SAW Resonator Enabled RF Remote Control Transmitter Design Guide

Before we discuss how to design RF remote control transmitters using SAW resonator, we have to know the basic principle, how it works.

Basically a simple transmitter – oscillator network, contains frequency control part, amplifier and data input for phase shift, the amplifier contains RF transistor and a few surrounding components, and frequency control part contains SAW resonator, part of output signal is feedack into input of amplifier, and insertion loss and phase of feedback depends on frequency.

SAW resonators application circuits can be divided into two methods, one-port resonators and two-port resonators, we always perfer one-port resonators since it’s simple application circuit and components, and it’s easier to source since major manufacturers such as Murata mainly make one-port resonators now.

The common-base colpitts oscillator is a preferred circuit for one-port resonators, this type of circuit is used in many applications and ideal for modulation with amplitude shift keying ASK and on-off keying OOK, the bill of material for a colpitts oscillator is very short, and transmitter antenna can easily be integrated on the PCB by using PCB inductor, colpitts oscillator also works with a two-port SAW resonator in a one-port configuration, but a real one-port SAW resonator has many advantages and is preferred.

For oscillation, the resonant frequency of the parallel resonant circuit should be near the SAW resonant frequency, neglecting the internal transistor capacitance, the load and the parasitic capacitance of the PCB, the resonant frequency Fp of the LC circuit comprising capacitors C1, C2 in series connected in parallel with L1 is determined by following formula.

$$F_{p} = \frac1{2\cdot\pi\cdot\sqrt{L1\cdot\frac{C1\cdot C2}{C1+C2}}}$$

This provides merely initial values for further optimization, the actual application circuit is much more complicated, because there are numerous parasitic capacitances such as transistor and PCB via etc. R1, R2 and R3 are used for DC biasing. C3 matches the high-impedance collector output to the load impedance. The feedback of the oscillator is set by the ratio of C1 to C2, which also determines the transient response and other specific oscillator properties.

As the circuit begins to build up the oscillation, the initial values such as DC biasing points, LC circuit and PCB with the antenna apply. For basic tuning, the SAW resonator is replaced by a 22 Ohms resistor and if necessary a DC blocking capacitor (33 pF…100 pF). The oscillator starts up with an oscillation at the LC circuit, the oscillator frequency is trimmed by C1, C2, C3 and L1, the frequency of the freewheeling LC oscillation should be close to the SAW resonator’s frequency.

When the SAW resonator is again used instead of the resistor and capacitor for next stage, the frequency is stabilized to the desired value, and for product certificate considerations, the signal frequency and the harmonics will be further optimized by fine tuning.

The SMD transistor should be a high-frequency type with a transit frequency of a few gigahertz, while using a very fast type, the oscillation will start under nearly all conditions, but the harmonics can be very high, if cost is concern, then 2SC3356 maybe a good choice.

The choke inductor L1 can often be designed like a copper line on the PCB without back-side grounding to the wiring side, that’s where we implement the transmitter antenna.

As for oscillators with two-port resonators, we will not discuss it here, since it is not very commonly used, and not recommended for most RF remote control transmitter applications due to complexity and higher cost.

Next we will show a sample application circuit which is 433.92Mhz SAW resonator enabled OOK remote control transmitter, the circuit and design can be found at following.

The transmitter is a SAW-stabilized oscillator with a PCB antenna, where a one-port SAW resonator is used, as the basic circuit form shown in above photo, the oscillator is switched on and off in step with the data telegram. The maximum modulation frequency is about 2.5 kHz regular or 5 kbit/s in NRZ code, the oscillator’s transient time of 30 μs prevents a higher modulation rate in OOK mode.

The additional components L01, C08 belong to the feedback circuit and are designed for harmonic suppression and matching the antenna to the output of the transistor, L02 is for DC biasing, the antenna is a loop antenna integrated on the remote control transmitter PCB.

The high ratio of C06 / C07 in combination with R07 prevents the oscillator to generate spurious oscillation at 50 to 100 MHz above the desired SAW oscillation frequency, this parasitic oscillation tendency is caused by slightly different LC resonance frequencies in the harmonics suppression – antenna circuit.

The oscillation frequency depends on C06, C07, C08, L01 and the layout of the antenna choke inductor L03, so it is necessary to use components with 5% tolerance for best performance. C04 is for DC blocking (33 pF…100 pF), R05 and R03 for DC biasing.

The transmitter requires a good common RF ground and short connections between the RF components, a closed common GND area at the bottom side of the PCB is very important to keep the critical GND nodes close together, and the most important nodes for excellent RF grounding are SAW Resonator, C06, C08, C05, C01. Separation of the digital from the RF circuitry on the PCB is also recommended, also keep in mind that back-side of loop antenna area must be free of ground layer.

So the best practice would be keeping the key components close together in layout, and use a bottom ground layer to connect all RF component’s common ground, usually the bottom ground layer size can be as large as whole circuit board, just leaving the loop antenna and PCB inductor area blank.

The circuit has been developed for low power consumption, which uses 3V supply voltage, measured current consumption about 3.5 mA for a continuous on oscillation without modulation and falls below 2 mA with a 50% duty cycle modulation, while at a supply voltage of 6V, current consumption is 6.5 mA in CW mode.

This circuit can also be used for other frequencies such as 315Mhz and 418Mhz, but the values of C5, C6, C7, C8, L01 and the SAW resonator will need to be changed accordingly, typical value is as follows

For more information on this topic, we recommend checking out TDK EPCOS application note, which can be found here