Intro

• When you are using SDR# you are looking at a chunk of radio spectrum. Standard radios only have the ability to process one signal at a time, you now have the ability to not only hear, but also see the signals.

 Sample Rates

• Your sample rate is your bandwidth. If you only want to see 1MHz of spectrum, choose 1.0MSPS. Higher sample rates above 2.4MSPS are not recommended
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 Interference

• Interference from very strong signals will show up as mirrors that run in the opposite direction of the actual signal or only appear at the edge of the screen.
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• Interference from very strong signals will show up at intervals corresponding to your MSPS rate
  • If you have a signal at 100MHz, and your sample rate is 1MSPS
    • You will see a duplicated signal at 101MHz, 99MHz, 98MHz, and 102MHz.
  • If you have a signal at 100MHz, and your sample rate is 2MSPS
    • You will see a duplicated signal at 98MHz, 96MHz, 102MHz, and 104MHz.

 The two display methods of SDR#

• What is the FFT and Waterfall?
  • FFT: Top half of the screen
  • Waterfall: Bottom half of the screen

 How to enter frequencies into SDR#

• Frequency entry
  • Type it into the VFO
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  • Click on the bottom or top half of a number
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  • Drag the waterfall or FFT
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  • Use your scroll wheel

 Eliminating the center signal spike

• If you want to get rid of that DC spike in the center, enable Correct IQ
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 Decoding digital tips

• For decoding digital signals
  • max out the volume, use WFM, change frequency step to 6.25KHz, and change the bandwidth to fit perfectly around the signal’s envelope.
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  • Set your sample rate to .900000001 MSPS (I personally have had the best results with this)
  • Also uncheck the filter audio box, chances are, your application will want a clean signal coming from SDR# to decode digital data properly.

 Decreasing latency

• If you are decoding time sensitive information (Trunked radio decoding), turn down your audio latency so it is low enough to where the audio doesn’t pop from buffer underruns, but is fast enough so that the beginning of conversations aren’t cut off.
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 Keeping things organized

• If you are going to be using your SDR for multiple uses, like HF, VHF, and Digital. Duplicate your SDR# folder for each instance. This will save configuration time, so you won’t have to change modulation, volume, or Gain every time you want to listen to different modes.
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 When to use AM

• If you do not have an upconverter, there are only very few times you will use AM,SSB,USB,LSB,DSB,CW,or RAW. These include
  • Listening to Airband (120MHz)
  • Listening to CB Radio (26-27MHz)
  • 12 and 10meter ham bands (24-30MHz)
  • Low tech gadgets that run on 27 and 49MHz (RC Cars)
  • Remote controls that run on ~400MHz (Car door unlocks)
  • some low tech ISM devices 902-928MHz (home automation, weather sensors)
• Otherwise use NFM or WFM

 Common interference in the home

• If you use Wi-Fi or Ethernet in your home, you may see lots of interference in the 140-160MHz range, when browsing these ranges don’t use Tuner-AGC unless you have already locked onto a signal you want to listen to. If you are just browsing, keep the gain somewhat low to avoid overloading the device.

 AGC

• When to use AGC
  • Use AGC when you are aware that checking the box may lead to visible interference, only use AGC when you are sure the frequency you are on, is the only signal you want to listen to.

• Tuner-AGC seems to work better than RTL-AGC at picking signals out from noise

 Calibration

• RTL-Calibration
  • Generally RTL sticks need calibration. If you are only listening to FM Radio, calibration probably isn’t needed, but for narrow and more precise signals, it is required.
  • It is recommended to wait several minutes before calibrating, when the temperature of the dongle warms up, the frequency could shift 4+ppm.
  • Calibration is calculated in ppm (parts per million) AKA Hertz per MHz
  • so, if your correction is 50ppm, a signal at 1,000,000Hz(1MHz), will be 1,000,050Hz(1.00005MHz)
  • Remember that this scales the higher in frequency you go. So at 900MHz, the correction will not be 900,000,000/900,000,050
  • It will be 900,045,000. At 900MHz, a difference of 45 KHz is huge, especially for narrow signals like voice.
  • When calibrating your dongle, always opt for using a higher frequency reference point.
  • Calibrating to a local NWS station (162MHz) is good, but calibrating to a local GSM tower, or trunked radio control channel is much preferred.
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 Getting a clearer picture

• FFT Resolution
  • If you are listening to narrowband signals, it may be helpful to get a clearer view of the signals you are looking at.
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• When you first start using SDR# turn your contrast down to a level where:
  • no signal = blue
  • weak signal = light blue
  • med signal = orange
  • strong signal = red
  • Note, while gain controls how the signal is heard and viewed, contrast is only visual.
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Log recorder has been updated to version 3.0

What does log recorder do?

• Records each call from DSD for archival purposes
• Logs each call in a database YYYY/MM/DD/UNIXtimestamp TGID RID.mp3

Changelog

• rewritten in PHP
• rewritten in preparation of the abandonment of SDR# in favor of Unitrunker
• utilizes sdrsharptrunking.log instead of single.log, so SDR# doesn’t need to be run
• Multiplatform (needs php5 and sox for Linux support)

Download

I oftentimes see people asking why there is so much interference on their SDRs. Most visible interference is in fact not caused by real signals, but caused by the RTL dongle overloading.

Eliminating visible interference

The first to troubleshooting interference is turning off any type of AGC. AGC maxes out the gain slider which makes nearby strong signals to appear where they shouldn’t be.

37.2 dB is a good level

Turning on and off AGC, Manual Gain 37dB

Change your sample rate. Understanding the sample rate is quite easy. 0.25 MSPS will only show 250khz of spectrum on your waterfall/FFT screen.

It isn’t recommended to use 0.25 MSPS because this can cause interference on frequencies every 250KHz. Take for example, a frequency on 850MHz and 850.250MHz. If you set your sample rate to 0.25MSPS, and either the signal is too close, or your gain is too high. Both 850MHz and 850.25 will mix causing bad decodes. The same can be found up to 10 iterations of 250KHz past the original signal. To see this, find a strong signal, set your gain high. Center it on your waterfall/FFT display, and increment the frequency by 1MHz. You will see the same signal every 1MHz, slowly decreasing in strength as you increase the frequency. Now use 2.048MSPS, you will notice mirrored interference every 2.048MHz

250KHz interference, 929.6125 MHz signal

.9000001 MSPS is recommended for digital modes because it provides a small enough chunk of spectrum for visually inspecting the signal, and it doesn’t cause even overlaps. So instead of 849MHz, 849.5 849.75 interfering with 850MHz, interference will only occur every 900KHz, which has a lesser chance of overlapping interference.

How to eliminate real interference

Real interference is caused by any type of RFI near your SDR receiver. Simply moving the dongle and antenna away from the source of noise (computer, TV. AVR) will lower interference.

To go even further, you can add a ferrite core, or an RF shielded case around your SDR to block unwanted signals.

RTL-SDR Enclosure

A few months ago I wrote a simple script in batch that allowed me to record each call in MP3 format with the TGID, RID, date recorded, into a folder that contained the proper year/month/date.

It’s still a little buggy, there are much better ways of doing this.

But what I recently did was make a web front end to read the folders and files, then spit out a table with all the data.

Right now it’s not very efficient, but I plan on fixing that by migrating from, a loop that scans every file from a directory, to a database application. I’ll be using a program called Everything Search by Void Tools. I chose this because it has an extremely fast indexing engine and a command line interface.

This should speed up my page load times by a lot. With my current setup a query takes about 5 seconds, with a directory that has a month of calls, 26,000 files, and almost 1GB of file space taken. If I don’t make this change, page load times will only increase.

I hope to be done with this migration by the end of the week.