Square-waves from a chip other than NE555, please?

[MrBungle]

That may be a combination of the probe capacitance and the output current driving capacity and your oscilloscope slew rate doing that.

Try stacking the rest of the gates in the 74hc14 in parallel. That will massively increase the electron pumping ability. See: http://www.epanorama.net/circuits/tdr.gif. Drop the resistors to about 10-47 ohms.

Agree with simpler things always

[David G4EBT]

Yes, I first came across the 74HC14 when I built a home-brew ESR meter which featured here:

https://www.vintage-radio.net/forum/...ad.php?t=54367

That design used the five spare gates as a buffer, as indicated on the circuit which is appended below. Ironic that in the ensuing years, Chinese 'multi-testers' ,which test resistors, caps including ESR, transistors and inductors have become so cheap as to render home-brewing such things hopelessly uneconomic. Fun while it lasted, and the ESR meter still works fine.

[MrBungle]

That's quite an interesting design. I actually have all the parts to build that lying around. Perhaps a weekend project coming up. Thanks for the reference.

If there's anything I've learned over the years, I'd rather homebrew something in this space and get the satisfaction of construction than the empty satisfaction of a purchase (unless it's broken and needs repairing!). I think SPRAT is a testament to that attitude

Anyway I shall not wander further from the topic (sorry mods!)

[Al (astral highway)]

Originally Posted by David G4EBT

As I said ..., a square wave generator doesn't get any simpler than an IC a resistor and a cap. I didn't make any attempt to shorten the wiring, just a 50pF cap that happened to be to hand, and a 22k resistor, the frequency coming out at 1.352MHz. Not a perfect square wave, as can be seen from the rise time and mark-space ratio, but clean, and not bad considering it's just tacked together. (about 4V p-p).

Hello David! I misssed your update somehow ! This is really interesting. And also relevant to me as this is around the frequency I'm interested in. I love the simplicity of that arrangement and am also a fellow fan of simplicity. Thank you

In the meantime , more questions than answers from here. I built an LC based transistor oscillator with an output stable to four decimal places at between 1.2 and 2MHz -- but then I ran into difficulties when I tried to digitise the output using an HEF40106BP.

The usual formula for gain in a non-inverting logic gate (gain =rF/rIN meant I picked values for a gain of around 300, enough to reach the gate threshold voltage of a MOSFET from the output of my oscillator. Only for some reason , the result was a signal about 150 times lower in frequency (see two 'scope traces - first is the output of the oscillator , second is the output from the logic gate .) This is obviously approximately half the voltage gain value and I wonder if there is some kind of relationship there ?

Anyone any insight into this behaviour ? Thank you

[Mooly]

I'm not so sure you can use a Schmitt invertor as an amplifier tbh, and I also have a vague recollection that even the basic CMOS gate type amplifier needs to be the type without input protection diodes ? Long time ago, but something comes to mind on that.

[G6Tanuki]

In times-past when building a cheep-n-cheerful Time-Domain Reflectometer I used one of the gates in a linearly-biased CMOS 4069 as a crystal-oscillator on twice the frequency I wanted, then followed it by a divide-by-two using a 4013 to 'square up' the resultant output.

Another 4069 with all the gates strapped together - followed by a ULN2003-type multiple-Darlington drive chip (again with everything strapped together) - gave a good amount of low-impedance drive.

[Truth is, this could have also made a good 2.5Watt 160-Metre transmitter. . ].

If you're after squareness-of-waveform and duty-cycle there's a lot to be said for generating the initial signal at a higher frequency, amplifying it a lot, then putting it through a divider network to get the frequency you actually want.

[MrBungle]

Originally Posted by Mooly

I'm not so sure you can use a Schmitt invertor as an amplifier tbh, and I also have a vague recollection that even the basic CMOS gate type amplifier needs to be the type without input protection diodes ? Long time ago, but something comes to mind on that.

You need UB suffixed 4000 series or HCU series 7400 logic to run in linear mode properly. Good for crystal oscillators!

[julie_m]

Possibly the most egregious abuse of NOT gates as a special kind of op-amp with no access to the non-inverting input, was a "percussion synthesiser"; using a birthday-card-type piezo transducer to produce a burst of exponentially-decaying noise, with the "amplifier" in a simple low-pass filter circuit, and the whole shebang six times over with different time constants for different sounds.

EDIT: Found it! Sick bags at the ready

[Mooly]

Originally Posted by MrBungle

You need UB suffixed 4000 series or HCU series 7400 logic to run in linear mode properly. Good for crystal oscillators!

Thanks MrB

[Al (astral highway)]

Before I retreat to my op, next week, UPDATE. After much experimentation and trial of various ideas , too many trials to mention in detail, -- thanks to everyone !!- I am, after all, going to square this waveform. This is from another LC design, not the one I posted previously. I had all the parts lurking in my parts collection....

Nice sinewave, stable to five decimal places, source-coupled FET design. (2 X NOS 2N4416's) The reason is that I need it to be finely variable across quite a high frequency range (1.3 and 2MHz ). This one is super stable and not an unintentional VFO, unlike some other designs. Using the build shown, I'm getting a pure sinewave stable to five decimal places. I will square the output with an IXDD414 gate driver.

I'll post an update on the digitised output tomorrow.
The component count is potentially lower than the as-built but I included several silver mica caps in the tank circuit. It has just one R.

The circuit draws a minuscule current. Other suggestions were brilliant for a known stable frequency but LC wins for smooth and easy variability. The main inductor is a high quality 56uH choke.

[Al (astral highway)]

'Unintentional VCO ', not VFO. Excuse the slip!!! Also, the photos didn't upload the first time . Here the are...

[G6Tanuki]

Originally Posted by astral highway

'Unintentional VCO ', not VFO. Excuse the slip!!! Also, the photos didn't upload the first time . Here the are...

I've used the valve equivalent of that circuit [with a 12AT7] in the past. It's good because both the L and C have one side earthed (so helping to reduce estraneous inductances that can raise the harmonic-content) and you can do band-switching using just a single-pole multiway switch to select different coils.

Selecting the correct value for the common-source resistor, and matching the two FETs closely, helps linearity/low-harmonic-generation too. JFETs aren't like traditional triodes in that they don't rectify well across the gate/source so any "mapping of a valve design directly into JFETs" that depends on a grid R-C network for bias is likely to be a bit problematic.

(Hint: It also works well if L and C are replaced by a crystal !)

[Bazz4CQJ]

I guess this may be a bit late, but wouldn't a SN74LS629N meet the need of the OP? I think it gives a square wave to about 30MHz.

B

[Al (astral highway)]

Originally Posted by G6Tanuki

I've used the valve equivalent of that circuit [with a 12AT7] in the past. It's good because both the L and C have one side earthed (so helping to reduce estraneous inductances that can raise the harmonic-content) and you can do band-switching using just a single-pole multiway switch to select different coils.

Selecting the correct value for the common-source resistor, and matching the two FETs closely, helps linearity/low-harmonic-generation too. JFETs aren't like traditional triodes in that they don't rectify well across the gate/source ...

That's really interesting, thank you... I vaguely recognised its aetiology as a double-triode circuit and if I get the time one day, I am curious to compare the performance of that version.

This ticks all the boxes - simple and super-stable and I like your idea of using a wafer switch and various inductors for changing bands.

[Ed_Dinning]

Hi Al, that circuit should also do nicely for a pantry transmitter.

Ed

[Al (astral highway)]

Originally Posted by Ed_Dinning

Hi Al, that circuit should also do nicely for a pantry transmitter.

Great idea, Ed! I love a multi-application build!!

[Al (astral highway)]

So I have this square wave output now from the gate driver...only after a minute or so , the amplitude diminishes and then the waveform collapses completely. Is this possibly a quirk of the probe? If so, why? I'm sure I noted a tiny spark when I connected the scope ground to circuit ground a couple of times.

The waveform is actually purer; this is via just a 2R resistor. 10k removes the apparent artefacts, the small overshoot and undershoot.

I am now powering both the oscillator and gate driver from the same power supply, both decoupled with a large electrolytic and a 10nF capacitor, one right on Vcc of the gate driver and same on Vcc of the oscillator. I thought the linear voltage regulator was cutting out from the large current needed by the gate driver chip but the LED power indicator says otherwise.

[Ed_Dinning]

Hi Al, probably worth monitoring the reg voltage in case it is drooping, also check its temp.
Try a separated reg, but from the same supply for your squarer.
Check your scope probe compensation setting to see I'd the rings on the waveform are true or if they are from the probe, use as short an earth lead on the probe as possible to keep inductance low when you are looking at fast rise edges.

Ed

[Al (astral highway)]

Thank you Ed. I will make those checks.
The rings are definitely false. They disappear if I make the coupling the scope probe more resistive and less capacitative. The probes are calibrated. I suspect the power supply is under-powered for the squarer chip. It's a beefy gate driver and produces 2A pulses I think. I'll check the voltage regulator later when I get home.
Cheers

[Al (astral highway)]

Shame. Total destruction of all the silicon.
On test, which was very short, the output from the linear regulator was 14v and rising by the second. I disconnected the power supply entirely and used my bench regulated supply but after undiagnosed over voltage yesterday, it was too late. No output from the oscillator and destroyed gate driver, a costly little chip. The linear regulator wasn't rated highly enough for the gate driver chip, which draws a hefty current.

The output was coupled to the gate of a big MOSFET which was not actually switching, no power yet to its drain. However, the driver chip was still working hard, apparently just by knocking out pulses into the load formed merely by its series 2R resistor and the MOSFET gate-source capacitance

These things happen but quite costly blow-up there. Unfortunately that's it for a while as my surgery will put a hold on any projecting for quite a while. This is a work in progress, so back whenever that may be.