Filter Shape Picture ( Re:BITX20A board finished (report JL1KRA)
Hi, Robby and AH6GItoggle quoted messageShow quoted text
Thanks your comments,
My chasis preparation need more time. So, here is some bench
pictutes in progress.
In this weekend I am doing filter alignment.
Since I couldn't adjust TX carrier point correctly
with my 2 tone generator, I used Frequency Responce Mesurement System
(FRMS) and checked the BTX20A filter.
The FRMS is popular homebrew tool among JA QRPer using DDS and AD8307.
Without spectrum analyzer this is a good tool to see filer shape.
As you see the first pic, the bandpass has 10dB ripple.
DF5SF fileter pics shows similar shape. I suppose the
BTX20As are showing same character.
DF5SF changed C54,55,56 to 150,200,150pF from original 82,100,82pF.
I am now reaching 68,0,68pF to minimize the ripple. But
the skirt of the filter is worser than before to supress carrier.
May be this problem had been discussed in this list.
I am just arrived the point. I will check the previous messages
Junihi Nakajima JL1KRA
--- In BITX20@..., "ah6gi" <ah6gi@...> wrote:
Hi, All again,toggle quoted messageShow quoted text
Thanks to previous filter discussion around November 7th,2007.
Back to basic, I red W7ZOI article in W1FB's Design Notebook(ARRL).
As recommendation, I replaced C54,55,56 to philips trimmer
as in picture,
I reached C54=82pF, C55=6pF, C56=82pF and changed it to
fixed capacitor. fc=11001000Hz
The ripple in bandpass was minimized less than 3dB.
After this, setting carrier OSC to 109995000Hz,
2-tone injection caribration give me a good result.
Now the both tone are modurated same level.
Still there is some instability the MOS-FETs behaviour
in higher drive level. It's better than before.
The above capacitor value are my case, I am not sure
this will work in other case.
For those who will adjust C54,55,56, I recommend you to
solder these in bootom side of the board. Once these are
soldered between the tall crystals, they are difficult to
JL1KRA Junichi Nakajima
--- In BITX20@..., "qrpjl1kra" <mx6s@...> wrote: