mk484test

All the capacitors are ceramic multilayer.

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 later.

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 bit 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 for 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 also 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 sales page (http://www.kitsrus.com/bits.html). 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.

An Enhancement

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 amplifier. 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 collector 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 resistor.

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 earphone connected.

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 would result in a collector voltage of one base-emitter drop (about 0.6 volts).

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.

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