Extreme Frequency Measurement

This will be a series of posts regarding my efforts to determine precise frequency measurements for a number of radio stations in the AM broadcast and High Frequency (HF) bands.

I’ll be using my ICOM R75 receiver for this. The specifications suggest that the frequency stability of the receiver is about 1ppm when warmed up (I’ve had mine on for several days) so we can not expect better results than that… but we may be able to witness this level of instability! I’ll also add, my receiving conditions are not good. I live on the lower ground of a triple-decker Boston home. My antenna is inside my ‘office’ room and is only a buddistick. To sample audio frequencies, I’m using an E-MU 02020 USB sound card which can sample at up to 192 kHz audio, but I will be using it at 96 kHz only. Using ‘baudline’ on a Mac Laptop, I can get very precise audio frequency readings from the soundcard. With sampling set to maximum resolution over the smallest frequency range, I can measure down to 1 milliHz. I’m hoping this sort of accuracy will be able to reveal the precision of broadcast radio stations. The radio and soundcard are below: Continue reading

Radio Havana Cuba 6000 kHz

This is more of a test post. Zero beating Radio Havana Cube on my Icom-R20. Looks like it is off by 40 Hz… Not too bad… I assume RHC is on frequency but you never really know.

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I can check this with my Icom-R75… Maybe tomorrow.

Update: radio been on for 10 minutes now. Zero beat now less than 10 Hz. Just needed to warm up most likely. Amazing such a small radio can be so accurate. Again, I can measure this tomorrow.

Update 2012/1/30: I have gotten very involved in doing this. This will turn into a series of posts. I need to start my explaining AM demodulation!!! Stay tuned

Graveyard Medium Wave Frequencies

I’ve been following the observations Chris Smolinski has been making with his netSDR radio on Medium Wave (AM) broadcast frequencies. Quoting Chris,

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.

Indeed, when monitoring these freuqencies for several hours on a number of these frequencies, several carriers can be seen +/- 50 Hz from the expected center. For examples see here, here and lastly here. The first two were recorded without the use of a high stability local oscillator, so some drift occurs over the length of the traces. The last one however was performed with the netSDR radio locked down with a highly stable 10 MHz frequency source. You can instantly see how much more stable this trace is.

The last trace also has an interesting phenomenon, four of the carriers (between 1.229970 MHz and 1.229980 MHz) appear to be echoed on the other side of the dead center frequency (between 1.230020 and 1.230030 MHz). See this image below:

Reflective echo patterns, where signals can appear as mirror images around a center frequency, is a common problem with Quadrature Sampling Detector (QSD) radios. One  such radio is the “SoftRock” SDR (software defined radio) kits -I have one and it will feature here again soon with an explanation of why the reflective echo effect can happen. However, the netSDR radio is a Direct Digital Sampling (DDS) style SDR, which, as far as I know, should not have this problem.

Closer observation shows that these 4 carriers are not actually mirror images around the center frequency. Look at the figure below:

Here I have copied the waterfall plot boxed in white from the top right  and shifted it down underneath the “echoes” on the left. You can see that these carriers align perfectly. But I have not flipped the section and made it a mirror image. A mirror image does not align well at all, so this is clearly not carriers being reflective across the center frequency. These carriers just simply appear to repeat with identical frequency shifts relative to each other, on the other side of 1230 kHz.

What could account for this?

Thought for the day…

“Decline starts with the replacement of dreams with memories and ends with the replacement of memories with other memories.” – Nassim Nicholas Taleb

Nassim Taleb’s books “Fooled By Randomness” and “The Black Swan”, especially the later, are some of the most enlightening books I have ever read. They are also the most eloquently written peaces of prose I have ever read. I love the above quote from him. I feels very prophetic at the moment.

Direct Conversion Receiver – si570 is wired up

Step 1: The VFO. The variable frequency oscillator is going to be based around the si570 chip. This chip is small, and literally does not have pins to solder to. I mean, not only does it not have through-hole pins, it does not have any pins that extend out from the body of the chip cover at all. Fortunately, the pads that are provided take solder very well and it’s fairly easy to solder some scrap wire to them. See below.

The si570 wired for 'sound'

Red and Black wires are positive and negative power (3.3V – the chip will not handle 5V!!!). Once power is provided, the chip happily oscillates away at its startup frequency. In the case of this chip, thats 56.3198 MHz. See below.

Weak signal, but its there. The chip is out of focus in the background.