75 to 300 Ohm L-Pad with switched attenuator.
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- Опубликовано: 31 июл 2024
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I use an ELV SUP3 Stereo FM test signal generator for aligning my tube radios, and have come across a couple of slight handicaps with this device: The output signal level is a little high for practical testing, so attenuation is needed. Another problem is the output impedance. The 75 Ohm output is fine for most modern sets, but not for the tube radios that expect around 300 Ohm impedance at the antenna input.
I decided to design and build a gadget that solves these two challenges. It is a stepped attenuator that can be switched up to 50dB of attenuation, as well as an impedance converter, from 75 to 300 Ohms.
The Gerber files will be on the PCBWay share section if you want to order boards for yourself.
Hope it proves helpful.
You can now support the channel:
/ mcaldeira
paypal.me/macaldeira
TimeStamps
00:00 75 to 300 Ohm L-Pad with switched attenuator.
00:11 Introduction.
00:45 The ELV SUP3.
01:30 Plugged attenuators and L-pad.
03:08 The PCBWAY boards.
04:21 The project goals.
05:10 The calculations.
10:25 Schematic and PCB design.
16:20 The build.
18:36 Testing.
26:03 Conclusion.
#tuberadios #antiqueradio #radiorestoration
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This is exactly what I've been looking for as I'm getting into fm alignment. Thank you very much.
pleasure
I always enjoy your home designed/built bits. I am also pleased that you include Through Hole PCB designs too. I don’t have a morale issue with SMDs, just a shaky hand and would spend most of my time picking the bloody things up off the floor ! Thanks Manuel. 😊
😊 I also sometimes have a problem seeing the little things, but I like the simplicity of it.
Thanks for sharing your knowledge
👍
Thank you for making this video, and putting the plans on the PCB site.
Unless I missed it, I did not see you verify the resistor values. I would like to have seen 1% or better matches to those you selected. I have not been especially impressed with bulk part consistency.
It would be very nice if you made a follow-up video, sometime in the future, using resistors that are closer to the calculated requirements.
It may be that more precise resistors are available as thru-hole parts, but that is only a guess.
The better Ham Radio step attenuators that I have seen disassembled, put vertical shields between the different stages. Sometimes they are only soldered-in copper clad board.
Manuel, I've been thinking about using the ELV stereo signal generator in lieu of a FM wireless transmitter or Bluetooth transmitter for rebroadcasting to my FM broadcast receivers. Though in the states I might be in violation of FCC part-15 especially with a big antenna.
I believe you probably will be. The quality will be there, but so will the excessive power.
Have you ever done a reactive load box for a tube amplifier? I cant find much online about it. Thanks for the video :)
I haven’t actually. Might look into that.
Manuel, didn't you consider to make the output switchable between 75 and 300 Ohms? Only one switch and one divider extra (to get the output levels match somehow)...
I actually kicked myself for not considering that. I’ll probably do a Version 2 one of these days.
Nicely done, and usable.
One thing... Why do you use close to the double value resistors if you want a more or less exact value ? I would use the first value above the wanted one, and then use a much larger for the second: you need 61,364Ω. You used 2x120Ω=60Ω. But after measuring the attenuation was to low, because the value is to low. I would have used a 68Ω (the first value above) and a 680Ω, which gives you 61.8Ω. Now measure and pick a low value from your stock, and you will be damned close to 61.364.
True, but with these attenuators, unless you are doing really precise experiments, a few dB is really not critical.
Thanks for good information all the time.
Do you think I can order same your pcb design at pcbway?
Yes, I will place it is the shared section tonight.
Wow! Thank you!!
I was retired Mechanical Engineer, but I really loved electronics, tube amp and music.
I built and modified many amps and got little or little deeper understanding on circuit but I got big huddle on tuning my MR71 Mcintosh tuner, Someday, I will fully understand fm circuit and fine tune my tuner with this attenuator.
Thank you again.
my pleasure
Have you ever checked the linearity of the box? Between 87Mhz and 107Mhz can be some drift.
Nice tool!
I haven’t checked, but expect some drift. I really don’t use this for any precision measurements, so it does the job.
Manuel, Where can i get an ELV signal generator
ELV sells it on their online store
You can buy the SUP3 here: de.elv.com/elv-hq-stereo-ukw-pruefgenerator-mit-oled-display-sup3-komplettbausatz-143310
There is a free manual (in english) available under "Downloads"
We currently deliver to:
• Germany
• Austria
• Switzerland
• Belgium
• France
• Italy
• Luxembourg
• Netherlands
• Poland
• Romania
• Spain
Interesting and great job. To bad the ELV SUP3 is sold only in EU, so I can't buy it.
Just a few quick questions/rants from me ;-)
Why 10 - 20 - 20 dB, and not 10 - 20 - 30 ?
Why not a 4th switch for the impedance stage, so you could switch between 75 and 300 ohms output?
And 300 ohms is supposed to be balanced, but with this you end up with one lead connected to ground on the signal generator.
Probably not an issue when you use the SUP3, but a mains powered SigGen might be an issue?
And just to top of my ranting... At 92MHz you should probably have your probe set to 10X, not 1X. but maybe the signal was too weak for that?
Again, thanks for constructing, sharing and posting this.
Hi Geir
I wanted around 60dB total, and counted on the 11 on the L-pad, hence the choice of attenuation steps. I can always change it easily if required.
The missing 75/300 switch was really stupid of me. Will probably have to make a V2.0 😊
As for the balanced, I would have to have made all the T-pads balanced as well, and the switches would then get complicated (4PDT), so I decided to take advantage of the footing SUP3.
@@electronicsoldandnew you can also add a balun at the end of the attenuator chain. And a switch 🙂👍