Grundig-Yacht-Boy-400-Service-Manual-2电路原理图.pdf

Grundig Yacht Boy 400 Low Cost DRM ModIntroductionIn this document we present a modification to the Grundig YB400 short-wavereceiver to make it suitable for DRM down-frequency conversion. As we do not haveschematics of this receiver, we have been forced to make tests by ourselves and fourreceivers later we have achieved a suitable modification which has been able to decodea -87 dBm channel power signal.This receiver adaptation allows a low cost solution (about a0a2a1a2a3a5a4a7a6 for DRMreception. The output signal can be decoded by a software receiver which gets it fromthe sound card line input.Receiver ModificationThe modification consists in the following three steps: Get a 455 KHz IF signal with a bandwidth greater than 10 KHz, tuned to thedesired 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).It must be taken into account that the signal level at this point is not affected bythe AGC (Automatic Gain Control), so it will not be very powerful. We are working itout by now but, for the moment, an audio amplifier at the output does the work. Thiswill be explained later.This frequency is set to be 455 KHz although it could be shipped with acentering error. We have found it to be as big as 300 Hz in one model. This can besolved just by adjusting a resonator blue marked in figure 2.Another important fact about this signal bandwidth is that it is only limited bythe RF and IF1 (55.85 MHz) filters, which is quite wide (a 50 KHz BW was measured).The effects of this greater bandwidth are a noise increment and the appearance of theimage frequency (current transmissions are isolated enough to avoid this last one). Asimple filter design will be presented soon, for the moment an unfiltered signal is stillsuitable.The last fact relating to this filter is that it may have not been correctly tuned inthe manufacturing process, causing an asymmetric response. This does not have avisible effect in the base band AM signal (aside from a Signal to Noise Ratio reduction),but it has for the 12 KHz DRM output. This effect can be solved just by tuning a coupleof resonators while monitoring it in a spectrum analyzer. These two points are redmarked in the following figure:Figure 2. IF filter tuning.The second step, the down-conversion to a 12 KHz IF, has been solved using the467 KHz mixer used in the other receivers modifications presented in this site. Thisoutput would suffice for the line input of the sound card, but without the AGC workingan extra powering is needed.In order to achieve this extra powering a CEBEK (PM-4) audio line amplifierhas been used. We have removed the internal radio speaker to make room for thismodule as shown in figure 3.The need for this amplification in the last stage, at 12 KHz, is determined by thedigital nature of the sound card. This card, a PC standard one, has an ADC after theanti-aliasing filter. The ADC ideally should be excited with a normalized signal to itsdynamic margin, otherwise the quantification error will increase about 6 dB of extranoise for each unused bit of the digital generated word. Although the S/N in the 12 KHzoutput may be good enough, this effect could derive in a smaller S/N in the digitalized12 KHz signal.Experimentally, without the BB amplifier and good RF conditions the programsynchronisation is possible and audio is available at the PC speakers. The ongoing workin filters and AGC use, to normalize the signal at the PC input, will minimize the RFconditions effect.ResultsThe complete modification with the 455 KHz down-converter and the audioamplifier is shown in figure 3.Figure 3. Complete DRM modification to the Grundig YB400 receiver.The extra connections have been made to present a nice front-end to this DRMmodification 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.AcknowledgementsThis receiver modification for DRM down-conversion has been carried out bythe DRM group of the Electronics and Telecommunications Department of theUniversidad del País Vasco.Any comments or suggestions will be welcome. You can contact us at our DRMmailing list: drmbips78.bi.ehu.es.