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VFO Output – Is This Right???

pietromartini2007@...
 

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.

Richard G4TGJ
 

What's the bandwidth of your scope? A square wave is formed from a lot of harmonics so if they are being attenuated then it won't stay square.
--
73
Richard
G4TGJ

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.

Graham, VE3GTC
 

Pietro,

What you describe sound correct for the way you were connecting your oscilloscope to have a look.

Oscilloscopes are very handy instruments but do require care in their use.

You may find these links helpful:

https://www.youtube.com/watch?v=zodpCuxwn_o  oscilloscope ground lead length effects.

https://www.youtube.com/watch?v=SX4HGNWBe5M   basic 1x and 10x scope probe tutorial

https://www.youtube.com/watch?v=-4q8geE5ef8  How to make a high performance oscilloscope probe socket

https://www.youtube.com/watch?v=OiAmER1OJh4  mysteries of 1x scope probes

https://www.youtube.com/watch?v=oxJQr4dKnys  importance of 10x probe compensation.

The first three videos are by W2AEW and quite well done. He has over 300 videos posted on youtube.

The last two are by Dave Jones of EEVBLOG fame. Also well done and he also has many videos posted.

You may also find this document useful:

https://www.sitime.com/api/gated/AN10028-Probing-Oscillator-Output.pdf

cheers, Graham ve3gtc


On 2019-06-21 18:40, 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.

pietromartini2007@...
 

OK, thanks Richard.  The scope is a 150 MHz HP.  I've been confused because my commercial signal generator always shows a nice square wave up to its limit of 50 MHz, but presumably it has buffers and amplifiers so that the signal doesn't get changed.  I'm going to redo my measurements with the suggestions I got today.

pietromartini2007@...
 

Thanks, Jim.  Scope is 150 MHz, but I just soldered some wires onto the CLK0 pads and attached the scope probes (1X) to the wires.  I'll try some of your suggestions when I redo the measurements.  What do you think of soldering some 50 ohm coax to the pads and then connecting the coax to the scope probe?

I was hoping to use this VFO for tuning a homebrew HF QRP transmitter (and driving a following buffer & amp).  I was more worried about the attenuation than the waveform.  Thanks again.

pietromartini2007@...
 

Thanks for the suggestions on scope use, Graham; they will make this an entertaining weekend!  I've been a long-time admirer of W2AEW's videos, but I didn't think to look at those on scopes.  But I'm reassured that the VFO is probably working correctly and that what I see on the scope is just a product of (poor) measurement technique. 

James Daldry W4JED
 

Hi, Pietro

The coax would be just gilding the lilly, and add capacitance. Just connect with as short leads as practical.

73

Jim W4JED

On 6/22/19 1:38 PM, pietromartini2007@... wrote:
Thanks, Jim.  Scope is 150 MHz, but I just soldered some wires onto the CLK0 pads and attached the scope probes (1X) to the wires.  I'll try some of your suggestions when I redo the measurements.  What do you think of soldering some 50 ohm coax to the pads and then connecting the coax to the scope probe?

I was hoping to use this VFO for tuning a homebrew HF QRP transmitter (and driving a following buffer & amp).  I was more worried about the attenuation than the waveform.  Thanks again.