EMRFD Message Archive 9270
Message Date From Subject 9270 2013-10-19 21:06:06 k6jq Unbuffered vs buffered CMOS inverters, with a twist The recent thread about the 'W6QIF Oscillator' was interesting. It reminded me thatI realized I did not entirely understand why the unbuffered CMOS inverters ('HCU04,'AHCU04) were superior to the conventional buffered ones ('HC04, 'AHC04), the W6QIFarticle made use of a buffered 74HC04.One a solderless breadboard, a single HC04 inverter with a 100k biasresistor from input to output would happily oscillate around 51MHz.This is because the HC04 is buffered, and consists internally of 3CMOS stages, each one inverting. In other words, a single HC04gate is actually a chain of 3 inverters. Connecting the outputto the input through the bias resistor creates a ring oscillator,the frequency of which is 1 / (6 * inverter delay), which yieldsabout 3.2nS delay per stage, or a total of ~9.5nS for the singleinverter - consistent with the datasheet typical delay of ~9nS.I don't have any HCU04s, but I do have a stock of AHCU04s.Other than being faster, they should behave the same as anHCU04. Replacing the HC04 with the AHCU04, I saw nooscillation, but the gate self-biased to 1/2 Vdd as expected.OK. So let's try the 74AHC04 I have. That ought to self-oscillateat a seriously high frequency! Except it doesn't, and won't.Because the input structure of the AHC family contains about100mV of hysteresis, which prevents oscillation in thiscircuit, as well as with any slow-transition input signal.So there you have it - a darn good reason to use unbufferedU04s in building an oscillator.Note that the AHCU04 when built into a ring oscillatorof three inverters runs around 116MHz, suggesting aper-inverter delay of ~1.4nS. AHC04 runs around 86MHz,or 1.9nS/gate.Dana K6JQ 9276 2013-10-20 08:31:53 k6jq Re: Unbuffered vs buffered CMOS inverters, with a twist Also, I recall the 74HC04 in a ring oscillator (three gates) runs at 37MHz, which suggestsabout half the propagation delay that I'm seeing in a single-gate "ring". The explanationI have for this is different load capacitance.Dana K6JQ 9283 2013-10-22 17:17:36 Dana Myers Re: Unbuffered vs buffered CMOS inverters, with a twist 9284 2013-10-22 17:56:42 ehydra Re: Unbuffered vs buffered CMOS inverters, with a twist How do you measure it? With a (loading!!) scope-probe or off-line via a
Dana Myers schrieb:
9285 2013-10-22 18:42:37 Dana Myers Re: Unbuffered vs buffered CMOS inverters, with a twist 9286 2013-10-22 20:12:41 Ashhar Farhan Re: Unbuffered vs buffered CMOS inverters, with a twist I have built vfos using the 74LS04 (not 74LS14). I have an unfinished
transceiver that uses these gates as rf amp. The gain was above 20 db.
The noise was low enough to bring up the atmosphereic noise on 14 mhz
About seld-oscillations. If the termination is proper (I used a
cure-all 6 db pad in the outputs) the oscillations go away.
For the vfo I used a butler configuration with smd parts. It was one
of the most stable vfos in the lab. I found no particular reason for
the stability apart from that iuse of smd minimized low stability
wiring and use of the 1N4007 pair as tuning element must have overcome
the drift due to variable capacitor's expansion.
On 10/23/13, Dana Myers <email@example.com> wrote:
9287 2013-10-22 20:29:55 Dana Myers Re: Unbuffered vs buffered CMOS inverters, with a twist 14472 2018-01-04 17:55:46 vk3hn Re: Unbuffered vs buffered CMOS inverters, with a twist Hi there,I've been planning an HF + 6m superhet receiver build, along the lines of the EMRFD general purpose monoband receiver front end (Fig 6.68), and want to use a 74HC04 hex inverter (Fig 4.31) as an si5351 clock buffer (on each of the BFO and VFO), followed by pad and LPF, driving +7dbm DBM ADE-1 receive mixer and product detector. All standard stuff.I'm interested in how high in frequency the 74HC04 will work reliably as a VFO squarer/buffer, as this determines the maximum VFO frequency and therefore the high/low side VFO frequency decision for higher bands.A quick search across this group trawled up a discussion from around 2008 and 2013 about 74HC04 as an oscillator, where people report it working up around 38MHz.Does this agree with other's experience of this IC when used as a buffer (not an oscillator)?regards Paul VK3HN. 14473 2018-01-05 04:42:06 Ashhar Farhan Re: Unbuffered vs buffered CMOS inverters, with a twist why would you need a buffer after the si5351? it has ample drive for a diode mixer.- f 14474 2018-01-05 06:10:18 Tayloe, Dan (Noki... Re: Unbuffered vs buffered CMOS inverters, with a twist I used this as a gated input squaring device for an audio output frequency counter as a club project, the Scqrpion Stinger Singer. I recall that it worked ok to 80 or 90 MHz. The AC version was faster yet going as high as 200 MHz in one case. No prescaler device was used.
The PIC processor clock input was only rated to 50 MHz, but always seemed good to at least 75 MHz. It was a special device that would read at 200 MHz. I estimated the software to be good to over 400 MHz if the clock input of the 4 MHz PIC chip could handle it.
The clock went through a PIC internal prescaler counter, which in turn drove a counter register. The secret to the high counting speed was to create a tight loop that only counted counter register overflows. I.e., when the MSB of the counter register toggled from 1 to 0.
The only down side was that when used in a linear mode as a squaring device with a 10K resistor from input to output, they would "free run" oscillate if there was no input. Since that frequency was 70 to 90 MHz, it was not really a problem identifying when this was happening.
- Dan, N7VE
Sent from my Galaxy Tab® A
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14475 2018-01-05 08:22:42 Mike Nothdurft Re: Unbuffered vs buffered CMOS inverters, with a twist
And the SI5351 already has a square wave output. It has rise and fall times of 1nS. The 74HC04 in worst case has rise and fall times in the double digits so it would only degrade the “squareness” of the SI5351.
14476 2018-01-05 08:43:27 Tayloe, Dan (Noki... Re: Unbuffered vs buffered CMOS inverters, with a twist
I could see using a “T” part such as an 74ACT04 to act as a level translator between the 3.3v out of the Si5351 and the 5v output.
- Dan, N7VE