The Realtek 2832U

In my spare time (lol!) I have been fooling around with one of those RTL 2832U dongles and a bit of software.  For those that don’t know, the RTL 2832U is a COFDM demodulator chip designed to work with a USB dongle.  When coupled with an R 820T tuner a broadband RF receiver is created  There are many very inexpensive versions of these devices available on Amazon, eBay and other such places. The beauty of these things is that for around $12-30 and a bit of free software, one can have a very versatile 10 KHz to 1.7 GHz receiver.  There are several good software packages for Windoze, Linux and OSX.

The one I recommend for beginners is called SDR-Sharp or SDR#.  It has a very easy learning curve and there is lots of documentation available on line.  There are also several worth while plugins for scanning, trunking, decoding, etc.  At a minimum, the SDR software should have a spectrum analyzer, water fall display and ability to record audio and baseband PCM from the IF stage of the radio.

Some fun things to do; look at the output of my reverse registering smart (electric) meter (or my neighbor’s meter), ACARS data for the various aircraft flying overhead, a few trips through the EZPass toll lanes, some poking around on the VHF hi-band, etc.  I also began to think of Broadcast Engineering applications and a surprising number of things came to mind:

  • Using the scanner to look for open 950 MHz STL frequencies
  • Inexpensive portable FM receiver with RDS output for radio stations
  • Inexpensive Radio Direction Finder with a directional antenna
  • Inexpensive Satellite Aiming tool

Using SDR sharp and a NooElec NESDR Mini+ dongle, I made several scans of the 945-952 STL band in a few of our markets.  Using the scanner and frequency search plugin, the SDR software very quickly identified all of the in use frequencies.  One can also look at the frequency span in the spectrum analyzer, but this takes a lot of processing power.  The scanner plugin makes this easier and can be automated.

950 MHz STL frequencies, Albany, NY
Analog and digital 950 MHz STL frequencies, Albany, NY

I also listened to the analog STLs in FM Wideband mode.  Several stations are injecting their RDS data at the studio.  There is one that appears to be -1500 Hz off frequency.  I’ll let them know.

Next, I have found it beneficial just to keep the dongle and a small antenna in my laptop bag.  Setting up a new RDS subcarrier; with the dongle and SDR# one can quickly and easily check for errors.  Tracking down one of those nasty pirates; a laptop with a directional antenna will make quick work.

Something that I found interesting is the water fall display for the PPM encoded stations:

WPDH using RTL 2832U and SDR Sharp
WPDH using RTL 2832U and SDR Sharp

Not only can you see the water marking on the main channel, you can also see the HD Radio carriers +/- 200 KHz from the carrier frequency.  That is pretty much twice the bandwidth allotment for an FM station.

WDPA using RTL 2831U with SDR Sharp
WDPA using RTL 2831U and  SDR Sharp

Those two stations are simulcasting.  WPDA is not using Nielson PPM nor HD Radio technology.  There is all sorts of interesting information that can be gleaned from one of these units.

Aiming a satellite dish at AMC-8 can be a bit challenging.  That part of the sky is pretty crowed, as it turns out.  Dish pointer is a good general reference (www.dishpointer.com) and the Dish Align app for iOS works well.  But for peaking a dish, the RTL 2832 dongle makes it easy to find the correct satellite and optimize the transponder polarization.  Each satellite has Horizontal and Vertical beacons.  These vary slightly in frequency, thus, but tuning to the correct beacon frequency, you can be assured that you are on the right satellite.  All of the radio network programming on AMC-8 is on vertically polarized transponders, therefore,  the vertical beacons are of interest.  Here are the vertical beacons for satellites in that part of the sky:

Satellite Position C band Vertical beacon (MHz) L band (LNB) Vertcial beacon (MHz) Comment
AMC-8 139W 4199.5 949.25
AMC-7 137W 3700.5 1450.25
GOES15 135.4W 2209.086 N/A NOAA WX
AMC-10 135W 4199.5 949.25
Galaxy 15 133W 4198 949.00
AMC-11 131W 4199.5 949.25
Galaxy 12 129W 3700.5 1450.25

For those in the continental United States, there is not much else past 139W, so AMC-8 will be the western most satellite your dish can see.  Of course, this can be used in other parts of the world as well, with the correct information. Bringing a laptop or Windows tablet to the satellite dish might be easier than trying to drag a XDS satellite receiver out.

AMC8 vertical beacon output from LNB
AMC8 vertical beacon output from LNB

In order to use the RTL-2832U, simply split the output of a powered LNB, install a 20-30 dB pad in between the splitter and the dongle.  Using the vertical beacon on 949.25 MHz, adjust for maximum signal.

Some other uses; look for the nearest and best NOAA Weather radio station.  Several times the local NOAA weather station has been off the air for an extended period of time.  Sometimes, another station can be found in the same forecast area.  Heck, couple these things to a Raspberry Pi or Beaglebone black and a really nifty EAS receiver is created for NOAA and broadcast FM.  One that perhaps, can issue an alarm if the RSL drops below a certain threshold.

I am sure there are plenty of other uses that I am not thinking of right now…

Unbalanced to Balanced Audio

There is a large number of things that amazes me on an almost daily basis.  To wit: a local mom and pop radio station called me because they couldn’t get their computer program to work right.  I decided that I’d give them an hour or two, in exchange for my hourly labor rate, and see if I could fix their problem.  The issue at hand was loud hum and other noise on the input source.  I knew before I even looked at it that the likely culprit was a ground loop.

It was worse than I imagined, with several unbalanced and balanced feeds improperly interconnected, line level audio going to a microphone level input and so forth.  I explained to the guy about putting line level into a mic level input, something akin to plugging a 120 volt appliance into a 240 volt outlet.  Improperly terminated balanced audio nullifies all of the common mode noise rejection characteristics of the circuit.

In any case, there are several ways to go from balanced to unbalanced without too much difficulty.  The first way is to wire the shield and Lo together on the unbalanced connector.  This works well with older, transformer input/output gear, so long as the unbalanced cables are kept relatively short.

simple balanced to unbalanced audio connection
simple balanced to unbalanced audio connection

Most modern professional audio equipment has active balanced input/output interfaces, in which case the above circuit will unbalance the audio and decrease the CMRR (Common Mode Rejection Ratio), increasing the chance of noise, buzz and so on getting into the audio. In this case the CMRR is about 30 dB at 60 Hz.  Also, newer equipment with active balanced input/output, particularly some brands of sound cards will not like to have the Lo side grounded. In a few instances, this can actually damage the equipment.

Of course, one can go out and buy an Henry Match Box or something similar and be done with it.  I have found, however, the active components in such devices can sometimes fail, creating hum, distortion, buzz or no audio at all.  Well designed and manufactured passive components (transformers and resistors) will provide excellent performance with little chance of failure.  There several methods of using transformers to go from balanced to unbalanced or vice versa.

Balanced to unbalanced audio using 1:1 transformer
Balanced to unbalanced audio using 1:1 transformer

Using a 600:600 ohm transformer is the most common.  Unbalanced audio impedance of consumer grade electronics can vary anywhere from 270 to 470 ohms or more.  The 10,000 ohm resistor provides constant loading regardless of what the unbalanced impedance.   In this configuration, CMMR (Common-Mode Rejection Ratio) will be 55 dB at 60 Hz, but gradually decreases to about 30 dB for frequencies above 1 KHz.

Balanced to unbalanced audio using a 4:1 transformer
Balanced to unbalanced audio using a 4:1 transformer

A 600:10,000 ohm transformer will give better performance, as the CMMR will be 120 dB at 60 Hz and 80 dB at 3 KHz, remaining high across the entire audio bandwidth.   The line balancing will be far better into the high impedance load.  This circuit will have about 12dB attenuation, so plan accordingly.

For best results, use high quality transformers like Jensen, UTC, or even WE 111C (although they are huge) can be used.  I have found several places where these transformers can be “scrounged,” DATS cards on the old 7300 series Scientific Atlanta satellite receivers, old modules from PRE consoles, etc.  A simple audio “balun” can be constructed for little cost or effort and sound a whole lot better than doing it the wrong way.

A brief list, there are other types/manufactures that will work also:

Ratio Jensen Hammond UTC
1:1 (600:600) JT11E series 804, 560G A20, A21, A43
4:1 (10K:600) JT10K series 560N A35

Keep all unbalanced cable runs as short as possible.  In stereo circuits, phasing is critically important, so pay attention to how the transformer windings are connected.

Proper termination of long audio wire runs

This is standard telephone company stuff, however, it would seem that several radio engineers have forgotten this.  I was reading on one forum where an AM station was using 1000 feet of 12 gauge romex to send audio from the studio to the transmitter out back.  The owner was complaining that the audio sounded bad.

Longer wire runs need to be terminated with the characteristic impedance of the cable being used, normally 110 ohms or so for typical audio wire.  This is because impedance mismatches can cause return loss just like in an RF circuit.  Exactly what the effect of the mismatched impedance depends on the length and frequencies involved.  On shorter cable runs of less than 100 feet or so, this usually is not an issue.

The result of return loss is part of the audio energy gets reflected back to is origin (a standing wave), where it mixes with newer audio.  This can cause out of phase issues and usually the result is high tinny sounding audio with distortion in the mid range frequencies.  In other words, it ain’t pretty.  This can really become an issue with digital audio because of the higher bandwidth requirements for high sample rates.  It has always struck me as odd that AES/EBU audio uses XLR type connectors.  An XLR connector does not maintain the characteristic 110 ohm impedance of most digital cable and itself can cause pretty significant return loss. But anyway…

There are a number of options for proper termination:

1.  Transformers are often used to match the impedances of circuits. A transformer converts alternating current at one voltage to the same waveform at another voltage. The power input to the transformer and output from the transformer is the same (except for losses). The side with the lower voltage is at low impedance, because this has the lower number of turns, and the side with the higher voltage is at a higher impedance as it has more turns in its coil.  Western Electric 111C audio transformers were often used in equalized TELCO circuits sending audio over long distances on copper pairs.

WE 111 repeat coil, one of the best such transformers ever made
WE 111 repeat coil, one of the best such transformers ever made

2. Resistive network impedance matches such as H or T or L pads are the simplest to implement. They limit the power deliberately, and are used to transfer low-power signals, such as unamplified audio or radio frequency signals. Almost all digital circuits use resistive impedance matching which is usually built into the structure of the switching element.

H pad impedance matching network
H pad impedance matching network

3.  Active balanced converters using opamps with high input impedances (10 Kohm bridging resistance) that first greatly reduce the voltage, then amplify it are often used an audio circuits.  They have the advantage of active gain control and are often used in conjunction with gain reduction and limiting circuits.

Unbalanced to balanced audio converter
Unbalanced to balanced audio converter

The above diagram shows an active unbalanced to balanced audio converter.  The advantages of such a circuit are active gain controls can be added to set levels.  With additional feedback circuit elements, it can also be used for automatic gain control, gain reduction, limiting and so forth.

For most inter and intra studio wiring, professional audio equipment is designed for 0 dBm 600 ohm balanced audio (AKA line level audio).   Audio cable such as Belden 8451 or like multi-pair cables terminated on punch blocks or connectors works well.  Cable impedances and matching are generally not a design consideration.  Long cable runs, longer than 150 feet or so, do need to be terminated in a high quality audio installation.