Dear Ken and John Sorry to have taken so long to reply to your emails about my 200MHz oscillating HCMOS example. I hope it is OK to reply to you both in one email, too.
Replying to Ken... My example was not an urban myth, it was a real example (although I didn't measure its RF field or estimate its ERP so can't vouch for their accuracy). It was an unlikely chain of events that caused the problem, and it was not the HCMOS device that was doing the actual radiating. The large PCB on which the HCMOS hex inverter was located had a 0.5 inch wide ground and power trace running all around its perimeter, one on each side of the two-layer PCB. Thin traces ran from these 'power buses' to all the ICs on the PCB. The dimensions of the perimeter traces were perfectly right for resonance at 200MHz (and there were no decouplers between the traces) and they made a wonderful rectangular frame antenna at that frequency too. The HCMOS device that suffered the unterminated gate was in the centre of the PCB and got its +5V from the trace at the top of the PCB, its 0V from the trace at the bottom, thereby making an excellent driver for the resonant circuit. The end result was a high-Q resonant 'tank' circuit being driven by the hard-switching device, and setting the basic oscillation frequency. Because the device was hard switching, it didn't heat up. Because the tank circuit had such high-Q and was also a great antenna it radiated the 200MHz component but not the other harmonics of the hard switching device. The result was a very efficient 200MHz transmitter design that RF transmitter designers would understand. I don't find it unusual that it could create quite strong field strengths. My point in making this example is that events can sometimes combine to catch designers out. Replying to John I'm sure I said in my original posting on this example, that the HCMOS was 'hard switching' and not producing a sine wave. A hot device was, of course, the first thing I looked for, and didn't find any. See the additional information above. Regards, Keith Armstrong PS: It will be another week before I can reply again to postings in this thread. > Date: 07/01/02 06:35:46 GMT Standard Time > From: [email protected] (John Woodgate) > Indeed, and assuming a 5 V supply, the current would be around 400 mA. > At 200 MHz, the dissipation would be several hundred milliwatts. > The absence of harmonics even suggests that this gate was producing a > sine-wave, which makes the figures even higher and less credible. In a message dated 07/01/02 02:37:12 GMT Standard Time, [email protected] writes: > Subj:Re: EMC-related safety issues > Date:07/01/02 02:37:12 GMT Standard Time > From: [email protected] (Ken Javor) > To: [email protected] > CC: [email protected], [email protected], > [email protected], [email protected] > > Quote : "Who would have expected an unterminated HCMOS gate to be able to > emit 2W at 200MHz?" > > Not me - 2 Watts of effective radiated power implies over 2.5 V/m at 3 m! > I guess I have a hard time believing that was transmitted from an HCMOS > gate. I think a little common sense will go a long way towards retiring > some of these EMC-urban legends. > > on 1/6/02 10:40 AM, [email protected] at [email protected] wrote: > > >> A.2) A portable computing device used in an automatic change machine on >> board transport was tested to be fully compliant with EN 55022 (approx = >> CISPR 22). >> I helped the manufacturer investigate complaints of interference and >> discovered that sub-fitted variant, which had not been tested for EMC >> compliance, left an HCMOS inverter IC with an unterminated inverter - >> which promptly decided to self-oscillate at 200MHz. (Many manufacturers of >> products with a number of build variants only test the fully-loaded one >> for EMCD compliance and assume the others are at least as good.) >> >> The very interesting thing about this example is that the power-ground >> structure of the PCB made a beautifully tuned antenna and resonant circuit >> at 200MHz, so although the inverter was hard-switching and did not run >> hot, the only emissions were at the 200MHz fundamental - no harmonics were >> emitted at all. >> >> Another very interesting thing is that some of the complainants had >> measured the equivalent radiated RF power from these devices as 2W. >> > Who would have expected an unterminated HCMOS gate to be able to emit 2W at > 200MHz?
