Operation Edit This article needs attention from an expert on the subject. The specific problem is: The description is confused about the operation of the circuit, and the diagrams do not show crucial phase lag components or the need for a high gain amplifier. The output of the amplifier should show a series R driving the first shunt C. That lag network is beyond cutoff so it has almost a 90 degree phase shift and significant attenuation.
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Not entirely safe, but never bored Thar be dragons! There are other versions of this kit in other colors that use the same circuit. This post explains why. It is billed as a DC MHz frequency counter and crystal checker. The idea of a crystal oscillator coupled with a frequency counter is a good one—plug in an old crystal you got in the flea market and it will not only tell you if it is oscillating, but it will also tell you at what frequency it is oscillating. Some might question why you need to measure the frequency when its written on the case, but this is often not true when it comes to old amateur crystal holders or WWII surplus that was later modified; labels fall off, cases are switched, quartz blanks are reground, etc.
Sometimes the case is empty or the crystal has been cracked. We often want to know if there is a good chance that a crystal will work in our gear, and the best test always seems to be in the equipment its intended for. Besides, kits are fun. This adventure started when a friend of mine with an interest in restoring WWII communications gear was using one of these testers to screen crystals. He commented to me that he had a hard time finding good meter FT crystals.
While a little harder to find, their reliability generally pretty good as these things go, as they thicker less likely to be cracked and the manufacturing tolerances are looser. He had more faith in a modern crystal oscillator that had an apparent track record of service more than the BC that might or might not be working right. Was this bad luck or is something else going on? Suspicious, I ordered a couple. False advertising? The kits were identical as to the parts and went together in under an hour.
The parts placement diagrams were all you needed and they were correct! The Ebay package came with instructions in absolute pidgin English, along with a photocopied schematic that could fit in a fortune cookie.
Despite its lexicographic shortcomings, it had one piece of helpful information: Before measuring crystal please switch the J1 jumper cap, in JP1 insert 4 m to 40 m crystals can measure the crystal frequency sic Frequency counter installation instructions Mirabile dictu! Well, someone admits that the crystal measuring range more limited than that of the counter, regardless of whether this note refers to wavelength or frequency.
Often I got nothing on the display and the scope confirmed no oscillation, or I got the third and sometimes fifth overtone MHz , which is usually a little below the arithmetic harmonic of the fundamental…Sometimes I got a random number generator. None of my crystals below 3 MHz would oscillate at all. Everything above 6 MHz behaved much better, although it was a rather stingy tester at 40 meters. Like I said, these are all known crystals that work wonderfully in any of the transmitters I have and I even use with some regularly in the old Adventurer in the picture.
The PIC simply counts waveform peaks each second and shows you the number divided by a million to get MHz more on that in a bit. The Colpitts oscillator in the kit. Test crystal is across pins 1 and 3 of JP1. The choice of a bipolar transistor, bias resistors R1, R4 , and low-value capacitors all points to one thing: this circuit is designed more likely to oscillate with crystals whose fundamentals are above 10 MHz. This is a fabulous reference for transistor crystal oscillators that discovered as part of this project.
He says that at any given frequency there is a minimum shunt resistance to the transistor that will allow oscillations to start. He even gives us a handy chart with empirically derived values: From Matthys, Robert J. The shunt resistance in our circuit is a function of the bias resistor R1 the emitter resistor R4 and the gain of transistor. It also explains why the more modern vacuum-sealed crystals tended to score a little better on average.
Packaging in air like a pressure mounted FT increases the equivalent series resistance of the crystal about 3 times on average Matthys, p.
The scope showed the strength of the oscillations to only be about 1 or 2V P-P. Once again, let us ascend the mountain to consult the oracle… Matthys, p. Same circuit as the tester. We see that the bottom capacitor C1 in Matthys, C3 in our oscillator should be about an order of magnitude larger than the 22pF that came in the kit if we want to work at the low HF range.
Prophetically, Matthys warns against the exact properties our circuit has when used with crystals in the low HF region: In the [bipolar] transistor-Colpitts circuit parasitics will occur at some nonoptimum circuit values. In contrast, no parasitics of any kind have been found in the FET-Colpitts circuit. The parasitics turn out to be third harmonic oscillations or a combination of fundamental and third harmonic oscillations. The circuit values are rather critical for obtaining this harmonic oscillation.
Setting the time constant R,C, for the third harmonic frequency also helps. Both third and fifth harmonic oscillation have been reported by Bahadur and Parshad . The amplitude of oscillation obtained this way is rather low, and there is a better harmonic Colpitts circuit available, which is discussed in the following paragraphs.
Matthys, p. The spurs and multiple frequencies which in our case manifest as random digits on the display as we confuse the software. But even then you may run into trouble with older third overtone types in the MHz range where both the fundamental and overtone are encouraged more or less equally.
One variant of this board in the wild puts the full supply voltage on the transistor 9V? Not all is lost…I think you can make a much better crystal oscillator to drive a PIC- or arduino-based counter input for these purposes and do so very cheaply.
Crystal oscillator circuits
Background[ edit ] There were several efforts to improve oscillators in the s. Linearity was recognized as important. The "resistance-stabilized oscillator" had an adjustable feedback resistor; that resistor would be set so the oscillator just started thus setting the loop gain to just over unity. I would report on some applications I had thought up on negative feedback, and the boys would read recent articles and report to each other on current developments. This seminar was just well started when a paper came out that looked interesting to me. It was by a man from General Radio and dealt with a fixed-frequency audio oscillator in which the frequency was controlled by a resistance-capacitance network, and was changed by means of push-buttons.
Not entirely safe, but never bored Thar be dragons! There are other versions of this kit in other colors that use the same circuit. This post explains why. It is billed as a DC MHz frequency counter and crystal checker.
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