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Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems. |
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MK484 single chip AM radio question
I read in alt.binaries.schematics.electronic that Albert
wrote (in ) about 'MK484 single chip AM radio question', on Mon, 4 Apr 2005: I hope to use the MK484 single chip AM radio at 20 kilohertz for a very low power receiver. The spec sheet says it only goes down to 150 kilohertz however. Is there any reason to suppose it contains a coupling capacitor or something that would limit the low-frequency response? It seems rather unlikely. But the definitive test invokes a little-used technique in these days of calculations and simulations. It's known as 'trying it'. (;-) It isn't as if testing at 20 kHz is rocket science, after all. -- Regards, John Woodgate, OOO - Own Opinions Only. There are two sides to every question, except 'What is a Moebius strip?' http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk |
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John Woodgate wrote:
I read in alt.binaries.schematics.electronic that Albert wrote (in ) about 'MK484 single chip AM radio question', on Mon, 4 Apr 2005: I hope to use the MK484 single chip AM radio at 20 kilohertz for a very low power receiver. The spec sheet says it only goes down to 150 kilohertz however. Is there any reason to suppose it contains a coupling capacitor or something that would limit the low-frequency response? It seems rather unlikely. But the definitive test invokes a little-used technique in these days of calculations and simulations. It's known as 'trying it'. (;-) It isn't as if testing at 20 kHz is rocket science, after all. Since this is a superhet chip (IIRC), the problem is more likely one of LO bleed through into the IF. The LO frequency would have to be on the high side of the receive frequency, so in this case, with a 455Khz IF, the LO would be at 475Khz. The LO signal bleeding through the IF filter would probably be as strong, or stronger than the 20KHz signal you were intending on receiving. -Chuck |
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I read in alt.binaries.schematics.electronic that Chuck Harris
wrote (in ) about 'MK484 single chip AM radio question', on Fri, 8 Apr 2005: Since this is a superhet chip (IIRC), No, it's a TRF. -- Regards, John Woodgate, OOO - Own Opinions Only. There are two sides to every question, except 'What is a Moebius strip?' http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk |
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John Woodgate wrote:
I read in alt.binaries.schematics.electronic that Chuck Harris wrote (in ) about 'MK484 single chip AM radio question', on Fri, 8 Apr 2005: Since this is a superhet chip (IIRC), No, it's a TRF. Ahhh! Well, that changes everything. I took a look for the spec sheet, and found the sheet for the ZN414Z, which is apparently almost identical to the MK484. There are two things that I think would limit operation at lower frequencies: 1) the gain appears to be purposefully rolled off at about 40KHz. That is to say, it approaches zero at that frequency. 2) the detected audio stage has bandwidth out to 20KHz. Imagine what would happen if you had a signal with a 20KHz carrier frequency. The carrier frequency would pass right through the detector and into the audio channel. The AGC, which appears to be audio derived, would get seriously upset, and if the 20KHz signal was modulated, you create some very interesting hetrodynes. That being the case, it appears that the manufacturer tried to keep the operating frequency far enough away from the audio passband that a simple filter would suffice to prevent interferrence. It probably won't work well at all as a 20KHz receiver. -Chuck |
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I read in alt.binaries.schematics.electronic that Chuck Harris
wrote (in ) about 'MK484 single chip AM radio question', on Fri, 8 Apr 2005: Ahhh! Well, that changes everything. I took a look for the spec sheet, and found the sheet for the ZN414Z, which is apparently almost identical to the MK484. There are two things that I think would limit operation at lower frequencies: 1) the gain appears to be purposefully rolled off at about 40KHz. That is to say, it approaches zero at that frequency. Yes, having found a 'data sheet' (handwritten), I see it has four coupling capacitors between the stages. Not good news. 2) the detected audio stage has bandwidth out to 20KHz. That may be controllable, but it's beside the point of there is no useful gain at 20 kHz. In that case, a 741 would be usable, but a 709 would be better (small signals). (;-) -- Regards, John Woodgate, OOO - Own Opinions Only. There are two sides to every question, except 'What is a Moebius strip?' http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk |
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"Chuck Harris" wrote in message ... John Woodgate wrote: I read in alt.binaries.schematics.electronic that Albert wrote (in ) about 'MK484 single chip AM radio question', on Mon, 4 Apr 2005: I hope to use the MK484 single chip AM radio at 20 kilohertz for a very low power receiver. The spec sheet says it only goes down to 150 kilohertz however. Is there any reason to suppose it contains a coupling capacitor or something that would limit the low-frequency response? It seems rather unlikely. But the definitive test invokes a little-used technique in these days of calculations and simulations. It's known as 'trying it'. (;-) It isn't as if testing at 20 kHz is rocket science, after all. Since this is a superhet chip (IIRC), the problem is more likely one of LO bleed through into the IF. The LO frequency would have to be on the high side of the receive frequency, so in this case, with a 455Khz IF, the LO would be at 475Khz. The LO signal bleeding through the IF filter would probably be as strong, or stronger than the 20KHz signal you were intending on receiving. Two reasons for using an IF amp are to amplify a fixed freq even as the receiver is tunes, and to amplify a lower freq so that the bandwidth is narrower. But since neither of these is necessary, it makes less sense to upconvert to 455kHz, or to use 455kHz as the IF. So it seems more logical to reduce the IF amps to the receiving freq, by padding the IF tuned circuit caps. There is one barrier though: you can't pad a 455kHz ceramic filter. -Chuck |
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