Grundig Yachtboy400 Drm Modification Manual

							Gru ndig  Y ach t B oy 4 00  –  L ow  C ost D RM  M od
Introduction Inthis document wepresent amodification tothe Grundig YB400short-wave
receiver tomake itsuitable forDRM down-frequency conversion.Aswe donot have
schematics ofthis receiver, wehave beenforced tomake testsbyourselves… andfour
receivers laterwehave achieved asuitable modification whichhasbeen abletodecode
a  -87 dBm  channel power signal.
This receiver adaptation allowsalow cost solution (about

forDRM
reception. Theoutput signalcanbedecoded byasoftware receiverwhichgetsitfrom
the sound card line input. Receiver Modification
The modification consists in the following three steps: ·Get a455 KHz IFsignal withabandwidth greaterthan10KHz, tuned tothe
desired RF channel.
· Down-convert it to a 12 KHz IF signal.
· Amplify this last signal.
The desired 455 KHz IF signal can be found in the pad remarked in figure 1.
Figure 1.  455 KHz IF output (without filtering).  
						

							Itmust betaken intoaccount thatthesignal levelatthis point isnot affected by
the AGC (Automatic GainControl), soitwill notbevery powerful. Weareworking it
out bynow but,forthe moment, anaudio amplifier atthe output doesthework. This
will be explained later.
Thisfrequency isset tobe 455 KHz although itcould beshipped witha
centering error.Wehave found itto be asbig as300 Hzinone model. Thiscanbe
solved just by adjusting a resonator blue marked in figure 2.
Another important factabout thissignal bandwidth isthat itis only limited by
the RF and IF1(55.85 MHz)filters, whichisquite wide(a50 KHz BWwasmeasured).
The effects ofthis greater bandwidth areanoise increment andtheappearance ofthe
image frequency (currenttransmissions areisolated enoughtoavoid thislastone). A
simple filterdesign willbepresented soon,forthe moment anunfiltered signalisstill
suitable.
Thelastfact relating tothis filter isthat itmay have notbeen correctly tunedin
the manufacturing process,causinganasymmetric response.Thisdoes nothave a
visible effect in the base band AM signal (aside from a Signal to Noise Ratio reduction), but ithas forthe 12KHz DRM output. Thiseffect canbesolved justbytuning acouple
of resonators whilemonitoring itin aspectrum analyzer.Thesetwopoints arered
marked in the following figure:
Figure 2.  IF filter tuning.
The second step,thedown-conversion toa12 KHz IF,has been solved usingthe
467 KHz mixer usedinthe other receivers modifications presentedinthis site. This
output wouldsuffice forthe line input ofthe sound card,butwithout theAGC working
an extra powering is needed.  
						

							Inorder toachieve thisextra powering aCEBEK (PM-4)audiolineamplifier
has been used. Wehave removed theinternal radiospeaker tomake roomforthis
module as shown in figure 3.
Theneed forthis amplification inthe last stage, at12 KHz, isdetermined bythe
digital natureofthe sound card.Thiscard, aPC standard one,hasanADC afterthe
anti-aliasing filter.TheADC ideally shouldbeexcited withanormalized signaltoits
dynamic margin,otherwise thequantification errorwillincrease about6dB ofextra
noise foreach unused bitofthe digital generated word.Although theS/N inthe 12KHz
output maybegood enough, thiseffect couldderive inasmaller S/Ninthe digitalized
12 KHz signal.
Experimentally, withouttheBB amplifier andgood RFconditions theprogram
synchronisation ispossible andaudio isavailable atthe PC speakers. Theongoing work
in filters andAGC use,tonormalize thesignal atthe PCinput, willminimize theRF
conditions effect. Results
Thecomplete modification withthe455 KHz down-converter andtheaudio
amplifier is shown in figure 3.
Figure 3.  Complete DRM modification to the Grundig YB400 receiver.  
						

							Theextra connections havebeen made topresent anice front-end tothis DRM
modification as depicted in figure 4.
Figure 4.  External interface of the DRM modification to the Grundig YB400 receiver.
The software receiver output is shown in figure 5.
Figure 5.  Software receiver output.   
						

							AcknowledgementsThisreceiver modification forDRM down-conversion hasbeen carried outby
the DRM group ofthe Electronics andTelecommunications Departmentofthe
Universidad del País Vasco.
Anycomments orsuggestions willbewelcome. Youcancontact usatour DRM
mailing list: 
[email protected].