Hello Farhan, you are right. I think my problem is mainly caused by the shielding of the input BPF and the strong broadband signals here in Europe, coming from my active antenna, which I am using for lissening.
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A tuned dipole for 20m would be better, or a good attenuator between the BITX and the active antenna.
Because of the harmonics of the VCO, the harmonics of the mixer and the very strong broadcast signals, leaking along the BPF, I have some birdies and images. With the LPF between it, it improves a lot. But I think it is better to make a good shielding then a LPF.
--- In BITX20@..., Ashhar Farhan <farhan@...> wrote:
my mental model is that the diode mixers work by clipping the vfo into
a very distorted, odd-harmonic rich signal. this is because they _are_
non-linear circuits. an LPF that filters out is useful though. we have
grown to a casual addition of the LPF after the VFO 'because doug
(demaw) said so' without much thought of why it is required given that
the mixer would distort it horribly anyway.
try designing the filter to allow the second harmonic (even harmonic)
in by raising it's cut-off a little. if the birdies and the images
return, we would have tested this hypothesis.
On Sun, May 30, 2010 at 6:47 PM, Marcel <pe1lnx@...> wrote:
Hi to all,
I have made the BITX20 version 3 with the FLL counter and the Hartley VCO. It all works fine and is very stable. After doing some measurements, I discovered the rich harmonics of the VCO (up to 100 MHz). It is a well known disadvantage of the Hartley design.
So I decided to add a 7-pole LPF between the VCO and the BITX. I have used the SVCFilter freeware program. In my photobook you can see the results of the calculation.
After inserting the LPF, the BITX becomes more quiet. No mixing of strange signals anymore. Before, the first and second harmonic were only 26 and 29 dB down. After the modification, they were more than 60 dB down, and no harmonics up to 100 MHz.
So, if you are using this design, it works well, but take care of the harmonics.