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 Post subject: Getting ready to build Mike's log amplifier
PostPosted: Oct Thu 24, 2019 3:26 am 
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Thanks to his generous post a few weeks ago, getting ready to assemble this on a 2x3" board. I laid everything out before committing to solder and posting it to make sure if any mods need to be made. Thanks!

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 Post subject: Re: Getting ready to build Mike's log amplifier
PostPosted: Oct Thu 24, 2019 3:30 am 
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Put C4 closer to the log-amp chip and move the ground connection. You want the minimum of lead length between the VCC and GND pins on the chip and the C4 bypass capacitor.

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 Post subject: Re: Getting ready to build Mike's log amplifier
PostPosted: Oct Thu 24, 2019 4:06 am 
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stevebyan wrote:
Put C4 closer to the log-amp chip and move the ground connection. You want the minimum of lead length between the VCC and GND pins on the chip and the C4 bypass capacitor.


This better? Had a feeling that cap would need to be moved :oops: . Thanks for the tips!

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 Post subject: Re: Getting ready to build Mike's log amplifier
PostPosted: Oct Thu 24, 2019 4:21 am 
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Hello tbone8,
yes good tip for beginners is to keep leads as short as possible also traces


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 Post subject: Re: Getting ready to build Mike's log amplifier
PostPosted: Oct Thu 24, 2019 6:00 am 
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I wouuld suggest putting the power switch before the capacitor C1. Otherwise, any leakage in C1 will drain the source. And leaving it with its stress 24/7 isn't as reliable as not doing that.

Read the LM7805 data sheet to see what the best input capacitor should be.

I would put C4 across the bottom of the IC so its ground lead won't be so long. The ground at the left is pretty far from the output ground and so on.


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 Post subject: Re: Getting ready to build Mike's log amplifier
PostPosted: Oct Thu 24, 2019 12:22 pm 
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Hi Tbone8,

I'd suggest you use "dead bug" style construction mounting everything on a piece of copper clad PCB material. This type construction allows you to get the components much closer to one another, and ground is everywhere. BTW it's called "Dead Bug" simply because the ICs are placed upside down, on their back, with the pins sticking up in the air, looking just like a Dead Bug.

Image

I built mine quick and dirty, so theres NO "craftsmanship" in the build and its a "bare bones" implementation. None of the needed components to adjust the output to 20 mV/dB were included in my build to make the output easily compatible with an Oscilloscope's normal input attenuator range setting of 20mV/CM.

The Data sheet for the IC is HERE and I suggest you download it and look at the Applications information starting on page 15.

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 Post subject: Re: Getting ready to build Mike's log amplifier
PostPosted: Oct Thu 24, 2019 6:21 pm 
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You guys are great at giving tips on this as I am trying to learn about pcb board design. The short/direct ground leads are to keep ground loops from forming then? If a guy could arrange the board so all the components ground lead fell on a single ground line, would that be acceptable? I am not against the dead bug design if that is what should be done. I assume the main benefit from that would be the shielding provided? Thanks!

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 Post subject: Re: Getting ready to build Mike's log amplifier
PostPosted: Oct Thu 24, 2019 6:27 pm 
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tbone8 wrote:
You guys are great at giving tips on this as I am trying to learn about pcb board design. The short/direct ground leads are to keep ground loops from forming then?

No. It's to minimize the parasitic inductance in series with the power supply bypass capacitor.

tbone8 wrote:
If a guy could arrange the board so all the components ground lead fell on a single ground line, would that be acceptable?

Nope. The rules for RF are very different than for low-level audio. We aren't trying to avoid ground loops.

tbone8 wrote:
I am not against the dead bug design if that is what should be done. I assume the main benefit from that would be the shielding provided?

Nope, the dead bug construction gives you a very low inductance ground plane along with very low parasitic capacitance at the circuit nodes.

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 Post subject: Re: Getting ready to build Mike's log amplifier
PostPosted: Oct Thu 24, 2019 6:53 pm 
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stevebyan wrote:
tbone8 wrote:
You guys are great at giving tips on this as I am trying to learn about pcb board design. The short/direct ground leads are to keep ground loops from forming then?

No. It's to minimize the parasitic inductance in series with the power supply bypass capacitor.

tbone8 wrote:
If a guy could arrange the board so all the components ground lead fell on a single ground line, would that be acceptable?

Nope. The rules for RF are very different than for low-level audio. We aren't trying to avoid ground loops.

tbone8 wrote:
I am not against the dead bug design if that is what should be done. I assume the main benefit from that would be the shielding provided?

Nope, the dead bug construction gives you a very low inductance ground plane along with very low parasitic capacitance at the circuit nodes.


Wow, 0 for 3 in the figgering catagory :oops: . Sounds like you are trying to say a manhattan style board is the board of choice for this build :D . Fair enough, will figger a layout for that and follow up with y'all. 8)

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 Post subject: Re: Getting ready to build Mike's log amplifier
PostPosted: Oct Fri 25, 2019 12:56 am 
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Components and wires always form loops. The idea is to keep the loop from one circuit from intersecting with the loop from another circuit.

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 Post subject: Re: Getting ready to build Mike's log amplifier
PostPosted: Oct Fri 25, 2019 6:05 pm 
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Those log amps and detectors are now all over eBay for cheap prices... for those who want something that is more plug and play...

https://www.ebay.com/itm/1MHz-8GHz-70dB ... 1867413025

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 Post subject: Re: Getting ready to build Mike's log amplifier
PostPosted: Oct Sat 26, 2019 12:19 am 
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Looks like a simple solution, but you'll still need a Voltage regulator circuit if its going to be portable, and I'd guess there still are some output scaling issues to address so its easily compatible with a scope's attenuator input.

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 Post subject: Re: Getting ready to build Mike's log amplifier
PostPosted: Oct Mon 28, 2019 2:00 pm 
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Mike

Was thinking of using a regulated wall wart for power...

Pete

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 Post subject: Re: Getting ready to build Mike's log amplifier
PostPosted: Oct Mon 28, 2019 2:44 pm 
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Make that a Linear regulated wall wart. You don't want a switching regulator near that log amp.
-Chuck


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 Post subject: Re: Getting ready to build Mike's log amplifier
PostPosted: Nov Wed 20, 2019 1:38 am 
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Finally got around to building this thing. I have a Kenwood on the bench, alignment doesn't call for a sweep alignment. Not eggzactly sure where to probe the output, anyone care to take a stab at a measure point for AM and FM?

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