Leif Asbrink wrote:
Hi Christophe,
As for the calibration of flat response I use an HP 8012A pulse
generator. This works perfectly for the Softrock. However for
the Time Machine it seems like no matter how well I ground and
shield there is always LF noise getting into the system...
(the Softrock seems to be very noise insensitive)
Have a look at the schematic diagram here:
http://www.expandedspectrumsystems.com/prod2.html
In early versions C8 and C11 were far too small (0.1 uF I think)
Check that they are big electrolytic capacitors.
If I were responsible for the design I would break the DC
connection between the output ground and the power supply
as well as between the output ground and the RF input ground.
One way would be to connect J3, pin1 with wires to all of the
ground points of the AF circuitry so all of them would be
separated from the ground points of J1 and J5.
It means the groundings of U1,U2, R5, R7, C10, C22, R4,R13
and C11. Then connect a 1k resistor from the J3 ground to the
J1/J5 ground.
There will be a need to connect the U1 and U2 grounds to the
J1/J5 ground with decoupling capacitors to leave the RF current
path unchanged around the mixers. The U3 and U4 ground points
have to stay connected to the J1/J5 ground since they carry
DC currents.
These changes are a bit difficult to implement because of
the ground plane on the PCB but it works very well.
See http://www.qslnet.de/member/on4iy/linrad.html
This page has the before and after mod pictures of the spectrum.
My LF noise peak is mostly gone and the system can be
calibrated now. I still have to look at the calibration
in more detail but at least the routine runs fine.
73 de xtof, on4iy
#############################################################
This message is sent to you because you are subscribed to
the mailing list <linrad@antennspecialisten.se>.
To unsubscribe, E-mail to: <[EMAIL PROTECTED]>
To switch to the DIGEST mode, E-mail to <[EMAIL PROTECTED]>
To switch to the INDEX mode, E-mail to <[EMAIL PROTECTED]>
Send administrative queries to <[EMAIL PROTECTED]>
