Re: Bitx-40 DDS VFO


Hi Jonathan
Thank you very much for your prompt answer.
I have solved the problem increasing the trimmer from 20pF to 60pF and add
10uH in L5 again.
Now I can get the LSB in 40m as before to use the DDS.
Have a good one

2012/11/5 Jonathan point <jonpoint@...>



If your VFO was on the 'low side' of the IF (~1.2MHz), you should get LSB
from an xtal set for the LSB side of the filter. If you use a DDS which is
above the IF (e.g. 15.2MHz), you will now be on USB. You need to adjust the
xtal to the other 'side' of the filter passband.

Some DDS (PA0KLT for example) only work on the 'high' side, which means
that you have to be able to shift the BFO frequency to the 'wrong' side of
the passband for LSB. This is what I did with my 40m mobile BitX. N3ZI's
unit works on either side, but tops out at 34MHz. Other units have other
limitations, but I haven't played with them yet =]

So, what is the frequency of your DDS? If above the IF, you get a sideband
reversal. Think of it this way:

8MHz IF, 1MHz VFO, 7MHz signal frequency:
= 7.999MHz (1kHz LSB @ IF) - 1MHz VFO = 6.999MHz (1kHz LSB)
The LSB IF signal becomes an LSB signal frequency = no inversion
8MHz IF, 15MHz VFO, 7MHz signal frequency:
= 15MHz VFO - 7.999MHz (1kHz LSB @ IF) = 7.001MHz (1kHz USB)
The LSB IF signal has now become a USB signal = inverted.
Hope this makes sense =]

Note that ~1MHz is not a great choice for the HFO, because it will likely
cause lots of harmonics and birdies.


Jon Point

--- In BITX20@..., "Juan Carlos" <jcberberena@...> wrote:

Hi Fellows
We have completed a lot a Bitx-40 in Cuba using its own VFO in 40m with
the 8.192Mhz IF frequency using IF-VFO and we take off L5 because is not
neccessary to get the right tuning.
Now I am trying to add a DDS but working VFO-IF and with the same
configuration in the oscilator I can not find the right point and I think
now with the DDS I am in the USB.
Please some one can clarify me this problem And how to solve?
Thank you in advance
Jc WJ6C ex/CO6BG

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