MK484 single chip AM radio question

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.


Has anyone used the MK484 chip below 150 kilohertz?


I know it works well at lowfer frequencies...it runs circles around my
ICOM receiver! But, I'm not sure about the minimum usable frequency.


I have the spec sheet for it (and it's more modern variant, the
MLF501), if anyone wants a copy, please speak up.


Any comments appreciated.

A

 

====================
Have you considered a mixer arrangement with a xtal osc.

Frank GM0CSZ / KN6WH

 

Hi Albert,
What is the upper freq limit on this chip? I am always looking for a good AM
dtector. 9MHz is current interest.

Thanks,
BTW, if you are the same fellow asking about wire gauge- a
micrometer/calipers is by far the more accurate method. The gauges just do
not work well for small diameters. And, you'll find lots of other uses for
the calipers.

73,
Dale

 

I read in alt.binaries.schematics.electronic that Albert <[email protected]?.?.invalid>

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

 

I read in alt.binaries.schematics.electronic that Chuck Harris

No, it's a TRF.

 

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

 

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

 

I read in alt.binaries.schematics.electronic that Chuck Harris

Yes, having found a 'data sheet' (handwritten), I see it has four
coupling capacitors between the stages. Not good news.

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). (;-)

 

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

 

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.