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MK-484 Evaluation Board
The basic MK484 (Replacement for the ZN414) connected in its minimum
configuration as a radio receiver for the A.M. Broadcast band and the
half of the United States' FCC Part 15 Lowfer (1600-1750 meter) band.
There is still plenty of room on
board for later
A one transistor audio amplifer
was added after this picture was taken.
The Basic Circuit
<>Its a circuit straight out of
application note and I have fount it to be useful on the bench. Even
more convenient that using the Radio Shack short wave receiver, mainly
because the tuning is so broad on this that I don't have to tune
precisely to hear the signal.
All the capacitors are ceramic
I put this together to evaluate the MK-484 as a
possible remote control/data receiver for the 1600-1750 meter band. The
on-off switch is hardly necessary because of the low current draw of
the circuit, but one never knows if one is going to add more circuitry
When listening to the AM (MW) band, there is a lot of EMI to be heard,
but went it is switched to the longwave band (LW), the EMI disappears.
This agrees with the datasheet showing sensitivity dropping off quite a
a the LW frequencies compared to the MW frequencies. This chip would
certainly be a nice receiver or IF around 1 MHz but seems to come up
short in the
sensitivity departmentat 180 kHz. Still, it might come in handy
some very short range applications.
The cutoff frequency of the output RC is 1.6 kHz.
I tested the receiver with the 187 Khz RF Source
mentioned on this site. Indeed, checking out the MK-484 was the first
use of the 187 khZ RF Source.
I had acquired the
radio parts - the ferrite coil, tuning cap, and the
KM-484 from Peter Crowcroft's Kitsrus.com
web site on the component
As for the high impedance earphone, I found that in Bahnmo Plaza, and
electronic components market in Bangkok. You are on your own as far as
finding one elsewhere.
When I went to
actually use the receiver, I realized that a little gain
on the earphone amplifier would be helpful, so I added a small
See the circuit below.
A single transistor
biased into its class-A region provides
up to 28 db of gain when driving a high impedance load.
The amplifier is very simple. When the
load on grounded emitter amplifier stages is much higher than the
intrinsic emitter resistance (usually a few ohms or less) the voltage
gain (Vout/Vin) as measured between the base and collector is
approximately equal to
Gain, Vout/Vin = Rc/(26e-3/Ic, where
Vout is output voltage, peak-to-peak
Vi is input voltage, peak-to-peak
Rc is total collector impedance, and
Ic is the collector current.
This expression is approximately correct at room temperature, and the
gain varies directly with absolute temperature, assuming the collector
load impedance and collector voltage are stable.
If you look at this expression long enough, you'll realize a few of
things: The gain is a function of collector current, and therefore
output voltage, meaning that this amplifier is inherently
nonlinear. Another thing is that is the only load on the collector is
the load resistor (no earphone) then the gain is independent of the
value of the collector resistor -this is because the collector
current, and therefore the gain, varies as a function of the collector
load resistance, thereby canceling the effects of the resistance
change, assuming that the collector voltage remains the same. The gain
works out to about 38 x the number of volts across the collector load
In this circuit, with a batter voltage of 1.62 volts, the collector
voltage is 0.89 volts, meaning that the voltage across the collector
load resistor is 0.73 volts, so the maximum open loop gain is
approximately 38 X 0.73 = 27.7, which also works out to 20 log (38 X
0.73) = 28.8 db (if you like to see gain numbers in db). The maximum
open loop gain occurs when there is no
When used with a 2k Ohm earphone, the collector impedance is 667 Ohms,
so the gain is reduced proportionally, to 667/1k X 27.7 = 18. The
number "1k" is the value of the resistor load resistor and determines
the collector current.
I glossed over the 56k resistor. As long as this resistor is large
compared to the collector load resistor it can be ignored as part of
the collector load, but the larger it is, and the lower the
transistor's beta, the higher the stage's output voltage, and the lower
the gain. The value I used here seems to be a good balance between
affecting the collector impedance and letting
the collector voltage get too high. In the limit, a zero ohm resistor
result in a collector voltage of one base-emitter drop (about 0.6
This amplifier has problems: Its gain varies with temperature and it
has some inherent nonlinearity, but it only took a minute to stick on
the board and it gives enough gain for me to hear the background noise
generated by the MK848, assuring me that I can hear the faintest
signals the receiver is
capable of detecting.
With the amplifier on the board, I measured the power consumption.
Battery voltage: 1.62 volts
Battery current drain: 1.37 milliamps
Total power to receiver: 2.3 milliwatts
A crystal set would draw less power, but it would need a larger
antenna. My guess is that the battery will last for its shelf life.
Contents ©2003, 2004, 2005 Richard Cappels All Rights Reserved. http://www.projects.cappels.org/
Dick Cappels' web version first posted in December, 200;
updated February 2004, September 2005.
You can send email to me at projects(at)cappels.org. Replace
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