Crystal-controlled FM receivers for the 10m band

Simple superhet using the MC3357

The integrated circuit MC3357 was developed primarily for use in double conversion receivers for FM professional radio applications. It includes all stages required on the IF side, from the mixer and the oscillator, IF amplifier and limiter to the coincidence demodulator. In addition, a squelch circuit is provided which is controlled by the noise signal. Little external wiring is required for everything. In the standard application proposed by the manufacturer, an IF input signal of 10.7 MHz is fed to the input via a crystal or ceramic filter. In the mixer, this signal is converted to the second intermediate frequency of 455 kHz with the help of the built-in oscillator operated by a 10.245 MHz crystal. An LC circuit and thus a coil is only required for the FM demodulator.

In an arrangement that differs only slightly from the original application, the IC can also be operated well as a single conversion receiver for the upper shortwave range. In this way, receivers for CB channels in the 27 MHz range and of course also for the 10m amateur radio band can be implemented without the need for further stages. In contrast to corresponding arrangements with the TCA440, which without an additional demodulator are only suitable for AM demodulation, a complete receiver for narrowband FM is achieved with just one IC. In order to be able to use ordinary overtone crystals, they have to be operated in series resonance. An additional LC resonant circuit is required for this. The crystal is now in the feedback branch from pin 2 to the capacitive voltage divider of the resonant circuit that is tuned to the overtone frequency.

Suitable crystals for the CB radio range are still quite readily available today. As a rule, they oscillate around the IF lower than the desired reception frequency. To receive CB channel 19 (27.185 MHz), for example, a crystal with a frequency of 26.730 MHz is required. For the reception of frequencies in the 10m band, however, it is not so easy to obtain. In principle, the oscillator could also work 455 kHz above the receiving frequency. In order to be able to receive the relay frequency of 29.690 MHz, you can get suitable crystals for 29.235 MHz as well as for 30.145 MHz only as an expensive custom-made product. In order to be able to use normal CB crystals, I therefore used the arrangement presented here elsewhere, in which the frequency generated with a CB crystal is mixed with a freely oscillating oscillator in the range of around 2 to 3 MHz. The desired channel frequencies could be adjusted using a spindle trimmer using capacitance diode tuning. So the resonant circuit with capacitive voltage divider and the crystal connected to the MC3357 are omitted. The injection signal was fed externally to the IC via the 270 pF capacitor. If the level of the externally supplied signal is too low, pin 2 can be blocked against ground with a capacitor (e.g. 10nF).

The input sensitivity that can be achieved with this 10m receiver is already quite good without an additional RF pre-stage. In order to achieve a usable image frequency suppression, there should be at least a two-circuit band filter in the receiver input. With an RF preamplifier, you can work with looser coupling without loss of sensitivity and thus achieve better filtering. Furthermore, the filter arrangement is then no longer damped by the antenna. As a rule, an additional RF pre-stage hardly results in a significant increase in sensitivity, but it does produce a noticeable improvement in image frequency suppression.

Double conversion receiver using MC3361 and TDA7212

With the FM receiver now presented, CB or remote control crystals, as they were used in older AM devices, can be used directly as channel crystals. Among others, it can receive the 10m FM relay frequencies, so also 29.690 MHz. Here, for example, the 10m transmitter of the DF0HHH relay station installed on the radio tower in Rosengarten south of Hamburg transmits. Instead of the MC3357 used in the previously shown receiver, this circuit uses the somewhat more modern MC3361 circuit. Because this receiver is a dual conversion superhet, additional a front end, using a TDA7212, is inserted at the input side. This circuit has been developed for wireless telephones, pagers, remote control receivers and the like. In contrast to the widely used NE602, this IC also contains an RF preamplifier in addition to the mixer and oscillator circuit. The circuit can therefore be operated with separate resonant circuits as preselector and RF link, which improves the image frequency supression. It is achieved with the circuit shown excellent input sensitivity and a very usable safety against overdrive apparitions.

The used LC single circuit filter for the first intermediate frequency is actually intended for 10.7 MHz. It is brought to a lower frequency by the parallel capacitor (1nF). For mixing to the second IF of 455 kHz, an easily procurable 3 MHz crystal is used. So the first IF is 2.545 MHz. With this value, now standard 27 MHz overtone crystals (3rd overtone) can be used as channel crystals. For receiving at 29.690 MHz, a transmission crystal for 27.145 MHz is required, as it is available for radio remote control purposes. To receive the relay input (29.590 MHz), a remote control crystal would be suitable for 27.045 MHz. With TX crystals from old AM-CB radios, various other 10m frequencies can be received, e.g. 29.620 MHz with a crystal for 27.075 MHz. The resonant circuits at the input, between pre-stage and mixer and the oscillator coil are from old CB radios. It can be found in the following devices: Stabo Stratofon P3, P6, M12 and F12, Lehnert MS120, HS120, HS220, Topfunk / Universum CBR2000 and Waltham WT512S. The single circuit demodulator filter (455 kHz white) can also be taken from these devices. Among others the matching 10.7 MHz filter, core signed orange, is found in the Grundig CBM100 and CBH1000 and in many old Japanese AM-FM radios (e.g., clock radios). These devices incidentally also contain the already mentioned filter, required for the FM detector. It takes some practice and a good desoldering pump to desolder such filters. But it bypasses the otherwise necessary winding of coils.

The receiver is designed for loudspeaker operation and equipped with noise-voltage-controlled squelch circuit (squelch) as well as S-meter. There is also an RX busy output (logical H with squelch open) and an RX mute input. If this is connected to ground, the receiver is turned off. Complemented by a 10m transmitter, the receiver can be easily expanded to a complete transceiver. Although the largely according to the data sheet, respectively the application circuit constructed squelch switches crackle-free and has a adequate hysteresis, but it has the disadvantage that the noise is not completely suppressed. Reason is the switching transistor contained in the IC, which has a resistance of about 10Ω in the conductive state. A soft noise that is normally acceptable at this intensity remains audible. Complete suppression could be achieved, inter alia, by additionally switching an FET located in the audio signal branch via the RX busy output. The use of a CMOS analog switch is also conceivable here. Another option is to simply turn off the speaker with a relay switching stage. In this case, even the inherent noise of the audio amplifier is suppressed when the squelch is closed.

I have intentionally kept the concept of this recipient as simple as possible. As disadvantage it is resulting in a not all claims sufficient image frequency rejection. But compared to old CB radios with single conversion superhet, as I have listed above as part donor, it is still far better. The image frequency related to the first IF results in a mis-reception from the little-used frequency of 24.6 MHz (29.690 MHz - 2x 2.545 MHz). This can be quite well suppressed due to the fairly large frequency spacing to the receiving channel with the existing one pre- and intermediate circuit. Improvement would bring a band filter in the receiver input, which can be realized with two capacitively coupled single circuits of the specified type. The image of the second mixer causes a mis-reception point at the also of radio services also hardly used frequency of 30.6 MHz (455kHz + 3MHz + 27.145MHz). The only one IF single circle in the 1st IF causes a still usable suppression, since the wrongly formed IF of 3.455 MHz is relatively far from the correct value of 2.545 MHz. An additional suppression here causes the pre- and intermediate circuits, which namely come from single conversion superhets and thus attenuate there also the only 910 kHz remote image frequency. A sufficient suppression of the 30.6 MHz mis-reception point even under severe conditions can be achieved with two coupled via a 22pF capacitor IF single circuits for the first intermediate frequency.

The sample construction shown was on a dot board with 5.08mm pitch. Unappropriate components, such as ICs and filters, are adapted via wires or through intermediate plates with a pitch of 2.54 mm. I have soldered the wiring at all points, otherwise experience shows that faults can occur which are noticeable as annoying crackling. Unused raster dots are tinned to avoid oxidation. The receiver calibration is very simple: The oscillator circuit is adjusted so that the oscillator safely sets on. This can be controlled with a receiver tuned to 27.145 MHz. The link circuit (green CB coil) and the first IF filter (signed orange) should be adjusted to the strongest pointer deflection of the S-meter, using a weak input signal. The FM detector circuit (455 kHz filter white) should be trimmed to the best audio volume. In this setting, the demodulation should also have the smallest distortions. The input resonant circuit is adjusted to least noise with a weak input signal. An adaptation of the S-meter for pointer instruments from old CB radios, where the lower display area is mostly stretched because of the AGC required for AM, can be achieved with a 1N4148 diode connected in parallel with the instrument. If a trimmer (for example, 50kΩ) is connected to it in series, the display characteristics can be adjusted in addition to the display sensitivity.