The block diagram of the 7MHzCW transceiver made of MC3362P and the explanation

(September, 1998)

MC3362P is an IC for the narrow FM double super heterodyne receiver. This contains two sets of combinations of the mixer and the local oscilator to change input frequency to 200MHz into 1stIF (10.7MHz), 2ndIF (455kHz). Each mixer has conversion gain before and after 20dB respectively. Moreover, a 1stLO circuit builds in a variable capacity diode, and only connects a coil for the tank circuit with the outside, and it can compose a VFO circuit. But, I add the silvered mica capacitor of 180pF in the outside to make it VFO of 5MHz band with this transceiver, too. Because 7MHz CW band is narrow, I can secure the variable range that even such a combination is satisfactory as to this. And, the part which I am not using with this transceiver is omitted in the inside block diagram of the IC. It is a part only for FM such as the IF limitation amplifier and the quadrature detection.

MC3362P finally disappeared from Web sight of the Motorola company. But, the newer IC which looks alike is being supplied, too. If I can obtain it, I will try to make an experiment again by using it. You may still be able to obtain MC3362P with the mail order and so on. Or, you are to use two NE/SA612s instead of using MC3362P. I think that the result almost looks alike. This is the composition adopted with the kit of the QRP transceiver in the U.S. such as NorCal-40A and SST.

You may be thought an IF amplifier to be really all right only on 1 stage of 2SK241 in the doubt. But, because the conversion gain of the mixer is high (In the upper IC inside block diagram, it is the expression of adding an amplifier behind the mixer .), I can get the sufficient sensitivity at least in low band. But, as for the AGC circuit, you had better not expect high performance very much. It has only the minimum effect that I protect both ears from the very powerful signals. QSO of 7MHz could be received well in the early experiment though I didn't insert an IF amplifier and I connected a crystal earphone with the output of the 2nd mixer directly. Therefore, I think that it is interesting to say that you make it as a 7MHz pocket receiver, too, without making it a transceiver. You can compose a receiver of few parts further in this case in such cases as MC3362P and an input tuning circuit, the tank coil of VFO, the crystal filter of about 3 element, the crystal for 2ndLO, and a power supply is 006P or four UM3 dry cells. Because the IC itself works from about 2V, even two UM3 dry cells of the power supply may be good.

When you show the circuit figure of another page, I have the point which I want some you to pay attention to. I explain it.

First, I compose the antenna switching circuit of the transmission-receiving by using the serial resonator in the receiving input circuit. The both ends of the inductor of the serial resonator fall through the diode switch into the ground at the time of the transmission. At that time, the capacity of the serial resonator becomes a part of the capacity of LPF. I ignore it because capacity is small though I should take this capacity into consideration in the design of LPF if I say strictly. A receiving signal is inputted to receiving tuning circuit through the serial resonator at the time of the receiving without loss (ideally).

Well, you may not know easily what kind of function the diode switch connected with 1 pin and 24 pin of MC3362P does. These two pins are prepared for balanced input of the 1st mixer originally. I drop these two input pins to the ground alternately corresponding to switching of the transmission-receiving. I am inputting a receiving signal or a transmitting career from the pin in the one that I don't drop it to the ground. I can fix one (for example, 24 pin) on the ground, and I change the connection of the other side (for example, 1 pin) with the diode switch, too, without doing like this. But, when I do like this, I can't get necessary switching-isolation because the transmitting career signal of 2 pin is near physically with the mixer input of 1 pin.

And, the diode which I am using as the switch by this transceiver is 1SS53. Because this diode needs a less bias current than the general purpose switching diode such as 1S1588 and so ons (Off character is conversely a little inferior.), the bias resistor is large value with the 10k-ohm. When you use 1S1588 and so on,you had better reduce the bias resistor by about 2.2k-ohm.

I am using the micro-inductor of the fixed inductance except for the inductor of the troidal core. As for BPF as well, I am doing so. This is the know-how which I learned from the kit of the U.S., too.

The simple 7MHz CW transceiver made with Motrola MC3362P