More adventures in filtering the power supply for an AFE-822 SDR

I frequency monitor and record the 285-325 kHz DGPS band, looking for DX beacons. Recently, I noticed a noise source centered around 315 kHz, almost 10 kHz wide, on my AFE 822 SDR with a 500 ft beverage antenna:

I tried hunting around the house with a portable radio, looking for it, but could never find it. I then checked on my netSDR, with a 670 ft sky loop antenna, and it was not visible there. Very curious. I then tried the beverage antenna, and could still not observe it. But it was there with the AFE822, with either antenna. This made me suspect noise was entering the AFE-822 through the power supply. I was use the USB input for power, and previously wrote about my attempts to reduce the noise from the power supply. This noise source was new since then, possible due to something else added to the shack.

I decided to put together a filtered DC power supply, using linear wall transformer, and adding filtering via capacitors and an inductor.

The circuit itself is fairly simple:

The output of the transformer I used is about 10 volts under load. I chose a 5 ohm power resistor to place in series, which dropped 2.5 volts, so the resulting DC power supplied to the AFE 822 is 7.5 volts. The value of this resistor depends on the output voltage from the DC supply. The AFE-822 draws 0.5 amps, Ohms Law can be used to calculate the desired resistance. The AFE822 has a voltage regulator inside it (it appears to be an LM7805 variant, possibly low drop out), so it can tolerate a wide range, the AFE 822 website specifies 7 to 10 volts.

The inductor is from the junk box, I don’t know what the value is. While I’m telling myself it helps to filter, I might try to find a known, larger value. The 1000 uF electrolytic capacitors provide low frequency filtering, the 0.047 uF ceramic caps provide RF filtering.

The filter circuit was constructed dead bug style on the lid of a small metal can:

Here it is mounted on the can:

And now the spectrum, with the new power supply. Certainly an improvement:

Yet Another !&*%$! Noise Source

The past few days, I have noticed higher than usual noise levels, generally on the lower frequencies, and particularly on the longwave band, including the 285-325 kHz DGPS band, where I run nightly SDR recordings, to later process the data and decode and detect DX DGPS stations using my Amalgamated DGPS app.

Thinking back to what new electronics devices have been added to the house, two came to mind, a new cable modem, and a new ethernet switch. The switch is up here in the shack, so it seemed to be a likely candidate. The switch is a D-Link DES-1008E 8-Port 10/100 Unmanaged Desktop Switch. It uses a mini USB port for power, using either the included AC adapter, or power from a USB port. When I installed it, I decided to not use the AC adapter, but rather a USB port on my UPS, figuring it was better to not add yet another potentially noisy switching power supply to the mix.

The test was easy, I just unplugged the power to the switch. Sure enough, the noise vanished. Great, the switch is a RFI generator. Or is it? As another test, I plugged it into a port on a USB hub. No noise. Hmm… so it seems that the noise is indeed from the USB port on the UPS. I did not notice any increase in the noise floor when I got the UPS a few months ago, but It’s something I should look into again, just to be sure. The UPS is a CyberPower CP1350PFCLCD.

Here’s a waterfall from the SDR, showing the DGPS band, 280-330 kHz. You can see where I changed the power to the switch from the UPS USB port to the USB hub, the bottom part of the waterfall is when the switch was still powered by the UPS (click to enlarge it):

I still have a noise source just above 305 kHz to hunt down.


I decided to see what I could do to improve things, and reduce the noise floor.

Here is the baseline, after no longer powering the switch from the UPS:

First, I relocated the AFE822 away from the computer and rats nest of assorted cables behind it, powered from an HTC USB charger:

The squiggly noise around 305 kHz vanished!

I then switched to an Apple USB charger / power supply, as their products tend to be a bit better made:

Another improvement, the overall noise floor is a bit less now.

But can we do better? I then switched to an older USB hub for power to the AFE822, that I thought might be better filtered:

I then changed to a linear supply plugged directly into the AFE822. I don’t notice any obvious improvement? Maybe it even looks like a little more noise? Difficult to tell. You can see a DGPS station popped up on 304 kHz while I was switching things around, between the last two tests, it was likely Mequon, WI.

Spying on your neighbor’s grill thermometer – Monitoring the 433.92 MHz ISM Band with an RTL Dongle

Remote weather stations, some car key fobs (although many in the US use 315 MHz), wireless grill thermometers, and many other devices use the 433.92 MHz ISM (Industrial, Scientific and Medical) band. Chances are good that if it is a wireless sensor, it uses this band.

Here is a waterfall showing transmissions observed here, using one of the inexpensive USB RTL DVB-TV Dongles:

The entire waterfall occupies 139 seconds.

You can observe several periodic transmissions. I have a remote weather station and a remote thermometer, so that accounts for two of them.

If you have an RTL tuner dongle, take a look and see what 433 MHz transmissions are occurring near you.

Chinese Firedrake Jammer

Firedrake is the unofficial name of a shortwave broadcast featuring loud oriental orchestral music. It exists solely to jam other signals, such as broadcasts by Sound of Hope, which broadcasts programming from Taiwan that is often critical of Chinese government policies and human rights abuse. The broadcasts only contain music; no form of on-air identification has ever been reported. Apparently, the source of the Firedrake shortwave transmissions is a China National Radio satellite feed.

While international regulations prohibit jamming, this has never stopped China (or the Soviet Union, Cuba, Iran, etc) from doing so. Rather than use a traditional jamming sound, apparently China believes that transmitting music 24 hours a day on dozens of frequencies doesn’t qualify as jamming. Or that no one will notice.

Here is a recording of Firedrake from March 29, 2012 on 14970 kHz.

Below is a relatively current list of known Firedrake transmissions:

Frequency     UTC Time
6280          2200-2400
7105          2200-2300
7280          1100-1300
7310          1300-1400
7310          2300-2400
7525          2300-2400
7565          2200-2400
7615          2200-2400
7970          0000-2400
9200          0000-2400
9450          1400-1600
9540          0900-1100
9635          2200-2300
10300         0000-2400
10965         0000-2400
10970         0000-2400
11500         0000-2400
11550         1200-1300
11760         0900-1100
11820         1330-1400
11980         2000-1700
12130         1500-1630
12160         1130-1200
12175         1300-1330
12175         1600-1700
12230         0000-2400
12300         0000-2400
12600         0000-2400
12670         0000-2400
12980         0000-2400
13060         0000-2400
13130         0000-2400
13270         0000-2400
13500         0000-2400
13850         0000-2400
13920         0000-2400
13970         0000-2400
14400         0000-2400
14700         0000-2400
14900         0000-2400
14970         0000-2400
15070         0000-2400
15500         0000-2400
15745         1230-1300
15750         1300-1330
15750         1400-1500
15800         0000-2400
15900         0000-2400
15970         0000-2400
16100         0000-2400
16700         0000-2400
16980         0000-2400
17100         0000-2400
17250         0000-2400
17450         0000-2400
17560         1400-1430
17920         0000-2400
18180         0000-2400

How Wide Can You Go (And Does the FCC Let You Spew QRM Over HF)

Here’s a waterfall I just made at 2155 UTC today, March 28, 2012, of WWCR Nashville TV on 6875 kHz, as captured by my netSDR running SdrDx software:


The sidebands extend all the way out to 6850 and 6900 kHz. That’s +/- 25 kHz wide. I inserted up to 30 dB of attenuation on the input signal, and the wide sidebands didn’t go away, so I don’t think this is an overloading issue.

Does the FCC have limits on the channel width SWBC stations can occupy? Is this really necessary?


Here’s a waterfall from 2327 UTC, showing both WWCR on 6875 and WYFR on 6915. Both are of similar signal strength, but only WWCR shows the very wide signal. Double click on the image to open it full size:


Over modulation?

FWIW, you can see that with both of these stations on, there isn’t a lot of space left for pirates on 43 meters. At 6925, you run into possible interference from WYFR on 6915. WWCR takes out at least 50 kHz, from 6850 to 6900. There’s several UTEs scattered around as well.

An Afternoon in the 1230 kHz Graveyard

Below is a waterfall of 1230 kHz, captured with the netSDR. The recording starts just before 1700 UTC (at the bottom of the image) and runs until about 0030 UTC (top of the image), click on the image to expand it:

The total frequency width of the graph is 100 Hz, that is it extends +/-50 Hz from 1230 kHz. Now that I have the Rubidium Reference on the netSDR, I don’t have an issue with the radio itself drifting over time.

1230 kHz is a “graveyard” medium wave frequency in the US. There are six graveyard channels, 1230, 1240, 1340, 1400, 1450, and 1490 kHz. These channels were set aside as local channels by the North American Radio Broadcasting Agreement, which went into effect in 1941. The term graveyard comes from the weird mix of sounds often heard at night, as dozens of stations mix together. Graveyard stations are restricted to 1000 watts maximum, and some use well under that at night, sometimes under 100 watts.

As you can see by the graph, even at 1700 UTC (local noon) there are dozens of carriers present. Locally I have WRBS 33 miles away and WKBO 40 miles away. Within around 100 miles, there’s quite a few stations.

As it gets later and the D layer starts to go away, new stations appear, and the existing stations get stronger. At about 2200 UTC (5 PM local time) the background noise becomes more obvious as well.

Two of new carriers have an interesting sawtooth pattern to the carrier frequency.

The FCC requires a +/-20 Hz frequency accuracy for medium wave broadcast station carriers. It looks as though most if not all of the stations maintain that, it is impossible to say for sure what some of the outliers are, they could be MW stations or they could be a semi local QRM.

What’s All This SDR Stuff, Anyhow?

The Software Defined Radio (SDR) has become very popular in the radio hobby scene over the last few years. Many hobbyists own one, certainly most have heard of them. But what is an SDR, and why might you want one, over a traditional radio?

First, a very brief explanation of how the traditional superhetrodyne radio works. This is the type of radio you have, if you don’t have an SDR (and you don’t have a crystal radio).

Here’s a block diagram of a typical superhetrodyne receiver:

superhetrodyne block diagram

The antenna is connected to a RF amplifier, which amplifies the very weak signals picked up by the antenna. Some high end radios put bandpass filters between the antenna and RF amplifier, to block strong out of band signals which could cause mixing products and images.

Next, the signals are passed to a mixer, which also gets fed a single frequency from the local oscillator. A mixer is a non linear device that causes sum and difference frequencies to be produced. I won’t go into the theory of exactly how it works. The local oscillator frequency is controlled by the tuning knob on the radio. It is offset by a fixed amount from the displayed frequency. That amount is called the IF frequency. For example, the IF of an radio may be 455 kHz. Time for an example…

Say you’re tuned to 6925 kHz. The local oscillator generates a frequency of 6470 kHz, which is 455 kHz below 6925 kHz. The mixer mixes the 6470 kHz signal with the incoming RF from the antenna. So the RF from a station transmitting on 6925 kHz gets mixed with 6470 kHz, producing a sum (6925+6470=13395 kHz) and difference (6925-6470=455 kHz) signal. The IF Filter after the mixer only passes frequencies around 455 kHz, it blocks others. So only the difference frequencies of interest, from the 6925 kHz station, get paseed. This signal is then amplified again, fed to a demodulator to convert the RF into audio frequencies, and fed to an audio amplifier, and then the speaker. The IF filter is what sets the selectivity of the radio, the bandwidth. Some radios have multiple IF filters that can be switched in, say for wide audio (maybe 6 Khz), and narrow (maybe 2.7 kHz). Perhaps even a very narrow (500 Hz) filter for CW.

This is a very basic example. Most higher end HF radios actually have several IF stages, with two or three being most common. The Icom R-71A, a fairly high end radio for its time (the 1980s) had four IF stages. Additional IF stages allow for better filtering of the signal, since it is not possible to build real physical filters with arbitrary capabilities. There’s a limit to how much filtering you can do at each stage.

Now, onto the SDR. I’ll be describing a Direct Digital Sampling (DDS) style SDR. The other style is the Quadrature Sampling Detector (QSD), such as the “SoftRock” SDR. The QSD SDR typically mixes the incoming RF to baseband, where it is then fed to the computer via a sound card interface for processing. The main advantage of the QSD SDR is price, it is a lot cheaper due to fewer components. The sacrifice is performance and features. You can’t get more than about 192 kHz bandwidth with a sound card, and you suffer from signal degradation caused by the sound card hardware. Some try to compensate for this by buying high end sound card interfaces, but at that point you’re approaching the price point of a DDS SDR in total hardware cost anyway.

Here is a block diagram of the SDR-IQ, courtesy of RF Space, you can click on it to see an enlarged image.
sdr-iq block diagram

The RF input (from the antenna) goes in at the left end, much of the front end is the same as a traditional radio. There’s an attenuator, protection against transients/static, and switchable bandpass filters and an amplifier. Finally the RF is fed into an A/D converter clocked at 66.666 MHz. An A/D (Analog to Digital) Converter is a device that continuously measures a voltage, and sends those readings to software for processing. Think of it as a voltmeter. The RF signals are lots of sine waves, all jumbled together. At a very fast rate, over 66 million times per second in this case, the A/D converter is measuring the voltage on the antenna. You’ve got similar A/D converters on the sound card input to your computer. The difference is that a sound card samples at a much lower rate, typically 44.1 kHz. So the A/D in an SDR is sampling about a thousand times faster. It is not too much of a stretch to say that the front end of an SDR is very similar to sticking an antenna into your sound card input. In fact, for many years now, longwave radio enthusiasts have used sound cards, especially those that can sample at higher rates such as 192 kHz, as SDRs, for monitoring VLF signals.

The output of the A/D converter, which at this point is not RF but rather a sequence of voltage readings, is fed to the AD6620, which is where the actual DSP (Digital Signal Processing) is done. The AD6620 is a dedicated chip for this purpose. Other SDRs, such as the netSDR, use a device called a FPGA (Field Programmable Gate Array), which, as the name implies, can be programmed for different uses. It has a huge number of digital logic gates, flip flops, and other devices, which can be interconnected as required. You just need to download new programming instructions. The AD6620 or FPGA does the part of the “software” part of the SDR, the other part being done in your computer.

The DSP portion of the SDR (which is software) does the mixing, filtering, and demodulation that is done in analog hardware in a traditional radio. If you looked at a block diagram of the DSP functions, they would be basically the same as in a traditional radio. The big advantage is that you can change the various parameters on the fly, such as IF filter width and shape, AGC constants, etc. Automatic notch filters become possible, identifying and rejecting interference. You can also realize tight filters that are essentially impossible with actual hardware. With analog circuitry, you introduce noise, distortion, and signal loss with each successive stage. With DSP, once you’ve digitized your input signal, you can perform as many operations as you wish, and they are all “perfect”. You’re only limited by the processing power of your DSP hardware.

Since it is not possible feed a 66 MHz sampled signal into a computer (and the computer may not have the processing power to handle it), the SDR software filters out a portion of the 0-30 MHz that is picked up by the A/D by mixing and filtering, and sends a reduced bandwidth signal to the computer. Often this is in the 50 to 200 kHz range, although more recent SDRs allow wider bandwidths. The netSDR, for example, supports a 1.6 MHz bandwidth.

With a 200 kHz bandwidth, the SDR could send sampled RF to the computer representing 6800 to 7000 kHz. Then additional DSP software in the computer can further process this information, filtering out and demodulating one particular radio station. Some software allows multiple stations to be demodulated at the same time. For example, the Spectravue software by RF Space allows two frequencies to be demodulated at the same time, one fed to the left channel of the sound card, and one to the right. So you could listen to 6925 and 6955 kHz at the same time.

Another obvious benefit of an SDR is that you can view a real time waterfall display of an entire band. Below is a waterfall of 43 meters at 2200 UTC (click on it to enlarge):
43 meter band waterfall

You can see all of the stations operating at one glance. If a station goes on the air, you can spot it within seconds.

Finally, an SDR allows you to record the sampled RF to disk files. You can then play it back. Rather than just recording a single frequency, as you can with a traditional radio, you can record an entire band. You can then go back and demodulate any signals you wish to. I’ll often record 6800 to 7000 kHz overnight, then go back to look for any broadcasts of interest.

For brevity, I avoided going into the details of exactly how the DSP software works, that may be the topic of a future post.

And yes, I borrowed the “What’s All this… Stuff, Anyhow” title from the late great Bob Pease, an engineer at National Semiconductor, who wrote a fabulous series of columns under that title at EDN magazine for many years.

A Day In The Life of 1470 kHz

This waterfall was recorded from 1840 UTC January 3, 2011 to 1840 UTC January 4, 2011. The beginning of the recording is at the top of the image. It shows the carriers of MW radio stations on 1470 kHz. The width is 100 Hz, so frequencies +/-50 Hz from 1470 kHz are shown.

Click on it to open it full sized.

1470 kHz Waterfall

The red line you see in the center is the carrier of semi-local station WTTR. During the daytime it is the only audible station on 1470, although you can see that carriers for about 8 other stations are always present.

I’ve annotated several events with UTC times.

At about 2213 UTC you can see a sudden reduction in received signal strength, and the start of a drift in frequency. This is most likely due to a station switching to nighttime power levels. The change in transmitter power causes a change in temperature inside the transmitter, causing drift in the frequency. My suspicion is that it is WLOA from Farrel, PA, since they are supposed to switch at 2215 UTC.

At about 2252 UTC a carrier suddenly disappears. It is possible that this is KMAL from Malden, MO. They are supposed to shut down at 2300 UTC.

Note that these are hunches of mine, I am not 100% certain that these are the identified stations. These events do suggest that it may be possible to identify and DX stations based on carrier transients, if the actual times that the stations make the changes are known.

At 0400 UTC a carrier suddenly disappears. I am not sure who this could be. This would be 11 PM EST. The station started to fade in around 2300 UTC (6 PM EST). That suggests to me a station in the central US. I’m not sure why they would be shutting down at this late time of the evening, vs around sunset. Perhaps some more experienced MW DXers have some theories / candidates?

There’s also some transients in the morning.

First at around 1200 UTC (7 AM local time) you can see the received signal strengths of the distant carriers decrease. The Sun is rising, and the D Layer is forming again, attenuating MW skywave signals.

At 1230 UTC one of the carriers suddenly disappears. At 1251 a carrier appears.

At 1326, it looks like a a transmitter is changing power levels.

At 1406 UTC there is another transient on another carrier.

There’s also noticeable differences in the frequency regulation of transmitter carriers. Several of the carriers have a periodic cycling to the frequency. I thought is this is due to some temperature cycling in the transmitter.

The 1610 Zoo

Below is a waterfall centered around 1610 kHz in the extended MW (AM) Band, 100 Hz Wide (click on it to see an enlarged image): 1610 waterfall

1610 kHz is used by only two broadcast stations, both in Canada: CJWI in Montreal, and CHHA in Toronto. It is also, however, used by many TIS (Travelers Information Stations) stations, which broadcast traffic reports, weather, etc. These are low power stations, typically in the 10 watt range.

Locally, the dominant station on 1610 kHz is a TIS that relays NOAA weather transmissions, and is located somewhere in south central PA. I’ve never heard an ID.

This recording was made between about 2100 and 1700 UTC, you can see the increase in signal (and background) levels overnight, and then the weakening of the signals as the Sun rises and the D Layer reforms, attenuating distant stations. Looking at the waterfall, you can see dozens of carriers, each a different radio station. It’s interesting to note how many radio station signals are present, even during the daytime.

The horizontal lines are due to static bursts, and there’s some changes in the signal level due to the wind blowing around the antenna.

Some of the wandering of carriers you see that is in unison is due to drift of the A/D clock in the SDR. Other drift you see is due to the carriers themselves. Note that each major division at the top of the waterfall is only 10 Hz (and the entire width is 100 Hz) so there’s really only a few Hz total drift. Eventually I’ll get a more stable reference clock for the SDR, and receiver drift should go away.

Let’s have a contest. How many carriers can you count?