Crystal

My FM Wireless microphone V5 has been very popular, both as a beginners project and as a kit. It is interesting that it has also been the biggest cause of lots of e-mail, the most common question being How can I make it crystal controlled?. The oscillator can be made crystal controlled; replace the 2p7 with a 5th harmonic crystal. The oscillator will then be very stable, so stable in fact, that it cannot even be modulated! This means that you could only use it to send CW (morse code). One solution to the problem would be to use a low-frequency crystal oscillator and modulate it. Since a crystal can be modulated by typically only 1KHz or 2KHz, then the multiplication factor would have to be around 72. This would require a 1.3888MHz crystal and several multiplier stages, each individually tuned. The first stages would also have to be double-tuned if the transmitter were to have low spruious signals; at total of eight tuned stages (plus output filter)! For example:

This converter allows reception of six metre signals on a two metre receiver. It should therefore be useful for those with single or dual band sets that do not cover 50 MHz. To eliminate the need to obtain a special crystal, the local oscillator is a computer crystal oscillator module operating near 48 MHz. The output of these modules is rich in harmonics. In this case two tuned circuits are being used to pick off the second harmonic near 96 MHz.

This project is a simple transmitter using only one crystal and will cover 145.00 to 146.00 MHz. The crystal is a 44.9333 MHz crystal for 145.500 receive, as used in the Trio (Kenwood) 2200, PYE, Motorolla, Tait equipment, to name but four. The frequency of the crystal is not critical as almost any other xtal for the 2-meter band will function. The circuit is given above and simply mixes the output of a (more or less) conventional receiver multiplier (x3) with the output of a 10.7MHz VFO that is modulated with true FM.

Using the circuit in Fig 1, a 68HC11 µP's stop instruction can put the µP's external RC-oscillator clock, as well as the µP itself, into a low-power mode. On receiving an interrupt, the µP will exit the stop condition and enable the RC clock. The RC clock, being a low-Q circuit, will start up immediately. Crystal oscillators, on the other hand, can waste precious milliseconds coming up to speed and stabilizing.

One of the nicest features of the 8-bit KX8 microcontroller (manufactured by Freescale Semiconductor) is that it includes an internal clock generator (ICG). This allows the chip to run without the trouble and expense of an external crystal or canned oscillator.

Bugdozer is an autonomous mini-Sumo robot. Her main board consists of a MC68HC908GP32 microcontroller along with input/output support chips, a voltage regulator, a crystal oscillator, and the usual assortment of resistors, capacitors, and switches. All chips are socket mounted. I'm not experienced at soldering, so soldering the sockets avoided damage that could have resulted from soldering the chips directly. Also, if a chip becomes damaged in battle, it can be replaced easily.

This 40KHz crystal controlled oscillator circuit drives an infrared LED with powerful 40ma pulses. The circuit can be used to test optical communications circuits, designed to receive 40KHz modulated light signals.

I have used this circuit many times when I needed a low frequency reference, which did not draw much power. With the components show, the current from a 3v battery is less than 1.2 microamps.

The circuit gates the output of a continuously operating 32KHz crystal oscillator to the input of a C-MOS buffer when clock pulses are needed. The technique gets around the problem of a slow starting crystal oscillator by keeping the oscillator going and switching on a transistor power stage only as needed. The method keeps the standby power consumption to a very low 1uA when used with a 3v supply.

The AD9850 complete direct digital synthesizer (DDS) is a popular device used to implement a low cost, high speed, digital synthesis system. Clocking the AD9850 at its maximum rate of 125 MHz may present some technical challenges especially in applications where the phase noise of the sine output is a major concern.

Oscillators are an important component of radio frequency (RF) and digital devices. Today, product design engineers often do not find themselves designing oscillators because the oscillator circuitry is provided on the device. However, the circuitry is not complete. Selection of the crystal and external capacitors have been left to the product design engineer.

The RF engineer sometimes has to look for an instrument that will check a low frequency quartz crystal unit reliably and rapidly. This is a difficult piece of equipment to find and the engineer often has to consult an electronic circuits handbook for the schematic of a circuit that will perform the task.

This is an image Schematic. No Description available.

This is an image Schematic. No Description available.

The world is full of xtal oscillators twiddled by digital designers lacking in the analog design knowledge necessary. Just look at all the PC real time clocks that lags or leads by several minutes per day. And they eat backup batteries too! IC's with pins that say "Xtal here" can't be trusted either!

This is an image Schematic. No Description available.

With the advent of high speed HCMOS circuits, it is possible to build systems with clock rates of greater than 30 MHz. The familiar gate oscillator circuits used at low frequencies work well at higher frequencies and either L–C or crystal resonators maybe used depending on the stability required. Above 20 MHz, it becomes expensive to fabricate fundamental mode crystals, so overtone modes are used.

Starting up oscillator circuits and getting them to maintain oscillation in a Spice simulation is difficult. Some high-frequency crystal circuits require days for the oscillation to reach steady state. Thus, most designers separate the crystal's circuit simulation from the rest of the system design. However, a technique that gives a "kick" to an RLC equivalent circuit solves this problem. This method makes sure the simulation starts fast and quickly reaches the steady state.

The Pierce oscillator shown above is essentially a common emitter amplifier with a tuned circuit for a collector load and a quartz crystal as a feedback element. In order to determine whether the Barkhausen criteria is satisfied, loop gain must be determined at the frequency of oscillation. This is accomplished by drawing the AC equivalent circuit of the Pierce Oscillator as shown below.

The crystal used in the topology of Figure 1 can be either a fundamental AT-CUT or BT-CUT. A BT-CUT crystal has poor frequency stability over temperature compared to an AT-CUT. This topology uses a parallel crystal and not a series crystal.