The schematic below displays one of these circuits which, even though not a precision piece of equipment, will deliver a decent outcome in small price range labs. UJTs produce this kind of waveforms using straightforward and cheap circuits.
#Ujt simulation qucs generator#
2) Accurate Sawtooth GeneratorĪ basic sawtooth generator featuring pointed spikes is advantageous in a number of apps involved with timing, synchronizing, sweeping, and so forth. The capacitance of this unit must be approximately between 0.1♟ and 1♟, the most effective magnitude might be the one which brings about minimum distortion of the output waveform, when the generator is run through a specific ideal load system. The dc output coupling can be seen in schematic, but ac coupling could be configured by placing a capacitor C2 within the high output lead, as demonstrated through the dotted image. The circuit works with just 20 mA from the 15 Vdc source, although this range could be different for different UJTs and bipolars. Where t is in seconds, R1 and R2 in ohms, and Cl in farads, and f = 1/t To get additional frequency ranges, R1, R2, or C1 or each one of these could be modified and the frequency calculated using the following formula: When resistance setting is at minimum, probably with only R1 at 1.6 k the frequency will be, f = 1522 Hz, and t = 0.66 ms.
When the resistance is maximum with R1 + R2 = 51.6 k and with C1 = 0.5 ♟, the frequency f is = 47.2 Hz, and the time off (t) = 21.2 ms.
The frequency, or cycling frequency, is determined by the adjustment of a 50 k pot resistance and the capacitor value of C1. The maximum amplitude of the output signal can be up to the supply level, that is +15 volts. The UJT output voltage, obtained over the 47 ohm resistor R3, switches the bipolar transistor between a couple of thresholds: saturation and cutoff, generating horizontal-topped output pulses.ĭepending on the off time (t) of the pulse, the output waveform could be sometimes narrow rectangular pulses or (as indicated across the output terminals in Fig. The first design below demonstrates a simple pulse generator circuit made up of a UJT oscillator (such as 2N2420, Q1) and a silicon bipolar output transistor (such as BC547, Q2). The transient simulation takes a long time, but this is to be expected.The example application circuits using UJT which are explained in the article are: If you share your actual circuit files it will be easier for us on this side of the screen to understand where the problem might be.Įnclosed you will find a Qucs project with your Xtal model, an S-parameters simulation that gives the expected results and an oscillator with that Xtal that actually oscillates in this case the oscillator does not need help for starting up.
#Ujt simulation qucs series#
If you use another method it will be more difficult to have the circuit oscillate.ĭoing an S-parameters simulations with your Xtal equivalent circuit values I can see the parallel and series resonances, as expected. The transient integration method for oscillators should be 'trapezoidal', this is the default in Qucs. Your simulation time should probably be of the same order of magnitude. Your Xtal has a Q of about 1e5 at 7 MHz, that means its time constant is around 1e5/7e6=14 ms.
#Ujt simulation qucs simulator#
With a simulator they might start with more difficulty, due to several issues (try googling 'xtal oscillator spice'), mainly that in a simulation there is no noise to start it up (but there is 'numerical noise') and that some of the algorithm used for the transient simulations are inherently damped. Xtal oscillators are in general are slow to start, even in real life (check with an oscilloscope). You are not simulating for a long enough time. My conclusion at this point is that qucs can not properly simulate a crystal, or at least, not this one. Real test equipment with this setup shows both. The simulations both found the series frequency but neither showed any evidence of a parallel frequency. I then modeled a S parameter and a frequency sweep with the crystal between 3dB pads as one would do with real test equipment. Stop time 14.28 microseconds (100 cycles 7 Mhz)Įven when the oscillator starts, the output is much (several orders of magnitude) lower than reality.Īdding various voltage or current pulses does not effect the starting. Specified nominal load capacitance 20 picofarad I didn't know about the infinite DC resistance restriction thanks.