This is mostly for the new folk. Simple modifications to existing rigs, etc: This is about recent threads on the hybrid radio concepts and the good news some seem to be reporting in that regards. I can not comment on the qualitative aspect of audio through a filter or about the effects of filter group delay since its been awhile since I read on that, and my 2005 ARRL Handbook CD ROM oddly has no information on that, and I can not understand that? You would think that was in there wouldn't you? If we have a problem with adjacent channel interference I can see no way to resolve that other than to switch on a crystal filter to cut it out of the passband. Hence go from a 48 kHz wide spectrum down to a 3 kHz wide channel when needed. To move the desired signal into that position to pass through that filter you will need a LO IN with fine resolution to place the desired signal right in the center of the spectrum display. But if you are using your rigs sideband bfo signal for a LO IN you will not have to bother about the LO IN tuning resolution which I explain down below. It will be in the center of the spectrum in this case. Here, this is not a matter of whether there is going to be an extremely pure audiophile sound quality since communications quality for amateur radio (and government/military) means cutting off offending signals and noise at the expense of fidelity if required. Meaning switchable crystal filter selectity but in this case you can switch the crystal filters out entirely when not needed if you are so inclined. So a slight loss of fidelity can mean an increase in intellegence. And sideband is not exactly broadcast quality bandwidth in amateur radio. Tough its not bad, just listen on 9.330 MHz in the daytime and see if its ok with you? And if it is, then thats all you need. A strong local signal in the passband is going to modulate in the sound card passband or create strong images. I see this here often and know its true. But its not by and large the norm over time when using the radio. Sometimes this can be lived with; and some folk will not want to live with it such as Uncle Sam. So it is a matter of broadcast quality receiver concepts as compared to communications amateur radio and military quality receivers. The later will have the additional hardware features to fight offending interference. But they usually have the abitilty of broadcast quality general coverage receivers on AM which usually only means a wider i.f. channel width for broadcast AM. And so, this is where the hybrid SDR radio receiver is going. It is the first thing to be studied by the end user since the first SDR radios to be made available here in the USA are hybrids of existing radios made possible by SoftROck40 and similar kits. Tapping into the i.f. of existing radios, etc. And so I can see now that the first amateur radio reports are going to be discoveries made with hybrid radios. And this means reports from out there in radio user land from you, and not the reports that will be made by the ARRL and their big lab and projects. We already know that existing radio receivers are sufficient for adding a DRM Converter to. And now it is easy to see that DRM is just plain wide band Upper Sideband as far as the hardware mode is concerned and if you have a sideband receiver you just need to widen the bandpass of the crystal filter (or bypass it with a resitor or capacitor across it) and you can receive the 10 KHz wide DRM signal. Just make sure that you can switch back to the normal crystal bandwidth for sideband use. And watch for stay signal pick up in your switching control line. And you will sample the audio right at the output of the sideband balanced mixer at the low level and where the audio bandpass is most likely the widest. Roger W3SZ here just told me he is using a dual conversion hybrid set up and likes it allot and I believe he said he is using roofing filters. I was not thinking yet of going past single conversion but he reports his dual conversion set up is excellent for his uses. I am ready here today for such experiments of this kind and will be studying this along with learning the software to analyze it. Now this is about cutting off adjacent channel interference in a contest enviornment or from a strong local station or two. *Over last year here we discussed the ideas of tapping into the 455 kHz i.f. and the 10.7 MHz i.f. and those were earlier hybrid radio ideas we were and many still are pursuing for kits and future applications. So the hybrid SDR radio is not new to discussion and I see it is living on elsewhere in radio land. *One thing that the hybrid SDR folk might want to consider in their approach which is something they can do with the SoftRock40, and we mentioned it here before, is to use the bfo signal source of the receiver's sideband mixer as a ready made LO IN to the QSD accessory board or SoftRock40. Now this will be easy I think on older radios that use crystals as the source for the bfo mixer. Newer radios will have a PLL derived bfo or some other source, perhaps off of a micro processor as I had in a Heath Kit once. I do not know what current Icom, Kenwood or Yaesu radios are doing on their boards for sideband mixers signal sources. But if there is a easy way to use existing sideband bfo signals in new rigs to derive a ready made LO IN source then a hybrid modification would be really simple. This should be interesting to all amateur radio owners and shortwave experimenters. Just study your rigs circuit diagram to locate the LO IN to you sideband balanced mixer and then that is the signal you want to use for you QSD LO IN. Study also to see where your AM LO IN is coming from also. But the sideband bfo signal should be ok for use in AM or FM since the spectrum display is wide banded and you can just snap the cursor anywheres you want in the spectrum. Folk should publish their rigs bfo circuit as an aid for others to follow suit with the modifications on that model radio. Most rigs have incremental receiver tuning anymore so it does not matter if you use the sideband bfo mixer all the time, you can adjust it with the incremental tuning or in the software. Providing the incremental tuning adjust the bfo and if not, don't worry much if not. Just know what your LO IN frequency is to add that into SDRadio or Winrad. I have not seen the newer versions of SoftRock40 but the board has a divide by N flip flop chip to divided the on board crystal down. You will not need this chip if you use your rigs bfo signal for you LO IN. Just run the bfo straight into the QSD chip. Addition of attenuation to the bfo signal level is expected to make it work right. You also must make a provision to prevent feedback in transmit mode: which if nothing else use a small dp dt relay switch to prevent feedback. Future SRD kits then would merely have to be a QSD and there would be no onboard crystal oscillator or divide by N flip flop chip since in this scenario they are not needed when you have a ready made LO IN available in existing amateur radios. And this means there should be no technolgocial barrier when expanding the use SDR as it is now, thanks to Alberto and others. And in most cases the i.f. will be tapped into by the QSD before the crystal filter. But you can make it switchable to work before and after the filter if you want. Actually it should not be located so close to the filter before the filter, since you might get loading of the filter and this is likely to be the case positioned right before or after the filter. But if that does occur I guess you will resolve that. And well I reckon you would not want to effect the way your rig works in its original manner. Also you don't want to over load the QSD with excessive signal, so the location is important to study. After the filter I suspect you might need to attenuate the sample of the i.f. signal into the QSD. Finding that position will be experimental and you might not have over load, I can not say. Try it and just see what it does. I believe that the use of a mere resistor here between the input or output of the crystal filter and the QSD might attenuate the signal overloading of the QSD and reduce the loading effects on the crystal and that might make this simple and work right up close to the crystal filter in this manner. Someone will let us know something about this later on. You know, the feasibilty.
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