Re: LF: £10 Dongel-SDR Direct sample on 504 KHz - S/N TEST

["Gary - G4WGT" <g4wgt@tiscali.co.uk>]

Graham,

 

Maybe then....if the PCB was installed in a small aluminium box the capacitor could be removed & a switch fitted to select the original (VHF) arrangement or the HF/LF method. Possibly fit a pre-amp also.

 

73, Gary - G4WGT.

 

 

----- Original Message -----

From: Graham

To: rsgb_lf_group@blacksheep.org

Sent: Saturday, April 06, 2013 6:23 PM

Subject: Re: LF: £10 Dongel-SDR Direct sample on 504 KHz - S/N TEST

Follow  up

 

Difficult to get a  range  for  the  dongle  in DS mode  , the  main  gain is  based  in the  e4000   mixer  , some  40 db, needed  for  its intended  wide band  modes   ,but  direct to the  A/D produces a  trace  at  0.1 uV PD  (gen  set  to PD), both  plots  in USB   , dial  set 503 , 

 

With  60  dB dynamic  range after  processing  , compared to  120 +  dB  (no rf pre-amp)  for the  Racal  , the  D/S option is not  a  serious  reception  system , but  , with a  selective  pre-amp  and  what  .  say  zero to   20  db of  gain , would  make a  good  system ,

 

The  D/S mode  removes  the  need  for  a  mixer  and  second  oscillator , but perhaps  requires  a  pre-amp  and  depending  on qth  , some  pre-selection  , though  reception  is  possible directly  , HF at  night  is  quite  reasonable , but  for  $10  not  bad  !

 

NB -  I have  connected  directly  to  pin 1  of the  A/D for  these  tests ,  without  access  to the  chip  architecture , its not  possible to  state this  is the  ideal  situation  , some  arrangements   use  transformer  coupling  , one  point that  stands out , is that  the  out-put  of the  e4000  remains  in cct  and  could  shunt  the  Rf to  ground  , as there  is  a  coupling  capacitor  e4 ><  A/D   I injected  via  a  0.1 cap .

 

There  are  some  posts  around   removing the  cap  from the  board  , to  improve  D/S  performance  , by isolating  the  e4 , this  should/ could   improve the  sensitivity  but  at the  'loss'  of the  vhf/uhf  function,  but then again at this  price  range , is that a  problem ?

 

 

73 -G.. 

 

 

 

 

This plot  is  1uV PD  504 KHz    to   pin   '1'  of  the  A/D   , 'Direct sampling'  input 

 

 

 

 

 

This  plot  0.1 uV  PD   at  504  KHz 

 

 

 

 

 

For  comparison 

 

Racal RA6790GM ., with LF front  end , no  rf  pre-amp    at  0.1 uV  PD ,   USB ,

gain = full  agc  = on -med

 

easy   CW  copy  from the  Speaker  at  0.1 uV , [short  signal  , gen set AM -on]

 

 

 

 

 

From: Graham

Sent: Thursday, April 04, 2013 2:01 PM

To: rsgb_lf_group@blacksheep.org

Subject: LF: £10 Dongel-SDR Direct sample on 504 KHz

All

 

£10 Dongel-SDR  Direct sample on 504 KHz

 

The  latest  window  drives   for  the  ultra-cheap    sdr   dongle   project , now  have the  'direct  sample'  option 

 

all  that  is needed  is  a  wire  to  pin '1'  of the  AD chip  , [ ie  use the  decoupling cap  as a  terminal  post]

 

select  I/Q , and  you  have a  HF rx  , which tunes  down to   zero  (ish)

 

This is the  504  carrier  from my  main Ae  via tuner , showing  qsb 

 

navtex  from the  south coast last  night  518  ,100% copy  , so  not  bad  ..

 

73 -G..