Tag Archives: Using a synchronous detector

Pulling Radio Santa Cruz out of the interference

BoliviaI recorded Radio Santa Cruz early this morning around 05:00 UTC on 6,135 kHz using the TitanSDR I currently have under review.

Radio Santa Cruz‘s 10 kW signal from Santa Cruz, Bolivia, was very much audible here in North America, though RSC was competing with another station on-frequency at the time. Actually, Radio Santa Cruz was broadcasting slightly off-frequency–6134.8 kHz instead of 6,135 kHz. In this case, the fact that RSC was slightly below frequency helped me delineate the station’s audio from that of a competing station.

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In the screen-grab of the narrowband channel from the Titan SDR (above–click to enlarge) you can see two distinct carriers spaced only .2 kHz apart (represented by the two peaks in the spectrum display and two parallel vertical lines in the waterfall display).

Here is what the audio sounds like in normal AM mode when we center on the Radio Santa Cruz frequency of 6,134.8 kHz:

You hear a hetrodyne and garbled noise from a competing station. Not pleasant audio.

If we change from the AM mode to eLSB mode (essentially, the TitanSDR’s version of synchronous detection on the lower sideband) we are ignoring all of the noise in the upper sideband, allowing the desired signal of RSC to pop out.

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You can see in the screen-grab above that now only the lower sideband of the RSC signal is highlighted. Here’s a 21 minute recording:

Makes quite a difference!

It’s easy to see competing signals and interference on an SDR’s spectrum display, but if you hear something similar on your portable, try the techniques above to see if it clears up the signal.

If your receiver lacks a selectable synchronous detector, much of the same results can be gained by zero-beating (tuning in) the desired signal in lower sideband mode. Of course, if you have a receiver that lacks SSB mode, the best you can do is tune slightly below frequency in AM, in which case the results will not be as dramatic.

Conclusion? Listening in single-sideband or with a selectable sync detector might be all you need to dig a signal out of the interference.

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AM sync lessens noise in this The Voice of Greece broadcast

TheParthenonAthensSometimes, the Voice of Greece plays very little Greek music; October 10th was one of those occasions.  Nonetheless, I recorded that evening’s broadcast.

Using AM sync for sideband noise

In the first hour of the 10/10 VOG broadcast, you’ll hear a pulsating noise from an unknown origin (possibly a jammer?). The noise was centered about 20 kHz above VOG.

Fortunately, most of the noise was in the upper side band of the VOG signal, so I was able to mitigate it by using an AM sync lock on the lower side band. Without AM sync, this VOG broadcast––and its music mix––was almost inaudible.

If you have a synchronous detector on your receiver and tune in a station with interference, always try turning on sync lock and locking it on either the upper or lower sideband. If most of the noise resides in one of the sidebands, the lock can help tremendously. I often use this method while listening to AM pirate radio stations in noisy conditions.

A confession…

I have no idea what she’s talking about–it could be something absolutely mundane–but I love this radio host’s voice as she speaks and Pink Floyd’s Comfortably Numb begins(Start listening around 26:00)

Click here to download more than two hours of the Voice of Greece, recorded on October 10, 2013, starting around 03:15 UTC on 9,420 kHz, or simply listen via the embedded player below. Most of the noise disappears around 00:21:

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