MK484 single chip AM radio question
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From: John Woodgate on 8 Apr 2005 11:59 I read in alt.binaries.schematics.electronic that Chuck Harris <cf-NO-SPAM-harris(a)erols.com> wrote (in <a7idndBatMl7AMvfRVn-sg(a)rcn.net>) 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
From: Paul Keinanen on 8 Apr 2005 12:46 On Mon, 04 Apr 2005 11:53:59 -0400, Albert <> wrote: >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. At some (very) low frequencies the 1/f noise will make the stated noise figure invalid :-). Some microwave bipolar transistors will have a quite bad noise performance at HF (and some even VHF) frequencies due to the 1/f noise, but since the maximum frequency for that AM radio is not very high, I very much doubt this would be an issue at 20 kHz. After all, the environment noise is quite high at these frequencies, so unless the antenna is very lossy, the receiver noise figure is of very little interest anyway. Paul OH3LWR
From: Chuck Harris on 8 Apr 2005 13:29 John Woodgate wrote: > I read in alt.binaries.schematics.electronic that Chuck Harris > <cf-NO-SPAM-harris(a)erols.com> wrote (in > <a7idndBatMl7AMvfRVn-sg(a)rcn.net>) 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
From: John Woodgate on 8 Apr 2005 14:20 I read in alt.binaries.schematics.electronic that Chuck Harris <cf-NO-SPAM-harris(a)erols.com> wrote (in <l6udnal9Z9b-IsvfRVn-uQ(a)rcn.net>) 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
From: Albert on 8 Apr 2005 16:34 Thanks Frank, Yes, I have considered a mixer/superhet. Can you suggest any particular chip that runs on 1.5 volts and pulls less than 300 microamps (which is what the mk484 draws)?? Op amps don't have the gain/bandwidth necessary without drawing millamps from the battery. There are some low noise chips that do a little better than others, but they all pull way to much supply current. I'm wide open to suggestions, suggest away. A >Have you considered a mixer arrangement with a xtal osc.
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