View Single Post
Old 12th Oct 2017, 10:44 pm   #17
G0HZU_JMR
Dekatron
 
Join Date: Sep 2010
Location: Cheltenham, Gloucestershire, UK.
Posts: 3,077
Default Re: SSB Signal Generator

Probably the first option would be to post up a 39MHz version of the minikits elliptic LPF. See below for such a 5th order LPF. Filters like this give a fairly sharp attenuation slope but the price for this is that the component quality and tolerance needs to be fairly decent. Ideally, you need to use 5% parts. C3 will probably be the most sensitive part in terms of tolerance. The inductors need to have a reasonably high Q. anything over 70 would be fine.

Because you are using this to drive a mixer, you probably don't need to maintain a very level output across 5-39MHz and the circuit below will probably deliver about 2 to 2.5dB droop when driven by the DDS as there will be 1.5dB due to sinx/x and maybe 0.6dB in the filter. But the filter may give up to 1dB droop depending on the components you use. So it could easily mean 2.5dB droop overall.

If you want to add some equalisation then maybe try the very basic equaliser circuit below. Simply place it after the LPF.

The design below gives very little attenuation at 47MHz but I don't think this matters in your case. A 47MHz LO spur probably won't give significant problems when mixing with 9MHz for the 10m band.

If you want to make something with better rejection at 47MHz I can show you a 7th order LPF design but this will need very good quality components. It will punish any attempt to use sloppy parts because the full performance will not be realised.
Attached Thumbnails
Click image for larger version

Name:	minikits39.jpg
Views:	90
Size:	91.6 KB
ID:	150662   Click image for larger version

Name:	39equaliser.jpg
Views:	97
Size:	60.9 KB
ID:	150663   Click image for larger version

Name:	39equalised.jpg
Views:	88
Size:	93.8 KB
ID:	150664  
__________________
Regards, Jeremy G0HZU
G0HZU_JMR is offline