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[POTM] dAISy - A Simple AIS Receiver

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Hi,

 

Extract from the lna4all site:

 

I bought the filter, should I put it before the LNA ?

 

 

Inserting the filter before the LNA (between the antenna and the LNA) will spoil the noise figure roughly for the filter insertion losses expressed in dB. So the S/N will also be degraded for the same figure.

 

 

I bought the filter, should I put it after the LNA ?

 

 

Inserting the filter after the LNA (between the LNA and the dongle) will leave the LNA widely opened to the strong signals. This may be fine if you have the high IP3 LNA.

 

 

OK, where should I insert the filter then ?

 

 

If you aim for the weak signals reception and you have the high IP3 LNA you need to install the filter after the LNA. If your LNA can not stand the strong blockers (broadcast TV, Radio, Cell towers) you need to protect it with the filter, otherwise the LNA may produce reach intermodulation products that may be spread all over the frequencies masking the other signals on the band. More over the result may be a bunch of phantom signals that are not existing on the bands.

 

Hope it helps :-)

 

Enviado desde mi SGP321 mediante Tapatalk

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So in short: Put the amplifier before the filter, unless there are powerful local transmitters.

 

An external bandpass could be easily added after the fact if needed. On the other hand, filter before amplifier offers more robustness out the box.

 

Decisions, decisions...

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Turns out that most, if not all of the improvement is due to the bandpass filter.

 

I built up a second board without the amplifier, and performance in a field test was about the same. Without amplifier it was able to receive messages down to -100dBm. With amplifier, which adds about 23dBm gain, that number was around -75dBm.

 

My interpretation of these numbers is, that the radio is capable of decoding messages all the way down to the noise present after the bandpass filter. Any amplification should happen before the bandpass filter.

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So definitely mmic before filter will improve reception of weak signals that the filter attenuate below the threshold ... that will be great for dxing ships and greater range as filter reduced my coverage, but received more small ships (with less tx power?) near my location.

 

gmtii.

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The filter before or after is unfortunately a catch 22. You are building up a cascades noise figure for your receiver. As a general rule of thumb the first element will contribute the most to you noise figure so in this case with the Bp having 5 dB of insertion loss, your noise figure cannot be better than 5 dB. With amplifier before band pass, the amp noise figure will really dominate.

 

The problem as you mentioned is linearity of the amplifier. If there is a strong signal exceeding its 1 dB compression point. It will start to distort the signal. This can mostly be rejected by the band pass except for the 3rd order inter modulation components which will be in band.

 

In any case, I think that in most cases, you will see better performance with amplifier first

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You can look for a FBAR,BAW or SAW filter that will have better insertion loss.

 

Also if you don't care about power consumption, you want to look for a MMIC with as narrow of bandwidth that still be in band. Also a high iip3 or p1db will help linearity. Obviously noise figure is king.

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@@Lgbeno Thank you for the input.

 

My goal for dAISy is good performance for the lowest price possible, with small form factor a secondary goal.

 

I was looking for premade filters, but what I found so far is obsolete and/or more expensive than the rest of my BOM combined. If there are inexpensive parts they are for much higher frequencies than I need (cell phone and WiFi tech I guess).

 

I tested the Mini-Circuits SXBP-162+ ($16 in singles), a 4-element resonator bandpass filter. The good results of this test were the reason I added a similar bandpass circuit to dAISy.

 

When searching amplifiers I'm a bit overwhelmed by the selection. It doesn't help that I don't know what half the parameters in parametric search mean :)

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one thought, if you are using external filters , you can probably remove the onboard LC filter. it might add a few extra dB of loss and probably isn't really needed.

I'm going to tackle mine very soon, I did a complete equipment upgrade, new DSO, new PLL sig gen, new hot air work station, new laptop in preperation. Now I have all the bits I had ordered so it's time to play.

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These module are expensive but for those that don't want to replace SMD components in the filter they may be a good option

http://www.icstation.com/product_info.php?products_id=4624#.VJT4xbALs

Except it uses 100kHz steps?

I don't think this module is of any use for AIS without modification, if at all.

 

Tuning into the right frequencies is only tge first step. The chip/module also needs to support GMSK modulation.

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Yes, I was looking at si4463 modules with an onboard STM8 when I saw this module and really didn't look too hard. Just assuming since I was looking at Si4463s this was also one.

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