Software Defined Radio VLOG Part 3 - Unintentional Resonate Circuit

I accidentally set up a resonate circuit by having long traces and ferrites in the supply rails. It was interesting that it only showed up after adding the FPGA as a clock source and happened on all frequencies synthesized. Changing the decoupling by addind and additional cap for the 74hc74 fixed the problem.

Category:

People & Blogs

Tags:

• radio ham Jeri Ellsworth
• vlog
• SDR
• Software defined radio
• fpga
• tayloe detector
• decoupling

[w2aew]

I've done a lot of debugging tricky circuits by literally using my fingers to drop in a bit of capacitance or RF loss into a circuit to try to find sensitive nodes or oscillating circuits (or overheating parts - ouch!). Good stuff! What SDR software are you using again?

[JeriEllsworthJabber]

@w2aew Software - Winrad is a freeware SDR tool I found.

[FlyByPC]

This is why I prefer digital. With analog, it seems that there are so many ways that the circuit could decide to do something else that it's hard to know where to start. Nice job tracking it down, though.

It looks like the TTL oscillators are out (or maybe they're just hard to see from this angle) -- is the FPGA running the show now?

[JeriEllsworthJabber]

@FlyByPC FPGA is now generating the local oscilator. I'll have more now with it's internal PLL.

[markiduval]

Now I'm picturing you Jeri being a preschool teacher teaching little kids electronics. "Where did the harmonics go!? *gasp*"

[JeriEllsworthJabber]

@markiduval Hah. "Jack and Jill went up the hill to fetch a bucket of capacitors..."

[cfgosnell]

I agree with FLyByPC, you ignore analog(ue) effects in digital circuits at your peril...

[mikeselectricstuff]

@Afrotechmods Fine until you spend ages wondering why things have suddenly changed before you realise it's because the battery is going flat...

[mikeselectricstuff]

@FlyByPC There is no such thing as digital . Everything is analogue.

Even in something as 'digital as an FPGA you can get marginal timings... metastability... decoupling problems...

[jeriellsworth]

@vk2zay I'll have to play with the circuit a bit. I was trying to squelch high frequency noise coming from the 74hc74, since it has very fast rise times. I even series terminated the select lines to the 4066, which seemed to help.

[vk2zay]

The opposite sideband suppression looks pretty good judging by that moderately strong CW signal close-in. RC filtering is often way better at RF than ferrites, doesn't give odd resonances except for capacitor weirdness, the resistance de-Qs any resonances.

[whisk0r]

A random capacitor or a resistor to ground does seem to usually do the trick for warding off many of these little gremlins!

tymkrs

[Afrotechmods]

If you are not already doing so, may I suggest you use a battery to power these initial prototypes? That way you are guaranteed minimal power supply noise. Once everything is going well you can go back and tweak it for any source you want.