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THE CONVERTERS


A MIXER FOR ROADSTAR
TRA-2350P

MOSFET OSCILLATOR
MURATA OSCILLATOR



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A 12 kHz MIXER FOR ROADSTAR TRA-2350P
 

Roadstar TRA 2350 P
 

An Oscillator / Mixer with one FET Transistor and a Balanced Mixer

My 4th receiver, a Roadstar TRA-2350P, is a really good value for money receiver. It covers the Medium Wave (522 kHz - 1620 kHz) and has three overlapping Short Wave bands (1711 kHz - 29999 kHz) in 1 or 5 kHz steps. The receiver shows good performance, the selectivity is good and the sensitivity is excellent, even if you use the Telescopic Antenna.

The interesting thing about this receiver is that the people who designed it have thought of a lot of very useful things: Battery cases for LR20 or LR6 batteries (switchable), a 230 VAC power cord or an external DC adapter. The DC adapter is not included in the package.

There is a choice of a telescopic antenna or an external antenna, there is an RF Gain control, a DX / LOCal switch, Narrow and Wide IF filters and a 455 kHz IF output which can be used for, in my case, a DRM IF converter.

This circuit is simple, very stable and works fine down to less than 5 Volts supply voltage. The 467 kHz local oscillator frequency is generated by a 2SK241 FET and an IF transformer. The secondary winding feeds the balanced mixer in the S042P.


The MOS-FET local oscillator circuit . . .

A 467 kHz FET Oscillator and a S042P balanced mixer

Fig. 1

The 455 kHz IF from the short wave receiver is fed from the socket on the rear of the Roadstar and the IF voltage level at this point is about 30 mV p-p with 50 µV RF signal input. The IF signal is coupled via a Murata 455 kHz IF Filter (Type G) and fed to the balanced mixer via a 455 kHz IF Transformer, L1.

The 12 kHz IF output is recovered on the IC pin 2. The isolation between the receiver and the PC is provided by an interstage transformer from an old small portable CB transceiver. A small ceramic capacitor is soldered to the secondary winding of the transformer to reduce mixer noise.

The S 042 P provides enough 12 kHz output to feed the Line Input of the computer's Sound Card. Be sure to mute the Line Input in the computer's Volume Control Panel, otherwise the 12 kHz signal will be heard as noise in the speakers.

If you connect to the computer's Microphone Input, then it would be wise to add a 5 kOhm potentiometer to the secondary winding of the transformer to reduce the output level in order not to overload the microphone input.

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The component layout . . .

Fig. 2

 

The finished Oscillator and Mixer

The finished DRM FET Oscillator and Mixer

Fig. 3

The DRM Converter PCB was mounted in a Video Modulator chassis, which I took from an old SAT Tuner. There were 4 feed-through capacitors, of which one (1 nF) is used for feeding the 6 Volt supply and two feed-through capacitors (12 pF and 120 pF) for the 12 kHz IF signal output.


The slightly modified circuit with a ceramic resonator oscillator. . .

A 467 kHz Oscillator with Murata BFB455 and a S042P balanced mixer

Fig. 4

This modified circuit uses a Murata BFB455 ceramic resonator to get stable oscillation on or around 467 kHz. The nominal frequency of the resonator is actually 455 kHz, but in the circuit shown above it oscillates on or near 467 kHz.

Other Murata IF filters can be used, but they oscillate on a wide range of frequencies. Several Murata CFU455D oscillated at 442 - 443 kHz in the same configuration as shown in Fig. 4 and several Murata CFU455E oscillated at 444 - 445 kHz. Other common Murata 455 kHz IF filters can also be used with some modifications to the capacitors on the base and the collector of the 2SC1815.

The 1 nF capacitor can be adjusted to get the proper oscillator frequency. The 33 nF from 2SC1815 base to ground can also be used to pull the frequency, but should be kept in the range 22 - 33 nF. The supply voltage needs no stabilization. In my case, I used the same 9 Volt adapter as for the supply to the TRA-2350P.

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