[hpsdr] Ground: Some questions of a newbie

[Alexander Geckeler]

Sheldon Hands schrieb:
> ***** High Performance Software Defined Radio Discussion List *****
>
> 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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Dear Sheldon,

many thanks for your detailed description - this will help me very much!

My "layout" of bringing RX-HPSDR into a housing are as following:
- Atlas, Ozy and Mercury will be placed into a metal case
- Atlas will be mount with metal spacers of 10mm hight directliy to the case
- In the case I will mount a 12V fan to make sure that heat is 
transfered outside
- The power supply (an ATX micro) will be placed outside the case
- At the back of the case I will cut out the metal sheet as narrow as 
possible around the plugs of Ozy and Mercuy

As I understand you right, you are suggesting:
- connecting the schielding of BNC of Mercuy to the case.
- the case to Atlas (If so, where exactly?)
- the case to ground
Is this right, or did I miss something?

Looking forward to your feedback.

73, Alexander



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