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Thursday, October 1, 2015

Bryan KV4ZS's "Let's Build Something" Direct Conversion Receiver



Bryan: 
 
I think it sounds great!  There is nothing really wrong with it -- that is what 40 meters sounds like!   Sure there is static.  And those whistles you hear near the top of the band are the carriers from shortwave broadcast stations.  You might have a little hum, but that should disappear once you get it all packaged in a metal box.   Congratulations Bryan!  You have done something that very few hams have done:  You have built a receiver. 73  Bill N2CQR
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Hi Bryan,
 
First let me congratulate you. That is one fine build and you may actually have absolutely nothing wrong!!!!!! I really must applaud your “squares”. They look like they were made on a CNC machine. Bravo!!!
 
You are operating the LBS without an RF amplifier and as such you are trying to make up the gain in the audio amp. I would say that the results you are hearing are very consistent with the DCR without an RF amp. Get the RF amps stage working and then run your test –you will find with the RF amp that at the gain setting you have for the video will be room filling. It actually sounds pretty good. You might also try connecting a 1 NF across the audio trimmer pot as that will cut down on the “hiss’ sound.
 
Concentrate on the RF amp stage and then re-run your test –you will see the difference.
 
Great build – very nice job.
 
73’s
Pete N6QW
 
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Very nice!
 
Sounds pretty good to me in terms of noise – that’s what a direct conversion receiver sounds like (they tend to be very wide in terms of reception – static is normal... Welcome the world without noise reduction and DSP!!).  DCR’s – because they are not run through a narrow IF filter – allow a very broad range of signals to get to the audio stage. So, for example, if you tune that around during a CW contest, you’ll hear a LOT of signals at the same time – versus only one or two at a time, once you have this run through the 4.9152 crystal filter. That’s the nature of the beast.
 
The 1nf across the audio trimmer definitely will help with reducing the hiss, although I must say my Kenwood receivers all have a similar amount of hiss and I prefer my radios with more, not less, noise (it lets me know what the band conditions are like...). I have noticed on my builds, however, that if you have a very, very high pitch WHINE on the other hand, that tends to be a bad solder joint or bad capacitor somewhere – probably on a capacitor – introducing an offset into your RF someplace it shouldn’t. What that looks like on an oscilloscope is the audio signal will have a large DC offset versus ground – almost always a bad solder joint on a capacitor—or a bad/broken capacitor--somewhere in the audio amplifier. That’s the same problem you get when you try to record audio sometimes from an external source (TV, radio, CD player) on your computer – DC voltage offset on the audio line. Kind of like what you might have heard on a stereo if you ever tried to switch to a channel where the input was hanging open.
 
Ben
KK6FUT

4 comments:

  1. Looks and sounds good. As Bill mentioned, the static is normal for the 40M band, don't try to fix something that isn't broken. Don't get discouraged either, I have been working on my transceiver for a while. Transmitter isn't quite right yet, but the most important part is to have fun and learn.73's Dean AC9JQ

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  2. Nice work Brian, that is a lot neater than mine.

    I had quite a lot of very high pitched noise that sounds just like yours, but I found if I drop the audio gain a little bit with the trimpot it pretty much vanishes.

    It took me quite a while to get the balance right between getting enough audio volume and having the very high pitched whine present, which made it tiring to listen to.

    Based on Ben's comments I'm going to take another look at the soldering and caps on my audio amplifier board.

    73s
    Greg VK1VXG

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  3. Oh, if your interested there is a useful ap for android phones/tablets called Frequensee. It will give you a nice audio spectrum analyser. Right around the 3 minute mark in the video you can see a little peak right around 5Khz, and a smaller one up around 10Khz.
    Its interesting they appear to be a fairly constant tone, although the level does vary.
    73s again
    Greg VK1VXG

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  4. good job Brian....congrats ,9a3xz,Mikele.

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