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Re: U14-3 Measurement

PostPosted: Thu May 28, 2015 8:45 pm
by gtilford
Thanks Ron. I was wondering if I should try that and your post gave me the vote of confidence needed. I was wondering if the output is acting squirrely because it is unloaded. If that doesn't work then it may be time to solicit help form the Peaberry creator ....

73, Greg, KM5GT

Re: U14-3 Measurement

PostPosted: Thu May 28, 2015 10:06 pm
by DJ&OA
gtilford wrote:Well, I don't know what to think. I installed the new chip. The measurements look the same. I checked the calibration on my scope. Everything looks okay. I tried both channels on the scope, with 2 different probes. Same result. This is what my waveform looks like with DC coupling.

73, Greg, KM5GT


Hi Greg,

I am wondering what you are expecting from the Si570 and your scope and its probes.

If you have a look at the data sheet for the CMOS version of the Si570, you will find a voltage swing of about 2Vpp at a load of 15pF. Your scope image shows a divisor of 1:1 which corresponds with 200MHz probes mostly to an input capacitance thereof of about 45pF, i.e. about a third of the data sheet impedance definition (neglecting the impedance of the ground strap of the probe which in combination with the input capacitance results in the sine wave). In any case and if possible, you should switch the probe (and the scope) to a divisor of 1:10, but even then the results may be questionable.

Therefore, please follow the proposal of Ron to install U14 and measure the output of pins 5 or 8 of U14 at about 14MHz. If you have a signal at pins 5 and 8 of U14, everything is fine.

73, Rainer DJ6OA

Re: U14-3 Measurement

PostPosted: Fri May 29, 2015 2:59 pm
by gtilford
I wouldn't believe it if I hadn't seen it. I installed U14, and the output of Q is nearly a rail-to-rail, 14MHz signal, just like it should be (see below). How that flip-flop takes the cruddy input signal and turns it into something usable is a mystery to me. Thanks for all your help. The build continues ....

73, Greg, KM5GT

Re: U14-3 Measurement

PostPosted: Fri May 29, 2015 5:24 pm
by W4MMP
Hi Greg,

I think is in line with what Rainer mentioned. It has something to do with the loading effect of the scope / probe themselves. I did not make the measurement in question when building my Peaberrys so it was a mystery to me ;) (RF circuits can be and most often are very sensitive to the device taking the measurement as I'm sure you are very aware of). I have been attempting to determine why 15M output is low on my 30/20/17/15 build. My budget USB scope was not making the grade for various reasons and needed to borrow a good Tektronix scope from a ham friend of mine to be able make reasonable measurements.

Great you found the (non) issue :D . Good luck with the remainder of the build. The receive portion of the build is really straight forward. It has been my experience with the issues presented on this board that guys experience very few problems with receive. The transmit portion on the other hand can be a bit tricky. Lots more components in the transmit portion. Take your time. Make sure all the transformers and chokes have good clean and tinned wires. Take your time winding the transformers. If you get that wrong you can get a lot of weirdness that is hard to track down.

Re: U14-3 Measurement

PostPosted: Sat May 30, 2015 1:55 am
by gtilford
Again, my thanks to everyone for your help. Rainer, I didn't see your post before I made my last one. Your explanation makes sense. Thank you.

73, Greg, KM5GT