Single Conversion 35MHz FSK Receiver

This first attempt at a receiver was intended as a learning exercise rather than a working system. A number of issues need to be dealt with including aerial choice and impedance matching, and measurement of receiver performance. The design is based on the MC3356 wideband FSK receiver chip which is still reasonably easy to obtain. It contains all of the active components needed for a single conversion receiver and therefore is suitable for small size applications such as a flight R/C receiver. The datasheet for the MC3356 gives suitable circuits that only need to be adapted to the available components and frequencies of the application.

Most commercially available R/C receivers are single conversion types. After some experimentation it turned out that an IF of 455kHz is used in these receivers and the receiver crystals purchased in model shops are set to be 455kHz above the nominal reception frequency. This very small IF means that the image frequency is only 910kHz away and therefore it would not be possible to use such a receiver where the allocated frequency band is broader than this. In the 36MHz band that is dedicated to R/C in Australia, this small IF can be used safely as long as standard transmitters and receivers are being used and no illegal transmissions are present.

Warning: do not use this design in a flying model. It is unproven and untested.

The MC3356 has no RF amplification and passes its input directly to the mixer. It includes the FSK demodulation circuitry and also a circuit that provides a very good logarithmic signal strength output that is useful for AGC purposes.

Aerial

The aerial normally used in R/C aircraft is a short monopole antenna without any substantial ground plane. This type of aerial has a high capacitive reactance and therefore requires good inductive matching to maximize the signal transfer to the receiver. It also has very low resistance, but this is not a problem for a receiver. A freely available numerical analysis program MININEC was used to compute the impedance of the aerial at 36MHz. Analytical results are available for this type of aerial but the numerical analysis may be able to provide more accurate results in a real situation. The aerial length chosen was 860mm, approximately 1/10 wavelength, as used by the JR Propo R700 receiver. At 36.33MHz this has a resistance of 1.26 ohm and reactance -10.7Kohm (equivalent capacitance 0.4pF). A series inductor of 4.7µH was placed in series with the aerial to cancel this capacitive reactance.wound on a 5mm diameter former with

Local Oscillator

The MC3356 provides the active circuitry for a common base Colpitts oscillator with a series resonant crystal. To simplify the biasing of this circuit, the base of the BJT is connected internally to the power rail. This means that the oscillator will only work if the collector is also connected to the power rail via the oscillator coil. The collector-base junction is therefore at zero volts bias. The effect of this is that the collector-base capacitance is large and can affect the oscillator frequency. This capacitance will also change with temperature, so component choices become somewhat limited. We will need to choose capacitances large to swamp this junction capacitance, and the coil inductance will be low. We could improve this if necessary by connecting the coil to a separate higher supply voltage.

The circuit is intended for 36MHz. A 4.7µH coil was wound but the oscillation frequency was about 17MHz, probably as a result of the high BJT capacitances. A second coil of 0.7µH with tuning slug allowed the frequency to be locked at 36MHz.

Mixer

This is quite a good double-balanced mixer with buffering on the local oscillator output. The datasheet application example showed a load resistor on the output of 380 ohms. Investigations with PSpice showed that this could not be varied much above that value without upsetting the bias point. This creates some difficulties with the filter stage which requires a higher impedance.

RF Frontend Circuit

The circuit shows only the RF/mixer stage. The MC3356 has two essentially separate circuits that can be powered separately to minimize interference. Pin 19 belongs to the second, low frequency circuit. The capacitors C1, C2 and C5 are ceramics. L1 is wound on a 5mm diameter former with 13 turns of 33SWG wire (about 3.5mm long). As mentioned above a tuning slug on this coil allowed the final frequency to be locked to the crystal. L2 is wound on a 5mm diameter former with 3 layers of 13 turns each of 26SWG wire (about 6mm long). The inductance of this is not critical however tuning this with the aid of a CRO should allow the signal to be maximized. It is strongly advised to enclose the local oscillator coil in a grounded metal can. The output of the mixer is taken from pin 5 and will be fairly small in amplitude.

IF Filter

A readily available ceramic filter Murata CFWLA-455E was used. This has a 455kHz centre frequency, ±20kHz bandwidth and 2K impedance. The latter means that both input and output impedances must be 2K. To satisfy this requirement a 2.2K resistor was placed in series with the filter and another in parallel with the output. Without these resistors the transfer characteristic of the filter has non-optimal passband ripple. In fact it can be extremely poor if both resistors are missing. A compromise could be to omit the input resistor, which will result in some ripple but this should not be serious with this type of receiver. It is also recommended to avoid dc bias voltages across the filter. This can be done by bringing the common lead to the power supply voltage, or by addition of decoupling capacitors. The latter could be chosen to provide the required impedance matching. The capacitors on the limiter bias pins were increased to 0.22µF because of the lower IF frequency.

Quadrature

The quadrature coil is just a readily available 455kHz tuned circuit coil as used in AM radio applications. This has a tapped transformer in which the tapped coil has a tuning capacitor built-in. The coils can often be purchased in packs that include a coil for the oscillator and some IF coils (see http://hem.passagen.se/communication/ifcan.html for some information).

IF and Digital Circuit

This circuit follows substantially the example given in the MC3356 datasheet. The MC3356 is particularly interesting in that it provides a very wide range logarithmic measure of the signal strength from the squelch pin 15, and can thus be used for a signal strength meter. The data out is provided at logic levels from pin 18.

Performance

The radio was tested functionally using a R/C transmitter. The crystal was borrowed from an existing radio receiver. This gave quite a strong signal at the output with the transmitter about 25m away. It would be interesting now to determine how to measure the minimum signal strength so that it can be compared with other radio receivers.

Further Work

Determine a measure of output quality to allow a measure of sensitivity to be made.
Rebuild the receiver on a PCB with better shielding.
Calibrate the signal strength output indication.