GPS Disciplined 10 MHz Reference

Some time ago, I wrote about the Rubidium reference that I connected to SDR. The reference supplies a very stable and precise 10 MHz reference clock to the SDR, so that the sample rate does not drift. Drift in the sample rate causes drift in the received frequency, much like drift in the various oscillators in a conventional radio causes drift.

Just today, I replaced the Rubidium reference with a GPS disciplined reference.

Here’s what I got:

The reference itself is the box in the center. To the right is the power supply, to the left is the antenna.

A GPS disciplined reference or oscillator uses timing signals from the GPS satellites to control,or “discipline” the oscillator built into the reference using a tracking loop. The 10 MHz output is continuously adjusted to keep it at the correct frequency, usually by making very small adjustments and using long time constants (averaging periods), typically around 100 seconds or more.

The 10 MHz output from the reference connects to an input on the netSDR. Internally, that 10 MHz signal is used to produce an 80 MHz clock that is used to drive the A/D sampling.

Here is what the inside of the reference looks like:

Here’s a plot of WWV on 10 MHz:

I believe the frequency shifts you see are due to doppler effects in the ionosphere.

Now I can figure out exactly what frequency Captain Morgan is on.

SdrDx – Software Defined Radio (SDR) App for Mac OS X

Software Defined Radios (SDR) have revolutionized the HF radio monitoring hobby. While most of the SDR manufacturers only offer Windows versions of their SDR application software, there are many third party solutions for other operating systems.

I use Macs, and there’s a great SDR app called SdrDx. It is based on the open source CuteSDR program (which fortunately uses the more permissive BSD license rather than the overly restrictive GPL license, allowing a much wider use of the code). SdrDx is free to download and use.

SdrDx works with all SDRs made by RF Space. I believe it may also work with some soundcard based SDRs, although I have never tried it that way.

As obvious by the screenshot (from a 24″ iMac) there’s a lot of controls. Besides the typical waterfall and spectrum displays, there’s also controls for memories and notch filters, as well as the ability to control the various AGC and noise blanker settings.

There’s a readme file that comes with the download, and recent versions of SdrDx have an optional tooltip display, so you can hover the cursor over a control to find out what it does.

The author of SdrDx is actively developing it, and has added quite a few features based on user suggestions. If you run Mac OS X, and are looking for an app to control your SDR, SdrDx may be the way to go. Download a copy and try it for yourself.

Graphs of Realtime HF Signal Levels

While it’s useful to forecast propagation conditions, it’s even more useful to know what the actual propagation conditions are. Propagation forecasting is somewhat like weather forecasting, although slightly less accurate.

I wanted to measure (and store for future reference) the signal levels from several shortwave stations from various parts of the world, transmitting on a range of frequencies. These stations can act as references for determining what propagation conditions are like to that part of the world, on a certain frequency or band, anyway.

One way to do this would be to step a radio through a list of frequencies, measuring the signal level and recording it. The problem is that you need to sit on a given frequency for at least some short period of time, to measure the signal level. And, if you’re only on the frequency for a second or two, that recorded signal level may not be characteristic of the actual signal level. You could have been monitoring while there was a burst of interference, causing a false high signal level reading. Or during a sudden deep fade, causing a weaker signal. And the longer you sit on one frequency, the less time you’re spending on other frequencies. You’re not continuously monitoring, but taking short snapshots.

But there is a solution. Once again, SDR (Software Defined Radio) to the rescue!

In this setup, an RF Space SDR-14 Software Defined Radio is connected to a 132 ft T2FD (Tilted Terminated Folded Dipole) antenna.

Custom software is continuously reading A/D (Analog to Digital Converter) values from the SDR-14 (in real, not complex, mode). The A/D sample rate is approximately 66.67 MHz. Blocks of 262,144 readings are taken, and an FFT is performed. The result is an array of signal strength readings for the entire HF spectrum, 0 to 30 MHz, in 254 Hz steps.


For frequencies of interest, the software then computes the signal strength in dBm over a +/-3 kHz bandwidth around a center frequency. This is computed approximately twice per second, and these readings are averaged over one minute. They are then uploaded to the web server, where they are stored, and graphs are made on demand when a user loads the page. These graphs show the last three days worth of readings. (Possibly less if I just started taking readings for that frequency)

The URL for accessing the graphs is

You will get a list of frequencies, with a brief description of the station on that frequency. Click on a frequency to get a graph of about three days worth of signal readings.

Several things can be readily observed:

For the lower HF frequencies, signal levels are very weak during the daytime, due to absorption by the D layer. They then slowly rise as the Sun sets, stay high overnight, and then go back down again as the Sun rises.

For higher frequencies, say 6 and 7 MHz stations operating in NVIS mode such as NAA on 6726 kHz, you can see a sudden decrease in the signal level as the station goes long in the evening, and then an increase again when the Sun rises, and the F layer increases in strength. The signal does not always completely go away during the night, but it is significantly weaker. There are also periods at night when the signal level goes up and down. This could be the actual signal from the intended station, or possibly another station located elsewhere in the world.

The effect is much more pronounced for even higher frequencies, such as WWV on 20 MHz.

The recording for 10 MHz often shows WWV cutting out around local midnight, then the signal level goes back up again in the early morning, but before sunrise. I believe in this case we’re actually getting a signal from WWVH in Hawaii.

I’m also recording 25,555 kHz, which is a mostly open channel. My goal is to observe radio noise from Jupiter on this frequency.

The aviation weather frequencies (6604 kHz for example) do not have continuous transmissions, there are gaps. This is readily observable by seeing the steps in signal level.

You will often see spikes in the graphs. This is mostly likely due to local noise, it could also be to brief transmissions on the frequency. There are ionospheric sounders that sweep across HF, as these pass by the frequencies being observed, they will cause a brief increase in the measured signal level.

You may also notice occasional gaps in data, the ancient computer the SDR-14 is running on (under Windows 2000) seems to develop USB issues every dozen or so hours. I need to try to set it up on another computer and see if that helps. Worst case I may try to rig up some sort of a Watchdog Timer.

Update: I’ve replaced the Win2k machine with a slightly less ancient machine running XP. We’ll see if that improves reliability. I know, reliability and Windows are rarely used in the same sentence. If only I knew someone with realtime OS experience like QNX…

As mentioned during the introduction, these graphs can be very useful in determining what time of the day propagation is open to various parts of the world on certain bands. It is important to remember that much like the weather, propagation changes from day to day, and with the seasons. The levels of solar activity, as well as the effects of solar flares, have a dramatic effect on propagation. Changes in the season affect the number of hours per day of sunlight on the ionosphere over the different parts of the Earth, and cause seasonal changes to propagation patterns.

Do you have a suggestion for a frequency to monitor? Ideally, the frequency should have a station that is on 24 hours a day. If so, please pass it along!

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.

Adding a Rubidium Reference to the netSDR

I recently acquired a FE-5680 rubidium frequency reference off eBay. This is a high stability 10 MHz frequency reference. I also bought the REFCLOCK option for the netSDR directly from RF-Space, which takes the 10 MHz as a reference input, and produces an 80 MHz clock for the A/D, locked to the reference input. The result is less drift in the radio. The stock netSDR drift is already extremely low, but I want to do some observations of RF carriers over a long time period, so I wanted to reduce the drift even further.

Here’s a picture of the reference I bought, it was around $55 shipped, from China of course.

The reference requires 16V at just under 2A peak, as well as 5V at a lower current. I used an old laptop power supply, and rigged a 7805 to produce the 5V. Unfortunately all input and output is via a DB-9 connector. The 16V is applied via the barrel connector on the left, and the 10 MHz comes out the BNC on the right.

Here’s the inside of the netSDR receiver:

And after installing the REFCLOCK module:

I’ll post some followup articles with long term waterfalls, and we’ll see how the drift looks.

But how does the rubidium frequency reference work? From the FE-5680 technical manual:

The Rubidium Physics Package incorporates a rubidium cell, rubidium lamp, and servo electronics to utilize the ground-state hyperfine transition of the rubidium atom, at approximately 6.834x GHz.The VCXO is locked to the rubidium atomic resonance in the following manner. The VCXO frequency of 50.255x MHz is an exact sub-multiple (36) of the atomic resonance frequency at 6.834x GHz. A microwave signal, having a frequency in the vicinity of 6.834x GHz, is generated from the nominal 50.255x MHz VCXO input. This microwave signal is used to resonate vaporized rubidium atoms within a sealed glass Rb resonance cell that is placed in a low Q microwave cavity.

The microwave frequency generation method is designed so that the VCXO frequency is exactly 50.255x MHz when the microwave frequency is exactly equal to 6.834x GHz. The frequency of the signal applied to the microwave cavity can be maintained equal to 6.834x GHz by generating an error signal when the frequency varies, and using this error signal to servo the VCXO via its control voltage.

The error signal is generated in the physics package. Light from the rubidium lamp, produced by an excited plasma discharge, is filtered and passed through the rubidium resonance cell where it interacts with rubidium atoms in the vapor. After passing through the resonance cell, this light is incident upon a photocell. When the applied microwave frequency is equal to 6.834x GHz, the rubidium atoms are resonated by the microwave field in the cavity; this causes the light reaching the photocell to decrease. The decrease in light, when the microwave frequency is equal to the sharply defined Rubidium frequency, is then converted electronically to an error signal with phase and amplitude information that is used to steer the VCXO via its control voltage and keep it on frequency at 50.255x MHz.

The VCXO operates nominally at 50.255x MHz. The VCXO has two isolated outputs; one output is provided to the Rubidium Physics Package for comparison purposes, and the other output is used as the clock input for direct digital synthesis within the Synthesizer.

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.