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.
So what is Synchronous Detection?
I like this concise Wikipedia answer:
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!
Tecsun H-501 also has SYNC.
Yes, and like its SSB performance its next to worthless, I have found the H501, S8800, & the PL330 to possess almost exactly the same exact characteristic, all three possess SSB that is impossible to calibrate for more than a couple of frequencies before you must yet again calibrate them, often wondering off within a few moments of achieving demodulation.
The synch circuits of the H501 & PL330 are worthless altogether!
However, the selectable SSB of the P:-990 & PL-880 portable sets are so rock solid stable you can do away with the two radios worthless synch feature altogether and simply stabilize any fading via ECSS, across the entire AM spectrum…..
It’s worth pointing out that all AM detection is mixing. A simple diode detector uses the incoming carrier to mix the sidebands down to audio. If the detector was just following the envelope, you’d get what you hear when listening to SSB with a diode detector.
With a crystal radio, little selectivity, you can hear more than one station. The carriers from each are translating the sidebands down to audio separately, so it all sounds fine, except one is talking over the other.
The carrier isn’t needed to send the AM signal, but it is needed to demodulate it.
Selective fading means the carrier may fade while the sidebands are still strong. You get a distorted signal, even to the point of sounding like SSB on a diode detector.
I think the first use of “exalted carrier” was in an article about the Q-multiplier. (A regenerative stage, like a regen receiver, added to a superhet receiver.) A Q-multiplier was a way to get better selectivity for cheap receivers, but it gave a narrow peak with a broad skirt. So it could regenerate a weak carrier. (I saw the article recently, it even mentions SSB, a puzzler until I realized at that early date many of the few SSB transmitters didn’t null out the carrier so much, hence a weak carrier to regenerate.)
With a double sideband signal, it’s not just an issue of “zero beat”. With no carrier, if you don’t place the BFO exactly between the two sidebands, the two sidebands translate to different audio frequencies, and the result is unlistenable.
After SSB came along, there was a wave of hams building DSB (suppressed carrier) transmitters. Better than AM, simpler than SSB. But unless you had a sync detector, it was hard or impossible to get the BFO right between the sidebands so reception was awful.
But, by then good SSB receivers were out there, with good filters. They’d receive those DSBsc signals fine, by turning it into an SSB signal inside the receiver. The filter was inside the receiver, not the transmitter. Most never even noticed they were receiving a DSBsc signal.
You no longer have to place the BFO exactly, though you can mistune the receiver so a voice doesn’t sound like it really does. (Music is way more noticeably off). It never bothered me, I didn’t know what the voice was like, so tuned to my liking. This is identical to receiving SSB.
But if you have a good sideband filter in your receiver, not only can it strip off the unwanted sideband, but it knocks the incoming carrier (of an AM signal) down, making the BFO dominant and less worry about the beat from the incoming carrier.
I think the older radios that are reasonably sensitive and don’t overload have a nice “headroom” and deal with fading, pumping signal with the gain opened up. The newer radios overload and distort easy, then you add a lackluster sync loop and sounds worse to my ears than fading.
I have a radio with synch detect.
Its a Grundig G3
It lives in my closet.
It failed the quality control numerous times after originally purchased from Rat Shack.
It was sent back to Grundig twice.
I now just use a TS 870, TS 430 ,Ten Tec RX320, or the SDRPlay RSP1A .
I just use SSB and don’t have to worry about a defective synch circuit.
Adjust c113 if the failure is the sync failing to lock or not holding lock. I have had to tweak it a couple of times over the years with my Grundig G3.
https://forums.radioreference.com/threads/g3-sync-fix.178931/
^FYI. That is not my “profile” as displayed next to my name.
Yes ECSS, Zero Beat or whatever you want to call it works really well unless you have a DSP radio, the Tecsun, PL-880, 990x and S-8800 sound absolutely horrible in SSB making ECSS impossible. The PL-680 on the other hand sounds really great in SSB making ECSS a real pleasure and essential tool against QRM from close stations, it’s a tool for the true DX’er and it’s a good tool against fading too sometimes even better than SYNC.
The DSP radios are a huge disappointment and I fear that many new to the hobby might actually think these performance issues on SSB on the likes of the PL-880/990x are normal, Tecsun and others are normalising bad performance and that, in my opinion, is really sad.
All good points made in these comments.
One other thing that should be thrown out for discussion – how modern digital filter speakers and add on digital filter boxes can “magically” eliminate heterodyne tones.
I bought a North Carolina made add on digital filter box in the 1990s and at first used it only on AM signals to clarify voices. One day “by mistake” I left the digital filter on while tuning in a SSB signal. I was amazed that all the Het tones were gone in a wide frequency range on either side of zero beat. Instead of the voice being “Donald Duck” anywhere except precisely at zero beat, it was now listenable how a wider range, and the receiver circuit no longer had to be perfectly temperature drift stable either.
I am slightly disappointed by the sync in the latest version of Tecsun PL-660. Does not hold lock like my Sony 2010 did. If I keep it in narrow bandwidth, it will work better. As someone mentioned, the audio does get a little harsh. If a manufacturer cannot make a sync circuit that works well, it might not be worth using. For what I need it to do (reduce fading on shortwave plus avoid sideband hash from digital mediumwave broadcast stations) the PL-660 is just usable. I have been looking for a way to make my SDR more portable but not sure if I want to get bogged down into trying it with a small Raspberry Pi setup. A portable should be grab and go, no hassles. So, it seems like we are stuck now with inferior sync circuits in portables until the end of time. And if digital modes (stupidly) are thrown into the same band spectrum as analog broadcasts, sync is not going to help get rid of the digital hash pollution.
Can someone explain the difference between ‘PLL’ (Phase Locked Loop), and DSP (Digital Signal Processing), and the effect on Sync Detection – I.E. Tecsun PL-680 (which I have) is PLL, but Tecsun PL-880 (which I also have, is DSP.
* Phase Locked Loop: basically, you take a known oscillator frequency and compare the phase of it to your incoming signal. The output is typically an ‘error voltage’ that represents the phase difference between the two. This can be used a couple of ways, depending on how you treat it. If you have a fast loop and average out the error, you can use it to tune your receiver circuit for minimum error by adjusting your known oscillator – this is how FM Automatic Frequency Control (AFC) is done (and an upshot is that you can if you choose use the *un*-averaged signal as your demodulated audio output! This was a common method for a while…). If you use a slower loop, you can use the error voltage to tune the known oscillator to substitute for an unstable, noisy, or weak AM carrier – which allows easier and potentially more reliable reception of weak signals. If you do that *and* arrange your filtering/demodulating to choose only one sideband and ignore the other, you have “Synch Detection” (the carrier regenerated by the PLL is “synchronised” to the weak incoming carrier). This can help where e.g. an adjacent channel or frequency-dependent fading, is affecting one sideband but not the other.
* Digital Signal Processing: All the above can be described mathematically. DSP is basically the process of digitising the signal then applying the maths to do it. The usual way is to sample the incoming signal – either the actual radio frequency (which requires fast A-D converters) like many SDRs, at some intermediate frequency (IF) after a frequency conversion stage or two (which can use slower A-D converters not too different to those in a computer sound card) like the common Si47xx chips, or sometimes after I-Q demodulation (which basically ‘splits’ the signal into its “in-phase” and “quadrature” components) which is common in some simpler SDRs (using e.g. Tayloe mixers). The heavy lifting of actually demodulating the signal – which can be much more complex than simple AM/SSB/FM – is then done mathematically, and the numeric result is either treated (in the case of simpler modulation schemes like AM/SSB/FM) as digital audio, or further processed (e.g in the case of DAB/DRM/other digital modes) to decode the encoded data into a digital audio stream. In either case, the output is a digital version of the wanted signal, which can then be converted back to audio.
Simples! 😉
AM is the modulation which transmits a carrier which is between 67 – 100 % of the transmitted power. It contains no information. The sync detection is to try and produce a replacement noise free version of the original carrier using a phase locked loop. This circuit requires a comparison of the frequency of the regenerated carrier with the carrier from the signal. How accurately the regenerated carrier will match the original depends on the noise level along with the speed at which the phase varies due to the multiple paths taken to the ionosphere and return.
With the exception of HD radio, no other radio modulation systems transmit a carrier and the receiver must reproduce the carrier. To guide this the digital signal spectrum contains pilot tones of a similar power to each digital signal but at a known frequency. These signals are used to control the phase locked loop.
Digital Radio Mondiale sounds much better than AM and in particularly LSB and USB. both of which contain unique digital signals.
Brasil DRM 11910 kHz 30-Dec-20 0330z DreamOut.wav is a demonstration of a signal path to NW Northern USA from Brazil 6730 km from a low powered 1.12 kW signal. Note that the signal is in stereo.
FM needs a carrier.
The phrase “Synchronous detection” has been used for a variety of schemes, which I suspect accounts for different results.
Some of the methods do regenerate the carrier, and if the carrier gets too weak, nothing to “lock to”.
This would be schemes that square up the carrier, or simple PLLs like the Signetic NE561.
But other schemes use the two sidebands to define where to place the carrier. From the schematic of the Drake R8, original, that locks to the sidebands.
Sony’s 2010 locks to the sidebands, I think that scheme is most common in recent receivers. I think any receiver that offers selectable sideband is locking to sidebands. (I make the assumption because once the circuitry is in place for selectable sideband, locking to the sidebands is easy).
Of course, some are better implemented than others.
Michael,
FM does not contain a carrier. When transmitting silence the FM transmitter produces a sine wave at the frequency on the dial of the radio. As sound is added the frequency is changed depending on the volume of the sound. This produces side frequencies. There is particular frequency and volume combinations which causes the transmitter to have no signal at all at the centre frequency of the broadcast. The centre frequency is the frequency shown on the dial.
Synchronous detection is not used on FM because it is not required. Remember that AM has a carrier which is always present during broadcast. Diode demodulation is used from crystal set to all AM radios except synchronous detection will go silent if the transmitter carrier disappears. Broadcasters use a limiter to prevent the loss of the carrier if the sound volume is too high
f
With no modulation, there’s no difference between the outputs of an AM, FM, or CW transmitters. There will be a carrier.
A PLL can demodulate an FM signal, since it follows the carrier as it moves around, so the DC out of the phase detector varies with the modulation.
But I already showed there were two broad groups of “synchronous detectors”. Some regenerate the carrier, others get from the two sidebands where to place the carrier at the receiver. The latter kind will properly demodulate a DSB signal with supressed carrier.
You can’t synchronize to an SSB signal because there’s no carrier, and no extra sideband to show where to reinsert the missing carrier.
It’s great that I can almost always rely on the community when it comes to saving me lot of typing on a polarizing topic like sync detectors. 🙂
Nothing to add here, to reiterate some points in my own words – sync detectors are for the most part a “convenience feature” – when they work that is. When they do work right (like on the venerable ICF-2010/2001D) they save you fumbling with zero-beating the signal in SSB (which isn’t even possible with the 100Hz steps of the 2001D so it really needed the sync detector just like other radios of that time) and as such they are most valuable when one sideband has more interference than the other. Sync detectors even go one step farther than ECSS in that they automatically lock on the phase of the signal (if they have a chance) so they are better than the best zero-beating mere humans can achieve….if they even let you select a sideband and unless they lose their phase lock. Then it’s howling and whining and hearing the signal less than before until you retune the radio, which isn’t necessary with ECSS, with the BFO just staying on the very-close-to-the-beat frequency instead of helplessly trying to find a lock again.
So yes, “sync” is a “magic button” that in some cases returned instant spectacular improvements and sometimes it just didn’t even with the best of their kind. But unfortunately 80s marketing was very effective and consecutive nostalgic glorification hyped that into an eternal “must have” feature for any radio with some ambition to cater for the BCL crowd where in fact they were just a shortcut to an improvement that could be achieved without and that isn’t even sooo necessary anymore in today’s rather overseeable channel allocations, at a pretty high price in additional parts and the research to make it work right. Surely a nice-to-have feature if it’s a good one that actually works sometimes and maybe even without mangling the signal in some other way (e.g. reducing the audio bandwidth further), then it’s always worth trying if it improves something. Crucial? Absolutely not, for a couple of reasons that would warrant their own article.
The description, pros, and cons of synchronous detection have been adequately described, here and in literature dating back at least to the introduction of Sony’s classic ICF-2001/2010 in the ‘80s. I will only add what I consider to be a preferable work around in place of synchronous detection, which I call “virtual synchronous detection.” Let me call it VSD to save some typing. VSD is simply operating a receiver in its CW or SSB mode when listening to weak AM signals.
In the case of sync, the incoming signal carrier is augmented or substituted by a locally generated carrier in the receiver. In the old analog days, this was done by the beat frequency oscillator (BFO). If I’m not mistaken, in the case of modern synthesized receivers, this local oscillation is now introduced at audio frequencies in the demodulation process. In either case, being much steadier in amplitude than the incoming carrier, the AM modulation is not distorted due to random signal fading.
Receivers having selectable sideband capability (LSB and USB) have an additional benefit in VSD, which is also part of synchronous detection. Adjacent signal interference can be eliminated or at least minimized by tuning either the upper or lower sidebands of the incoming AM signal.
Thus VSD has both major benefits of normal synchronous detection, i.e., minimizing distortion due to periodic fading and selection of the cleanest set of sidebands.
What about heterodynes you say? With normal synchronous detection, any heterodyne or beat frequency between the incoming carrier and the local carrier is suppressed. You don’t hear it. Not so with VSD. An audio tone or squeal will be noted as you approach the carrier in the tuning process. Obviously, with VSD you have to keep the incoming and local carriers at zero beat. Even inexpensive radios nowadays are quite stable, and this is not a problem. On the other hand, the heterodyne can be used to advantage by providing a telltale indication that a carrier signal is present. This ‘flag’ will often be present when you cannot otherwise detect a signal with the receiver in the AM mode.
A final advantage of VSD is that it does not cut out and in like many normal synchronous detectors when tuning exceptionally weak signals.
Finally, one old trick I learned from a ham operator many years ago. It was commonly used to receive single sideband signals on receivers without SSB product detectors. Simply tune the BFO control about 1.5 kHz higher or lower than its center zero-beat position; then retune the receiver for zero beat. That will put you in tune with either the lower or upper sidebands – a poor man’s selectable sideband receiver.
Give it a try.
Bob Colegrove
This has been used by DXers for years and is called ECSS– Exalted Carrier Selectable Sideband.
It’s not just for program listening. Even a very weak carrier will produce a het when tuning across it, and this can be a sign of a sought-after DX station that might strengthen into audio if you hang around the frequency.
Love that pic of an ICF-SW7600GR, Sony’s last iteration of an excellent line of wideband receivers and, I believe, their last wideband shortwave receiver on the marketplace. Own one and the previous iteration and they have excellent synchronous detection.
The HF receiver with the best synchronous detection I’ve ever owned is the AOR-7030, just superb, like everything else about that radio.
The worst one?
My Icom IC-R75, never been able to detect any difference whatsoever when I activate its synchronous detection.
I’m also an Icom R-75 owner, John. I have a later model, without sync. Now I don’t feel as bad, though I’m still curious to know how it might work with the KIWA sync upgrade. Unfortunately I don’t have the option. I have decent success using ECSS.
My only radios with AM sync are the PL-680 and PL-880. The PL-680 sync was getting good press, so it was a selling point for me. However, I reach for the PL-880 more often because even with sync, the PL-680 audio can become fatiguing. I guess with all else equal, I’ll take pleasing audio over synchronous detection.
I guess that’s why they took it out, I never found it made a damn bit of difference.
I get what you mean by listener fatigue, sometimes the optimum audio shaping for copying weak signals becomes fatiguing to the ear after a while.
Not a deal breaker. I have it on the Sony ICF-2010, and the Grundig Satellit 800. I say this because it is not a “magic bullet”, and I have seen people think they are using it correctly, when often times, they are not. The benefits range from limited to great. As good as the sync detect in the 2010 is, in the Satellit 800, it is much better. But I’d venture to say that having it does not guarantee that it is useful.
Points made by others are good ones — SYNC basically became a must-have feature that other manufacturers leaped to put into receivers after SONY perfected it in the famous 2010/2001D. It was also put into some high-end premium receivers, for example, the EKD-500 series receivers by the former East German company RFT in the period just after the Cold War ended, and of course by Drake in its superb R8 series (8A and 8B had better implementation than the original).
It’s true that basically — i.e. just using SSB to exact zero beat a signal — accomplishes pretty much what SYNC does. But not many people who are new SWLs these days are familiar with the ECSS/Zero beat method and thus will want to obtain a portable that does have SYNC.
I’ve owned a Tecsun 880 for 6 years, with firmware 8819, the older software. I’ve found the sync detection to be adequate and as good as on my Sony 2010.
Synchronous AM detection is a nice feature that helps in some fading situations.
A good alternative is SSB reception with a very stable receiver, within some 20 Hz of mean frequency or so: You set the frequency in the 5 kHz raster and hardly ever worry about anything else. This might not be valid for higher-quality music reception.
Deal breaker? Not necessarily. But would I prefer to have it? Yes, as long as the quality of that sync detector guarantees its usefulness and, preferably, it is coupled with dual sideband synchronous detection, the ability to choose either sideband as a means of reducing or eliminating adjacent channel interference. By example, the Sony 2010, Eton E1, Drake R8B, Lowe HF-150 — all receivers no longer manufactured and only available on the used market — have stellar, stable sync detectors with the ability for the user to choose either sideband individually. In my experience, no portable receiver currently on the market has what I would consider to be a useful sync detector. I find that somewhat odd given excellent examples of this feature have been a part of receiver architecture for over three decades. Nonetheless, if anyone can name a receiver at any price point today that has it at the level of quality and usefulness that existed in the radios I named, I’m all ears.
I have had 4 radios over the years that had synchronous detection.A 2010,7600,Satellit 800 & 660.The 2010 was the only one that was useful.On the others I find that going to narrow bandwidth and tuning plus or minus 1 khz works better.
Synchronous Detection is one of those features that will make a great receiver shine. However, if done poorly, Synchronous Detection can actually make a station sound worse. Basically, a conventional AM detector can hide a world of ills in your radio. Synchronous Detection will expose them. These exposed problems may include AGC not working well with the detected output, poor filter selectivity, poor audio amplification, or increasing internally generated background noise.
The detection systems in the Airspy’s SDR# program are excellent. Used in combination with the AM Noise Reduction feature on a strong, solid AM signal, and the main limitation of the audio you recover is the quality of your speaker.
The synchronous detection ECSS in my PL 680 is not bad. It is most useful in cases where one sideband is trashed from adjacent channel activity, but the other one is okay. For example, sometimes New York’s 660 AM can splatter a signal on to the upper sideband of Nashville’s 650 AM. In cases like that I use ECSS to select the lower sideband, as there usually isn’t nearly much going on at 640 AM in my location.
Synchronous Detection is also useful when receiving signals such as CHU, which broadcast a carrier and only the Upper Sideband. The other sideband just adds noise, so using Synchronous Detection can actually improve intelligibility when the signal is weak.
If your receiver is stable, your filters are good, and you can zero-beat a carrier well, then you can always use SSB to tune in one sideband of your AM signal. This works if selective fading is severe enough to completely null the carrier of a signal. Most synchronous detection systems can’t handle that situation well.
Short answer: Yes, for any receiver I’d rely on in hurricane season or out in the field, sync is pretty important. Not so important for the bedside radio though.
I haven’t use it very much to remedy unstable signals, though after reading this post I will be using it for that (thanks, I learn something new all the time here). Rather, I’ve been using the sync USB/LSB on my Tecsun PL-660 to eliminate heterodyning and other interference from stations close by on the dial. For that, it’s a champ.
But yeah, it seems like there’s always a MW-AM station with a signal adjacent to the one I’m trying to listen in on. It’s great to be able to scrape it off and not listen to the heterodyne whine.
I’ve assumed that one reason synchronous detection became popular is that generally it’s implemented with selectable sideband, which helps for SSB.
Until the Sony 2010, synchronous detection was only in very high end receivers (actually, I can’t name any). And there were projects for adding them to receivers.
For awhile, GE was a proponent for DSB with no carrier. Simpler transmitters, no carrier, and there were advantages when received with a synchronous detector. Webb was part of that work at GE, and in 1957 or 58 he had a construction article for a synchronous detector in CQ magazine. Lots of tubes, I doubt many built it, but it was just an extension of “sideband slicers” that were appearing at the time. Phasing type SSB reception, an add on for receivers designed for AM, you get a product detector, and opposite sideband reception.
Once ywiu could put it in an IC, it was “cheap” and took up little space. It became a selling point, and good for SSB too (most portable radios have rather broad “narrow” filters). And Sony set a barrier for others to follow.
I would prefer a radio with synchronius detection, but it only became “needed” once it was a selling feature.
As the owner of a older Kiwa MAP unit, that style of synchronous detector seems to work differently but very effectively. Craig Siegenthaler‘s design was expensive but can be added to nearly any radio with a 455 kHz I.F. As you can imagine the two bandwidth filter choices on the MAP are sharp and the speaker is good.
https://www.kiwa-electronics.com/about-kiwa-electronics/
It would be interesting to attach a MAP to one of the new portables with built in sync and directly compare the two.
I have not tried one yet but the sync on the Sony ICF-EX5MK2 gets high praise and is Sony’s special design.
https://www.amazon.com/Portable-ICF-EX5MK2-Nikkei-Tunning-Japan250/dp/B001QHLKE6/ref=sr_1_1?ie=UTF8&qid=1490379900&sr=8-1&keywords=SONY+ICF-EX5MK2
There’s always ECSS …