[hpsdr] Ground: Some questions of a newbie

[Sheldon Hands]

Hi Alexander and any other  "Newbie"

A few notes about what to expect as you get beyond the rx mode hi!

If you intend to run anymore than the 500mW from Penelope you will need to 
address the case issue.
Merc, Pen and Atlas open on the desktop are fine for first impressions but 
you wil quickly run into trouble
as you increase the transmit power level. Without adequate sheilding it 
becomes very difficult to discover
if sources of RFI affecting the transmit modulation are due to periphals or 
direct injection to the boards.

Homebrewing the HPSDR setup is slightley different from a analogue stand 
alone QRP or medium power tcvr where
everything is in one box. The notes below detail a few of the problems I 
encountered in getting my setup going
driving a 1KW triode amplifier. I still have a few issues to resolve but I´m 
currently enjoying dx contacts 80 to 17 mtrs.


The best inital fix is to mount Atlas on a sheet of 1.5/2mm alloy sheet with 
5 or 10mm threaded spacers.
A boarder of 10mm angle stock will allow you to attach a one piece top and 
side cover made from  1.4mm FR4 double side pcb seam
soldered together. There´s no point doing all this unless you also fit a 
rear cover. Only open the card slots around what slots
are filled and only as wide as needed the connectors. This arrangement will 
also be very suitable for inclusion in a Midi tower case
to isolate the Rf part from the pure digital of the any embeded PC.

On the bench use a wide earth strap to both the PSU and any following 
amplifier to insure a common ground,
don´t just reley on the coax shields to do the job. Any leads out or into 
the box will benefiet from extra inductanace in the form of beads or rings.
 I recommend this even for the the rx\tx coax, I use minature coax with some 
43 material sleeve beads for this purpose.


Once you get up to the KW level in the following amps there can often be a 
fair amount of rf flying around the shack.
Without adequate sheilding it then gets very difficult to trace sources of 
rfi into the microphone circuits.
Initially bad cases will show up on Merc as distorted low level audio on the 
phones or speaker in transmit even with monitor switch off.
if you use open wire feeds with a 4:1 Balun make sure they are Gunella style 
on two seperate cores.
 Bifilar on a single core will match but balence is very poor with high 
shack currents.

It´s economical to use PC Skype style headsets and powered speakers, they 
work ok but do need some help in high RF enviroments.
Usually some beads and a 1nF on audio leads cures problems. Many PC 
microphone headsets have Electret inserts,
the need for bias volts is not stated on the packaging ( I suspect on board 
audio and sound cards have "phantom Power" as standard)
so you will need to fit the necessary Pen jumpers. (before I discovered this 
my headset microphone worked ok at LF but with bad distortion at hf, It must 
have been rectifiying enough rf in the fet buffer at some frquencies but not 
at others.)

If you have not used the your PC actively before when on the radio you might 
encounter some further RFI problems.
My Optical mouse has a habit of switching  back to receive  in a qso with no 
key press. It appeared my finger capcity
was inducing enough rf to cause switching. A SMD 1206 10nF fitted very 
nicely across the switch tracks and solved the problem.
Key down on 40mtrs caused error beeps on the PC and was traced to the 
keyboard, here winding a few turns of the lead on a
 ferrite ring solved the problem.


You may or may not need Alex in your system see the current meassages on 
this. If you don´t need it for input you will need some interface for
a LPF after any medium power amp. if you have a existing  LPF that  is high 
side switched (ie needs 12V to activate not GND) Penny will accomadate this 
via the DB25 lines by pulling down the base of a suitably  biased BC640 or 
similar transistor.

Shel CT1IZU








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