#1 2015-05-26 01:03:40

avisnofsky
Member
Registered: 2015-05-26
Posts: 11

OK for 1uF cap to be polarized?

I want to build one of these, however the only 1uF cap that I can find in my junk drawer is a tantalum polarized cap. Can I use this as a substitute, and if so, which way should I install it. Thanks in advance, and sorry if this is a stupid question. smile

-Andy

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#2 2015-05-26 02:30:31

avisnofsky
Member
Registered: 2015-05-26
Posts: 11

Re: OK for 1uF cap to be polarized?

I also managed to find an electrolytic cap. Maybe this would work better, still polarized though.

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#3 2015-05-26 09:10:02

markqvist
Administrator
Registered: 2014-12-01
Posts: 112

Re: OK for 1uF cap to be polarized?

Hi Andy!
I think you will need a non-polarized capacitor there, since it is there to act on an incoming AC waveform. If you use a polarized one, it will have a high leakage current when the incoming signals polarity is reversed in regard to how it is installed, and I think you won't get the clean DC-biased AC waveform on the other side of the cap!

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#4 2015-05-26 09:11:38

markqvist
Administrator
Registered: 2014-12-01
Posts: 112

Re: OK for 1uF cap to be polarized?

It shouldn't be hard to find one though, any electronics shop will be able to supply you one, and they are also quite common to salvage if you have some old junk boards lying around smile

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#5 2015-05-26 17:15:57

avisnofsky
Member
Registered: 2015-05-26
Posts: 11

Re: OK for 1uF cap to be polarized?

Thanks for your quick reply. After some more searching through my parts, I did find a big old 1uF metal film cap. I'm going to give that a try. Also, I figured that I may be able to get away with a .1uF cap in place, since the value isn't extremely critical, right? Anyway, thanks again. I will let everyone know once I have the circuit built and have started testing.

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#6 2015-05-26 20:57:14

markqvist
Administrator
Registered: 2014-12-01
Posts: 112

Re: OK for 1uF cap to be polarized?

Yes, you should be able to get away with using another value. If you have a scope, hook it up to the point after the cap, and see how an incoming signal looks. It should be a nice representation of the original signal, centered around 2.5V (or 1.65V if you're using a 3.3V ADC reference). Let us know how it goes!

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#7 2015-05-26 22:44:05

avisnofsky
Member
Registered: 2015-05-26
Posts: 11

Re: OK for 1uF cap to be polarized?

OK, so I've built the device. I am running into a couple issues though. The first is when I tell the micromodem to send any data, the arduino resets. I first thought that maybe putting a 22uF cap across 5v would help, but no dice. Could this have something to do with the mosfet that I'm using? I didn't have a 2n7000, so I had to use a IFRZ44 (big to-220 power mosfet). I have triple checked the circuit (built on breadboard so far), and everything checks out.
My second issue/question is this: I am in an area with very little aprs traffic (i do hear some every day though, just very intermittently), so while setting everything up to 'just run' for a day to hopefully hear something, I realized that I wasn't sure if I should be running open-squelch? I am using the kiss firmware with a copy of ui-view32.
Any input is very much appreciated. Thanks in advance. smile

-Andy
KC3BHG

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#8 2015-05-28 17:07:00

markqvist
Administrator
Registered: 2014-12-01
Posts: 112

Re: OK for 1uF cap to be polarized?

You can run open squelch if you want to. Just set the "OPEN_SQUELCH" option to "true" in "device.h". Also make sure you have compiled it with the correct ADV reference voltage for your build smile

Strange that it is resetting though.. Have you tried removing the MOSFET from the circuit and see if that helps?

If you just want to test if decoding works, download a copy of the TNC Test CD from this page: http://wa8lmf.net/TNCtest/
And feed the audio from your computer to the modem. When you're sure decoding works OK, try connecting to a radio!

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#9 2015-05-28 22:48:49

avisnofsky
Member
Registered: 2015-05-26
Posts: 11

Re: OK for 1uF cap to be polarized?

So, after some more testing, I have an update. smile First off, I discovered that I by adding a 1k resistor between d3 and the mosfet, and also increasing the distance between the antenna and micromodem, I was able to completely eliminate the reset issue. Secondly, I was able to successfully decode ~100% of the packets on the tnc test cd. Unfortunately, I have not been able to decode any on air yet. I believe that the issue is that none of the packets i receive provide full quieting on the radio. I have tried adjusting the volume, but I live in a rural area with relatively little aprs traffic. Sometime I have to wait for a full hour before hearing another. I plan on buying another radio (probably baofeng, which i already own one of) and building another micromodem.
So far, I am really enjoying this little gem. smile I am going to borrow my fathers o-scope this weekend so I can take a look at both the input and output.

Also, this may be a simple question, but are there instructions somewhere on how to compile this on windows? I do not currently have access to a linux machine. Thanks in advance!

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#10 2015-05-28 22:58:46

avisnofsky
Member
Registered: 2015-05-26
Posts: 11

Re: OK for 1uF cap to be polarized?

Actually, build instructions for linux would work too. I still have my desktop dual booting. Sorry for the beginner request. I've only ever used the arduino software. sad

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#11 2015-05-29 01:33:58

avisnofsky
Member
Registered: 2015-05-26
Posts: 11

Re: OK for 1uF cap to be polarized?

aggh....i give up. i managed to build the kiss version, 3.3v, open squelch....but no matter what i do/try, the serial version will not compile.

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#12 2015-05-31 18:42:48

Stanley
Member
From: Kuala Lumpur, Malaysia
Registered: 2014-12-01
Posts: 65
Website

Re: OK for 1uF cap to be polarized?

markqvist wrote:

Hi Andy!
I think you will need a non-polarized capacitor there, since it is there to act on an incoming AC waveform. If you use a polarized one, it will have a high leakage current when the incoming signals polarity is reversed in regard to how it is installed, and I think you won't get the clean DC-biased AC waveform on the other side of the cap!

Dear Mark,

I was using polarized (electrolyte) caps all these while at the audio input ...

How would this affect the packet decoding ... ??

Thanks


Stanley
9W2SVT/ N5SVT
http://9W2SVT.blogspot.com

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