Every time I discover myself harboring a consistently nagging thought, I do know I’ll ultimately should get busy and discover a method to put it to relaxation.
Once I learn EDN’s Design Concepts, I search for the vary of frequencies over which the circuit will function, if that is an applicable spec for the circuit. Mentally, I give the circuit a low rating if the working frequency vary is, for my part, restricted.
Wow the engineering world together with your distinctive design: Design Concepts Submission Information
My nagging thought was “can the working frequency vary for the Frequency doubler with sq. wave output be considerably prolonged above 2.82 MHz?” If “sure”, then can or not it’s performed for a “cheap” price? (Affordable price, in fact, will depend on the appliance, amongst different issues, a few of which can be considerably subjective.)
So, I began considering and researching doable methods to increase the frequency vary of the lately revealed sq. wave frequency doubler circuit.
To increase the working frequency vary , I wanted to start with a extremely quick one-shot circuit. Being unable to buy an acceptable machine, I designed one much like the ever-present 555 timer, however with a lot sooner parts. Quick XOR gates, a quick comparator, a quick flip-flop, and a quick discharge transistor are the primary parts of the one-shot. This one-shot circuit will function at frequencies as much as 50 MHz (and possibly even increased).
The comparator I used, the TLV3501 from Texas Devices, operates rail-to-rail (enter and output), and has a typical prop delay of 4.5 ns (6.4 ns max) when driving 17 pF.
It’s the costliest part utilized in my circuit ($1.62 in 1k amount), however it’s quick, moderately priced, and available. The opposite parts are cheap, quick, and broadly used within the electronics trade. (The MMBT2369 is the floor mount model of the 2N2369, which dates again to the early Sixties, however it’s fairly quick and it’s low-cost.)
The 74LVC1Gxx components have prop delay instances within the order of 1 ns when driving a couple of picofarads of capacitance, and they are often operated with a provide voltage of 5 V (which was my desire). I used the TLV9052 twin op amp, which has infinite enter impedance (nicely, virtually) and operates rail-to-rail, enter and output. The 74LVC1G86 XOR gate is helpful as a result of it might probably perform as an inverter or as a buffer, and I used a number of of them.
A easy description of the operation of the circuit: An ultra-fast one-shot is compelled by damaging suggestions to provide a 50% obligation cycle sq. wave output. I added a 50-ohm termination and a buffer/squarer on the enter and a 50-ohm driver on the output for comfort in testing.
The nitty-gritty description (Determine 1): A 50% obligation cycle sq. wave is the enter to the XOR gate, U3 (by way of U7), inflicting a 2 ns pulse output from U5 to be utilized to the /S enter of the flip-flop, U2. The /Q output of the flip-flop goes low and turns off the discharge transistor, Q3, which permits the timing capacitor, C4, to start charging. The output from Q3 is a voltage ramp which is utilized by way of R1 to the inverting enter of comparator, U1. The output of the comparator goes low when the voltage ramp reaches the reference voltage set by R4 and R5. This resets the flip-flop, inflicting the discharge transistor to activate and discharge the timing capacitor, C4, and the cycle repeats.
Determine 1 Circuit for an ultra-fast one-shot is compelled by damaging suggestions to provide a 50% obligation cycle sq. wave output.
The charging present to C4 is equipped by Q1 and the related parts. The charging present is managed by damaging suggestions from op amp U6A, which forces the one-shot to provide a sq. wave of fifty% obligation cycle, which, when low-pass filtered, produces a DC voltage of precisely 2.5 V (if the availability voltage is strictly 5 V). The tolerances of R18 and R19 will decide how actual this voltage is.
The reference voltage supplied by U6B and its related parts is about to 2.5 V by precision (or matched) resistors R18 and R19. This reference will observe the +5 V provide, in order that the 50% obligation cycle sq. wave output of the circuit stays at 50% if the availability voltage modifications. (The frivolously loaded output of the flip-flip additionally tracks modifications within the provide voltage.)
Simulation, implementation, testing, and outcomes
I used LTspice to design and simulate the circuit. Then I used Categorical PCB’s free instruments to design and lay out a two-sided circuit board with a floor airplane on the underside aspect. I used AppCad, which is freely out there on the internet, to simulate the sign’s overshoot/undershoot on important traces. I inserted resistors in collection with among the longer traces for quick alerts to cut back overshoot/undershoot. I used floor mount gadgets for all the parts besides Q1, Q2, the enter and output SMA connectors, and connecter P1 and P3. (P2 was not used.) The factors marked T1 by way of T12 are check factors (plated-thru holes on the PCB). The loaded circuit board is proven in Determine 2.
Determine 2 The loaded two-sided circuit board with a floor airplane on the underside aspect, the factors marked T1 by way of T12 are check factors.
The ten pF capacitor for C4 was left within the circuit after I examined with different values of C4. The opposite values had been soldered on the PCB, however had been related to a 0.100-inch heart connector mounted on the PCB, so I might choose them individually with a slide-on shorting tab. That’s the reason all the opposite values of C4 have a further 10 pF for the frequency vary assessments.
The circuit efficiency was fairly nicely predicted by the LTspice and AppCad simulations. Desk 1 reveals the vary of efficiency vs a number of values of C4.
Desk 1 The vary of efficiency with completely different values of C4.
This circuit extends the working frequency vary for the frequency doubler to 36 MHz, which is greater than 10 instances the higher frequency restrict of the unique circuit.
Jim McLucas retired from Hewlett-Packard Firm after 30 years working in manufacturing engineering and on design and check of analog and digital circuits.
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