|Gerard L'AMiGo||ECC40, EL41||PLAY|
|Loop Antenna Monitor||none|
This article describes the IC and some projects I built with it, and comments on the receiving quality. Some very useful additional information is in this note by Dan McGilles.
Data sheets of the MK484 are easy to find, but only have two pages with basically little more than the construction of the match box radio. Fortunately, it is as good as equal to the older ZN414 and its sheets can be used. Only be aware of the reversed pin layout! The IC has become very cheap; Spring 2017, I bought 50 TA7642's for 88ct!
The quality factor Q of such a tank is reasonnably high, and so is the input impedance of the IC, giving a fair selectivity. The IC amplifies and detects the AM signal and the current through its output wire has four components. First, a constant DC current of approximately 0.3mA to feed its transistors. The exact value of the DC current depends a little bit on the signal strength, and this was done on purpose; this second, signal strength dependent DC component makes the voltage drop over R1 (800R) signal dependent. Third current component is amplified RF current, which we don't want anywhere on the right side of the chip, so we short it through C1 (100nF). The fourth component is Audio Frequent, which can be taken as current through a transistor amp, or as voltage over R1 for a tube amp.
R2 and C2 form the AGC circuit. R2 (120k) connects one side of the tuned circuit with the DC voltage of the output, so the DC level of the input now depends on the signal strength. Amplification in the IC depends on the DC level, which closes the control loop. We don't want the AGC to react too quickly (it would counteract the modulation), so C2 serves as a buffer.
Not too much can be expected from the AGC operation. To avoid overloading on strong stations, you should use a large AGC resistor (R2 is 1k) and the supply voltage should be low (1.3V). To avoid noise on weak stations, use a smaller R2 (400R) and/or higher supply voltage (1.6V). To receive strong and weak stations, some manual gain control will be necessary.
Because there is just one tuned circuit, selectivity is limited. If your region has a strong MW station, you may hear it all over the MW band, and it may even mask distant stations. So, your TA7642 project will be a typical local broadcaster receiver.
The antenna coil is hardly loaded by the IC (its input impedance is 4M), and may show a strong directionality. This is nice for handheld or small sets, but a table radio with fixed ferrite rod is not so easy to rotate to optimum reception!
I experience a quite good quality of sound.
With my own transmitter,
the quality is not limited by the usual 10kHz bandwidth.
Also, when a broadcast station comes in
with the right signal strength and uninterfered,
the sound is good.