I was pleased to hear Roger's 73kHz beacon on air earlier in the week.
Although a little down on its usual level, I was suspicious that my converter
wasn't performing as well as it does on 136k.
Using Roger's beacon and some transmissions on 136 as test sources, I
discovered that my receiver was being "de-sensed" by the level of local
oscillator feedthrough.
The converter upconverts the LF band to an IF of 10MHz. Unfortunately the
closer you get to "10MHZ" the lower the performance. 136 is OK but 73 was
poor in comparison.
As I have already fitted a multi turn pot to optimise carrier balance on the
MC1496, I appeared to need a very narrow filter on the output. normal coils
/ caps were no use, but the answer came in the form of an 86 pence 10MHz
ceramic resonator from RS components.
I fitted two small ferrite transformers with a 1:9 and 9:1 ratio back to back
on the output of the converter. From the centre point (at 350 Ohms) I
connected the resonator in series with a 65pF variable to ground. Listening
to the 10MHz breakthrough and tuning the capacitor I managed to reduce the
level by 27 - 28dB.
The performance on both bands has been improved. MDS on 136 has been
improved by about 1 -2 dB, but 73k is 10dB better than before!
The technique could be useful to anyone trying to build a robust RX (DL)
using high level diode ring mixers.
resonant frequency of the "notch filter" could be set anywhere from 9.6 MHz
to 10.05MHz.
73
David
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