Many thanks to SWLing Post contributor, 13dka, who shares the following guest post:
Revisiting the Belka’s “pseudo-sync detector”: A sync detector crash course!
“It’s usually hard to assess whether or not a sync detector helped with a particular dip in the signal or not, unless you have 2 samples of the same radio to record their output simultaneously and compare.”*
Since I was recently upgrading to the Belka DX in order to pass on the Belka DSP to a friend, I had briefly two examples of almost the same radio on the table at the dike. I tuned them to the same stations and recorded some audio clips with one radio on sync detector, the other in regular AM mode, to answer the question whether or not sync has “helped with a particular dip in the signal”. Then I thought that demonstration would be an opportunity to try an explanation on what exactly (I think) sync detectors are all about anyway, hoping to find a middle ground between “technical” and “dumbed down beyond recognition”.
The trouble with sync detectors
Perhaps no component of a shortwave receiver is surrounded by so much misconception and confusion as sync detectors. Full disclosure: Until quite recently, I had an, at best, vague concept on what they do myself. It seems it’s not so much that people don’t know how they work, what they actually do when they work is where the ideas often diverge. Continue reading →
Many thanks to SWLing Post contributor, Mike, who writes with the following question:
How important is AM Sync for a portable radio? Is it essential or a deal breaker?
That’s a great question, Mike, and one I don’t think I’ve directly addressed it here on the SWLing Post oddly enough.
Synchronous detection is actually a fairly deep topic to explore–and everyone has their own opinion–but I get the impression that you’d like a simple answer, so I’ll try to keep this as brief as possible. You might follow the comments section of this post as I’m sure some SWLing Post readers will share their thoughts on synchronous detection and how important it is for them.
In electronics, a synchronous detector is a device that recovers information from a modulated signal by mixing the signal with a replica of the un-modulated carrier. This can be locally generated at the receiver using a phase-locked loop or other techniques. Synchronous detection preserves any phase information originally present in the modulating signal. Synchronous detection is a necessary component of any analog color television receiver, where it allows recovery of the phase information that conveys hue. Synchronous detectors are also found in some shortwave radio receivers used for audio signals, where they provide better performance on signals that may be affected by fading. To recover baseband signal the synchronous detection technique is used.
How does synchronous detection help shortwave, mediumwave, and longwave listeners?
As the Wikipedia article notes above, sync detection can help “provide better performance on signals that may be affected by fading.”
In short: a solid synchronous detector can help stabilize an AM signal which then can help with overall signal intelligibility.
In some modern portable radios, at least, this could come at the expense of audio fidelity (see caveat below).
I use sync detection when the bands are rough, noisy, and QSB (fading) is affecting signals.
A good sync detector will help clean-up and stabilize the signal so that you can hear voice information with less listener fatigue. Sync detectors are also great tools for grabbing station IDs when propagation is less stable. If you have a receiver with selectable sideband synchronous detection, it can also be used as a tool for eliminating adjacent signal interference.
Caveat? Sync detectors vary in terms of quality.
The PL-880 has a synchronous detection “hidden” function. I’m sure it’s hidden because it’s so ineffective. The PL-880 is a fantastic portable, but don’t bother using the sync detector.
Many modern DSP portables sport synchronous detection, but they’re not terribly stable and the audio fidelity can take a big hit as well. Poor sync detectors can make audio sound “tinny” and narrow.
If a sync detector isn’t effective a providing a stable lock on a signal, then it’s pretty much useless. Why? If it can’t maintain a stable lock, it’ll produce very unstable shifting audio, often with the occasional heterodyne sound popping in as well. In those cases, it’s better to turn off synchronous detection.
Benchmark legacy tabletop receivers and modern Software Defined Radios (SDRs) typically have solid, effective sync detectors. Indeed, I rarely have the AM synchronous detector disengaged on my WinRadio Excalibur–that particular SDR and application enhance audio fidelity through sync detection.
I find that I use sync detection less with my Airspy HF+ Discovery and SDRplay RSPdx, for example, because the OEM applications natively does a brilliant job managing unstable signals.
In terms of portables, I’ve always considered the sync detector of the Sony ICF-2010, Sony ICF-SW7600GR, and PL-660/PL-680 to be pretty solid. I’m sure readers can suggest even more models.
Is sync detection an essential feature on a portable radio?
Not for me. But I do admit that I value the radios I own that sport a good sync detector.
For some SWLs and DXers, however? It might very well be a deal-breaker if a radio doesn’t have a sync detector, or if its sync detector doesn’t function well.
What do you think?
Is the lack of sync detection a deal-breaker for you? When do you employ sync? Please comment!
The lighter shaded side of the AM carrier indicates a lower sideband sync lock. (Click to enlarge)
A few days ago, I tuned to 9,420 kHz and found a relatively strong signal from the Avlis transmitter site of the Voice of Greece. The broadcast was quite clear until a heterodyne (het) tone popped up out of nowhere.
I checked the spectrum display of my Excalibur to find two steady carriers located about .5 kHz off each side of VOG’s AM carrier. I assume this may have been a faint digital signal centered on the same frequency as VOG.
The noise was annoying, but SDRs (and many tabletop radios) have tools to help mitigate this type of noise.
The het tone was originating from both sidebands of the VOG AM carrier (see spectrum display above). I had planned to use my notch filter to eliminate the noise, but I had two carriers to notch out and only one notch filter.
Synchronous detection to the rescue…
The simple solution was to eliminate one of the carriers using my SDR’s synchronous detector which can lock to either the upper or lower sideband. In this case, it didn’t make any difference which sideband I locked to because both had similar audio fidelity and were otherwise noise free. In the end, I locked to the lower sideband, thus eliminating the het in the upper sideband.
Next, I enabled my notch filter and moved its frequency to cover the annoying het carrier in the lower sideband; I kept the notch filter width as narrow as I could to preserve VOG’s audio fidelity. You can see the notch filter location and width in the spectrum display above (the notch filter is the thin yellow line).
I should note here that the great thing about using an SDR–or tabletop receiver with a spectrum display–is that you can see where the noise is. I was using my WinRadio Excalibur, but pretty much any SDR in my shack could have handled this task.
The results? No het tone and I was able to preserve the great audio fidelity from the Voice of Greece broadcast!
Here’s a 3.5 hour recording I made after cleaning up the signal. I believe at one point in the recording, I switched off the notch filter to demonstrate how loud the het tone was:
I 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.
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.
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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