Configure your Realtek USB Device


Read this explanation of the different classes and roles of receivers for decoding and voice following.

If you have not yet calibrated your RTL stick, skip ahead to the Calibration section at the bottom of this page. After calibrating, come back here to resume your configuration.

  • Select the Receivers tab on the main program window.
  • Click "+".
  • Click "RTL2832U".
  • If the RTL2832U button is disabled, plug the Realtek into a USB port.

  • You should see a new receiver window. In this window ...
  • Verify the receiver type is "RTL2832U".
  • Select the RTL Device.
  • If no RTL devices appear, click the Refresh tool to update the list of devices.

  • Set the gain values - Baseband AGC and Tuner Gain / AGC. Use your favorite values from SDR# or try these: Baseband AGC on, Tuner Gain at 300 and Tuner AGC off.
  • If your processor has a clock speed of 1 Ghz or more, set the sample rate to 2.56 msps. For slower, older hardware, try 1.024 or 0.96 msps.
  • Click the Start tool button.
  • Verify the receiver's State is now "running".
  • If the recevier won't start:
    • make sure it isn't reserved by another program like SDR#.
    • Another suspect is incomplete device driver installation.
    • Run the Zadig driver utility to pair the Realtek USB stick with Microsoft's WinUSB driver.
    • Note that there is a special version of Zadig for Windows XP.
    • The tuner chip inside the Realtek USB stick must be a Raphael Microelectronics R820T or R820T2.
  • If your receiver starts, stop it and follow the instructions below for Voice and Signal VCOs.

You can edit the Model field. The default text is "R820T" but you can edit that to whatever you like. Useful if you have more than one RTL stick.


Unitrunker supports multiple VCOs for some SDRs. Each VCO tab is paired with a Scope tab to the immediate right. A VCO can tune to an separate RF channel. The acronym VCO stands for Voltage Controlled Oscillator - the part of a heterodyne receiver used to tune a specific frequency. An alternate name is VFO or Variable Frequency Oscillator. In SDRs like the Realtek, the only constraint is all VCOs must fit inside the usable bandwidth of the receiver. This is about 80 percent of the sample rate. At 2.5 msps, that's about 2.0 Mhz of usable bandwidth. You can set the number of VCOs to anything from 1 to 8.

Idle VCOs

Any VCOs surplus to your needs should be set to signal role and the park frequency set to 0.000 Mhz. This prevents the VCO from taking up compute and memory resources.

Signal VCO

  • Set the Role to Signal.
  • Set the Audio Output to your speakers or headphones.
  • Set the Digital Output to "Unspecified".
  • Click the Start tool to start the receiver.
  • Use the Pick Site tool to select a previously logged site.
  • For a fresh install with no logged systems, type a in control channel frequency as the Park frequency.
  • After having accumulated systems from your area, use the Pick Site mechanism to easily choose among them.
  • You should hear the control channel buzz over your speakers or headphones. You should also see an active waveform on the adjacent Scope tab.
  • Check the mute button to silence this noise.

Analog Voice VCO

  • Set the Role to Voice.
  • Set the Audio Output to your speakers or headphones.
  • Set the Digital Output to "Unspecified".
  • Check "Analog".
  • Check "Deemphasis".
  • Un-check "Mute".
  • Un-check "ProVoice" (in the Voice section).
  • Un-check "P25" (also in the Voice section).
  • Start the receiver. If it was running when you made these changes, re-start it.

Digital Voice VCO

The program does not provide decoding of digital coded voice channels. It can provide baseband audio suitable for other applications. Piping audio between applications requires a device driver that mimics a pair of audio devices - one input and one output. Two popular utilities are VB Cable and Virtual Audio Cable. The two popular digital voice programs are DSD and DSD+.

  • Set the Role to Voice.
  • Set the Audio Output to your speakers or headphones.
  • Set the Digital Output to the input of your virtual audio cable (eg. "VB Cable Input 1").
  • Remember this choice so the third party voice decoder can be configured to get its input from the virtual cable's output device.
  • Check the "Analog" check box.
  • Check "Deemphasis".
  • Un-check "Mute".
  • Check "ProVoice" (in the Voice section) if you monitor an EDACS system with digital voice calls.
  • Check "P25" (also in the Voice section) if you monitor a P25 system or a Motorola Astro (Motorola with digital voice) system.
  • In the Voice section (not Decode), check the digital audio formats relevant to you - most likely P25 or ProVoice.
  • Start the receiver. If it was running when you made these changes, re-start it.


For analog voice, set the squelch level so that the voice receiver becomes silent at the end of a transmission. It's also a good idea to do this for digital voice as the digital voice decoder won't mistake noise for a transmission.

Squelch Procedure

If you change the receiver's gain or sample rate, you will need to readjust squelch. The same squelch setting will generally work for all VCOs on the same receiver.

Usable Bandwidth

The RTL2832U samples RF at 1.0 or 2.5 msps. The usable bandwidth is about eighty percent of that sample bandwidth. The program tunes the Realtek to a center frequency (as displayed on the receiver window's Info tab). Individual VCOs tune to channels inside that usable bandwidth. If a VCO is directed to a frequency outside this usable bandwidth, the VCO is effectly muted. The program uses a sliding window to fit as many VCOs as possible. It is possible to monitor multiple sites provided the voice and control channels are relatively close together. For example, monitoring a 851 Mhz trunking site, a 935 Mhz trunking site, and 455 Mhz site won't work. You would need three or four Realteks.

In many cases, the voice and control channels for one or more sites all fit inside the Realtek's usable bandwidth. However, many sites have voice and control channels that are more than 2.0 Mhz apart. The good news is Realtek devices are very inexpenside. The most flexible configuratiois one running separate Realtek USB sticks for voice and control channel as each device can be independently tuned. When running a Realtek device where only one VCO is used, set the sample rate to 1 msps (not 2.5 msps) to reduce computation demands on the host PC.

Calibration Procedure (First Time)

The program does not provide a way to calibrate the Realteks. You will need to run an SDR program like HDSDR or SDR# - something with a waterfall display.

Copy this final value into the Realtek's receiver window in Unitrunker.

Calibration Procedure (Fine Tune)

Drift Correct

The calibration procedures above describe how to set the correction value to compensate for the inaccurate timing reference built into these low cost devices. However, even with this correction applied, the timing reference can drift over time. The VCO's window value can display the magnitude of this drift.

The Drift Correct option allows the program to apply any measured tuning error from the first VCO towards adjusting the receiver's correction value. The initial correction value and the program's applied value are displayed. The program can make adjustments as small as 0.0625 parts per million (yup, that's 62.5 parts per BILLION). At start up, the window value will likely be large. The program will adjust the applied correction value. After a few seconds, the window value will reduce to some value close to zero. As the device heats up, or as room temperature fluctuates through the day, the window value will increase. Once the error exceeds a few hundred hertz, the program will adjust the applied correction value. Note that attempting finer adjustments is pointless since the fractional-N PLL inside the R820T can only resolve to the nearest 200 hertz.

Bear in mind that this correction value is for the whole receiver. All VCOs will be affected. For best performance, run the first VCO in signal role on a strong control channel.

The drift correct feature cannot function without a working squelch setting. When the signal drops, you don't want random noise driving the applied correction value.

© Copyright 2014-2015 Rick Parrish