Re: VFO Output – Is This Right???

James Daldry W4JED
 

Hi, Pietro

First - what is the bandwidth of the scope you are using? To properly see a square wave the scope cutoff frequency has to be at least 5 times the fundamental frequency of the signal.

Next - how is the probe grounded to the source? If you are looking at signals of more than a few megahertz the ground should be a short wire wrapped around the ground ring on the probe and soldered to the ground side of the output connector on the device. Even a couple inches of wire is too much. It will add the overshoots that you are looking at.

Next - are you using a X10 probe? While the cable into the scope is coaxial, it is not terminated with a 50 or 75 ohm resistor inside the scope unless it is specifically made for looking at 50 or 75 ohm systems. With a 50 ohm impedance at the input and a 1 megohm resistance inside the scope the cable will ring like a bell. Using a properly adjusted (!) X10 probe will disconnect the input capacitance of the scope from the signal generator and make the display truer. If you don't know how to calibrate the probe, read the manual.

Good luck.

73

Jim W4JED

On 6/21/19 2:40 PM, pietromartini2007@... wrote:

I built the VFO/Signal Generator kit with no problems, and it started up right away. However, the output waveforms and amplitudes are not what I expected; according to the manual, they should be square waves with an amplitude of 3.3V p-p. Taking the output from the CLK0 pad, what I see on my scope is sort of a square wave at 3.5 MHz with a lot of overshoot and undershoot, and as the frequency increases the waveform becomes more of a triangle and the amplitude steadily decreases, until it’s only 246 mV at 150 MHz, and the waveform is almost a sine wave. I don’t have any load on the output, and I attached the scope probes directly to the CLK0 pads. If I had to guess I’d say stray capacitance may be the culprit. So can someone tell me if I have a problem with my build? I’m attaching a few photos of the scope images.

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