Newbie RF frontend protection Qs

Useful information regarding antennas for SDR products.
vk7jj
Posts: 126
Joined: Thu Mar 02, 2017 9:56 am

Re: Newbie RF frontend protection Qs

Postby vk7jj » Wed Jun 06, 2018 8:18 am

Incandescent bulbs take far less than 150 milleseconds to achieve maximum resistance. In 5 milliseconds
they are at 1/2 maximum resistance and at 10 milliseconds have reached about 80% of maximum resistance


We sure are terriers in search of a resolution aren't we? Have to laugh at us!

It looks like a case of duelling citations then, the first one I quoted from https://www.picotech.com/library/results/lamp was of a 6 volt globe, showing the experimental setup and a graph of the response time along with the statement (on the second page teacher's notes)

The instant that the p.d. was applied can be clearly seen. What is also very clear is that the lamp takes some 150 ms to reach maximum brightness.


Obviously I'm unashamedly trying to bolster my case by finding slow globes, though to be fair I did try hard to find a definitive as possible reference but couldn't [PDF warning]: http://www.wwnorton.com/college/chemist ... 1155-3.pdf

The 12 volt brake lights tested show graph and table listing 0-90% to-turn-on times ranging from 118 to 256ms. No doubt the smaller 200mA filaments we are actually discussing would likely be a faster but unlikely to be orders of magnitude faster.

Rather fun to think that the slow turn on time of filament brake lights is considered to be a significant factor in driver reaction time and that LEDs have turned that around.

Anyway, since I'm still fretting, as I mentioned, about whether a resistance in the order of 40 ohms would make any difference to voltages of unknown but potentially pretty high values, I'm running away to spend more time at play :-)

Please be congratulated on putting up with me, thank you for a civilised and amiable discussion and may all your front-ends continue to live happily ever after!

Regards, Phil VK7JJ

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OH2BUA
Posts: 63
Joined: Mon Aug 29, 2016 8:58 am

Re: Newbie RF frontend protection Qs

Postby OH2BUA » Tue Jun 12, 2018 2:52 pm

Is it for sure proper way to calculate two 1N4148 diodes in series simply like 0.7 + 0.7 = 1.4V ?

If we deploy 2 volts over that, is the forward voltage over one diode 1V... or perhaps just plain 0V?

(Like to hear that 1.4V is ok calculation... as also the (unwanted) capacitance is halved.)

73, Jukka

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vk7jj
Posts: 126
Joined: Thu Mar 02, 2017 9:56 am

Re: Newbie RF frontend protection Qs

Postby vk7jj » Wed Jun 13, 2018 11:03 am

If we deploy 2 volts over that, is the forward voltage over one diode 1V... or perhaps just plain 0V?


Hi Jukka, I'd never have thought of that.

http://www.nutsvolts.com/questions-and- ... -in-series

I guess in the case of series RF protection diodes, at any voltage higher than the combined forward voltage internal diode leakage causes current flow with a differential between the diodes which doesn't matter for a short duration spike. Leakage means there is no "zero volts" case.

I've never used series diodes because without really thinking it through I've felt that the RF front end needing protection may present only a single junction (perhaps even a Schottky with a very low forward voltage) causing it to bear the brunt of the current. As a case in point the front end of my old Atlas 210X is the diode ring mixer and I replaced the originals with Schottkys.

Regards, Phil

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Mike2459
Posts: 71
Joined: Mon Jul 24, 2017 8:24 pm

Re: Newbie RF frontend protection Qs

Postby Mike2459 » Thu Jun 14, 2018 3:08 am

Two identical forward biased 1N4148 diodes in series with 2 volts applied to the circuit would result in a forward voltage of 1 volt per diode (assuming they are identical in characteristics). This would result in a forward current of about 50ma (from Figure 2 in the data sheet). The absolute maximum rating for If or forward current for this diode is 300ma. Got this info from a 1N4148 data sheet which I tried to post but the board isn't accepting any attachments at this moment but it can be found here:

https://www.vishay.com/docs/81857/1n4148.pdf

The use of .7 volts for the Vf or forward voltage of a silicon diode is a rough approximation, it is in the real world dependent on temperature and forward current.

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OH2BUA
Posts: 63
Joined: Mon Aug 29, 2016 8:58 am

Re: Newbie RF frontend protection Qs

Postby OH2BUA » Thu Jun 14, 2018 11:12 am

Thanks for your replies.

I'm planning for a 'major upgrade' for my 2*1N4148 protection system, to go for 4*1N4148. That's mainly to push down the unwanted capacitance, as I feel that my monitoring system is a little bit hard-in-hearing above 18MHz.

https://www.pskreporter.info/pskmap?preset&callsign=oh2bua&timerange=3600&showgrid=1

73, Jukka

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9a4db
Posts: 29
Joined: Sun Jan 24, 2016 6:44 pm

Re: Newbie RF frontend protection Qs

Postby 9a4db » Thu Jun 14, 2018 2:46 pm

18 MHz only?!
My circuit (above) is working nice up to FM broadcasting band.
Above that, on Aviation AM band can hear the difference.
One day I may measure real performance...
73
Djani

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OH2BUA
Posts: 63
Joined: Mon Aug 29, 2016 8:58 am

Re: Newbie RF frontend protection Qs

Postby OH2BUA » Thu Jun 14, 2018 3:00 pm

18 MHz only?!


Above 18MHz, my friend. I'm monitoring between 1.8-50Mhz (with that antenna), and I feel that on 6m I'm almost deaf.

I have other gear going nicely over 1GHz.

73, Jukka

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harley040
Posts: 2
Joined: Fri Jul 06, 2018 2:32 pm

Re: Newbie RF frontend protection Qs

Postby harley040 » Fri Jul 06, 2018 2:44 pm

You could purchase a small SO-239 lightning arrestor. Should give some protect from lightning strike and static discharge. I wouldn't leave any antenna hooked up to a receiver in a thunderstorm even with the arrestor. The other thing is if you are operating transmitters that share a RF line with the SDR receiver, MFJ-1708SDR is a great item to use. It grounds the receiver antenna line when the Transmitter in in use, saving you receiver from damage.
Last edited by harley040 on Thu Jan 01, 1970 12:00 am, edited 0 times in total.
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