notch filter, a frequency
shift to a 700 Hz CW sidetone and a narrow CW filter can be realised.
It is a wideband receiver that displays 125...149 kHz if a sample rate of
48 kS/s is used. Some internal filter are focusing on the LF amateur band
and provide a limiting of strog outside BCD stations such as DLF, DCF39 and
so...
I'm running a comparable concept of converting down to baseband.
The problem of choosing 125 kHz as LO-frequency *here* was LORAN-C from
Sylt, located ~100 km NW from the rx. The whole signal bell was mirrored
up, centered around 150 kHz even though I'm using a 5-pole bandpass filter
between preamp and mixer (NE612). My frequency region of interest is not
the ham band and some sidebands from BC stations do not bother me compared
to that steady forest of LORAN lines. So shifting the LO to the upper
(other) side elsewhere around 165 kHz made me happy. Using the HC4060 like
you it was easy to find a sufficient crystal of 1 or 2 octave(s)
higher/lower in frequency.
Peter, df3lp
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