Equipment Reviews, April 2006
Last Updated on April 3, 2006 by Ralph Brandi
###Sherwood Engineering SE-3 MK III Synchronous DetectorHave you ever noticed in the receiver reviews in Passport To Worldband Radio that the reviewer may think that a particular radio is very good, but that the addition of the Sherwood SE-3 synchronous detector makes it great? What does the SE-3 do that makes it such a useful addition to an already good receiver? I had used an earlier version of the SE-3 with a JRC NRD-515 many years ago, but I decided to see what was different with the new model and see how it worked with more modern receivers.
What Is It?
The SE-3 is a replacement for a receiver’s detector and audio output stages. It is designed to make listening to shortwave more pleasurable by significantly reducing the distortion induced by the selective fading of an amplitude modulated radio signal’s components. It does this by substituting a locally generated carrier that is phase locked to the signal’s carrier. This local carrier is stable and immune to fading, unlike the signal’s own carrier. The SE-3 uses a product detector which generates less distortion of the audio signal than the usual envelope (diode) detector used in most receivers for AM reception. The SE-3 also provides for passband tuning to select either sideband of an AM signal to reduce adjacent channel interference. It is an external box measuring 3.25 x 8 x 5.5 inches (HxWxD). It comes with an external power supply supplying 20 volts AC at 500 mA. The SE-3 contains an audio amplifier supplying five watts into an 4 ohm load with a frequency response of 30 to 15,000 Hertz. There are outputs for a speaker, ¼ inch stereo or mono headphones and an auxillary line level output to drive a tape recorder or external amplifier. The SE-3 is plug and play, as long as your receiver has a 455 kHz intermediate frequency (IF) output—such receivers include the AOR AR7030, AOR3030, TenTec RX-340, Racal GM6790, Watkins-Johnson HF-1000, later versions of the Palstar R-30 and the military R-390A. Other receivers will require modifications to provide the appropriate output to interface to the SE-3—see the Sherwood Engineering website for more information. You will also need to make or obtain a cable with the appropriate connectors to connect your radio to the SE-3. The price for the SE-3 is $549.00, plus shipping. A rack mounting panel option is available for an additional $49.00. It appears that the SE-3 is assembled to order, so it may take at least a week or two for delivery. The overall quality of construction is excellent, although I found a ground lead to the headphone jack that had not been soldered in place on my unit.
If you have more than one radio that can be used with the SE-3, the SE-6 switch box is available to provide switch selection and level matching for the radios. The price for the SE-6 is $99, $125 or $149 for the 4, 8 or 12 input models respectively. The SE-6 is housed in the same size and color cabinet as the SE-3.
Fading
Fading is one reason that shortwave radio doesn’t sound as good as local medium wave and FM stations. Fading is a result of the shortwave signal’s propagation via the ionosphere. One form of fading, selective fading, is due to cancellation of portions of the signal by multipath propagation. The desired signal may be taking multiple paths through the ionosphere to reach the receiver. These paths may have different lengths which affect the phase of the signal and differing signal phases at the receiver result in cancellation or reinforcement of portions of the signal which vary over time. If the signal carrier is cancelled, severe distortion of the signal results. Fading can also result from alterations of the polarization of the signal as it is reflected by the ionosphere. Again, this phenomenon can affect just portions of the received signal, resulting in distortion of the received signal, not just variations in signal strength.
The effects of fading can be reduced by substituting a constant strength carrier in the receiver to replace the carrier that is being varied by propagation. This is the basis of the ECSS (exalted carrier selectable sideband) technique of tuning. This technique consists of tuning amplitude modulated signals with the receiver in USB or LSB mode. The receiver’s BFO substitutes for the signal’s carrier. The drawback to this technique is that the BFO signal is not completely matched to the signal’s carrier, so the technique works relatively well for voice signals and less well for music. One source of mismatch is the receiver’s tuning steps, which may be limited to 10 or even 100 Hertz, preventing an exact match to the signal’s frequency. Even with receivers tuning to one Hertz, there may be phase differences between the signal’s carrier and the BFO, resulting in residual distortion. This is where synchronous detectors have an advantage, as this type of detector generates a local carrier that is perfectly matched to the signal’s carrier both in frequency and phase. For more information on shortwave signal fading see Joe Buch’s article in the March 1993 NASWA Journal, which can be found on-line at: http://www.hard-core-dx.com/nordicdx/antenna/special/fading.html .
The SE-3
The SE-3 combines the advantages of synchronous detection and passband tuning, along with a fixed 5 kHz notch filter, treble tone control and a powerful (5 watts) high fidelity audio amplifier. It’s contained in a no-nonsense enclosure with several cryptically labelled switches and knobs. Along the middle of the front panel from left to right are the Sync lock/offset switch, the PLL guide switch, Sync (“Xâ€) or AM (“+â€) switch, BFO offset knob, a blank cover for the bass boost/cut (which has been discontinued due to component costs), volume/power knob and then the treble boost/cut switch. Along the bottom of the front panel are the flutter filter switch, AM/SSB switch, a blank cover for the optional circuitry to switch between a 455 kHz or 500 kHz IF input and a ¼†mono or stereo headphone jack. On the back are RCA jacks for IF in, speaker out and line level out, along with a Jones plug for the power supply. The no-longer available Radio Shack Minimus speaker is recommended for use with the SE-3, but any good quality small speaker will work. The auxiliary output can be used to drive an external amplifier if a larger speaker is to be used.
Basic operation is quickly mastered, once an appropriate cable to connect the SE-3 to a receiver is obtained and a speaker hooked up. The SE-3 is turned on, the volume is set to a low level, the mode switch set to AM and the detector switch to “+†for regular detection. The receiver is then tuned to the desired station, using AM mode. Once the signal is tuned in, switch to SE-3 to sync detection (“Xâ€) and then hold down the Lock toggle switch. There will be an audible whine that decreases in frequency as the phase lock loop locks. Release the Lock switch and adjust the volume to enjoy fade-free reception. The SE-3 does an impressive job of holding lock and eliminating distortion with even the deepest fades.
The passband tuning feature is utilized by holding the left-most switch up in the “Offset†position and tuning the receiver to either side of the carrier to reject interference—release the “Offset†switch and the detector is locked. The beauty of the offset feature is that it can be used to enhance fidelity by using a wide IF filter and off-tuning so that the entire filter bandwidth is used for a single sideband (instead of the filter’s bandwidth being spread over both sidebands as in normal AM mode reception) or to reject interference by using a narrow IF filter but maintain intelligibility by shifting the passband so the sound is not muffled as it is when a narrow filter is switched in with the receiver tuning centered on the signal’s carrier. The manufacturer states that the SE-3 can be used with any filter bandwidth from 1.3 to 8 kHz.
If the receiver is tuned to another frequency, the SE-3 will have to be relocked. The “+†detector setting can be used to avoid hearing a whine as the receiver is tuned. However, if the receiver can be tuned in 5 kHz steps (the normal channel spacing for shortwave signals) or if a new station is tuned to its carrier frquency via a keypad the SE-3 will still be in lock when the radio is tuned to the new station.
The flutter filter can be switched in when severe fading or flutter of the signal is present. It slows down the time constant of the lock to better hold lock on fluttering signals, such as those that are propagated via a trans-polar path. Engaging the filter will decrease the SE-3’s ability to initialize lock, so the flutter filter should only be engaged after the signal is tuned in and locked. The treble control provides a 5 dB boost or cut at 5 kHz (can also be set to flat). I tend to use the cut position the most, as most shortwave broadcast signals don’t have much signal content above 5 kHz. The 5 kHz notch filter is always in line. The Deluxe version of the SE-3, which is the only model listed on Sherwood’s website, includes a variable BFO control for SSB/CW reception. To use SSB mode, the AM/SSB switch is set to SSB while in sync detection mode and the BFO control is rotated until the SSB signal sounds natural. The stated range for the BFO is +/- 2.5 kHz. My only complaint about this arrangement is that the BFO and receiver tuning interact, so that it can be difficult to exactly determine the carrier frequency of the SSB signal. I tend to just use the receiver alone when listening to SSB signals—having the SE-3 in place doesn’t seem to affect the receiver’s own product detector and doesn’t affect the receiver’s built-in (or external) speaker.
What’s New with the MK III?
The older SE-3 that I have is labelled “Phase Lock Detector†on the front panel instead of “SE-3 Synchronous Detector†and is a Mark II according to the instruction sheets that came with it. The main difference between this model and the Mk III is that the earlier unit has a rotary switch to select four upper sideband and four lower sideband offsets. This arrangement is designed to allow for setting the offset of the SE-3 for each IF filter in the receiver so that the entire sideband is passed by the filter. This is done by adjusting trimmer capacitors inside the SE-3 for each offset setting. The advantage of the Mk III is that the offset can be continuously varied allowing for the best balance of interference rejection and frequency response for a given signal. The variable offset allows the Mk III to be used with various receivers without having to make internal adjustments. The Mk II also lacks the facilities for SSB/CW reception. This information regarding the Mk II is presented primarily for historical interest, but if you happen to come across a used unit for sale, snap it up if the price is right—the reception results will be the same, but it will be a little less flexible in operation than the newer version and some tweaking will be required to match it to your receiver.
But My Radio Already Has a Sync Detector
This is a valid concern, as the SE-3 is expensive, requires an external speaker, takes up space on the radio desk and increases the number of buttons to push and knobs to turn. However, the quality of receivers’ built-in sync detectors as regards to their ability to hold lock varies greatly. In my experience, the ones in the AOR AR-7030 and Drake R-8/SW-8 series are tops. The Drakes (at least the later models) have push-button selection of either the upper or lower sideband, while the AOR uses passband tuning for sideband selection. The Lowe receivers have fair sync detectors, the Ten-Tec RX-340 is less good and the Icom R-75 is poor, at best. The Sherwood SE-3 just does not lose lock. There may be some decrease in the audio level as the signal fades, but there is no distortion of the audio as the signal dips. Using an SE-3 with a good receiver, antenna and speaker combination is as close to high-fidelity as shortwave gets. It is very satifying to watch the needle of the S-meter dip and have no change in the quality of sound, as opposed to normal AM detection, where there is distortion of the signal, especially music, as the signal fades. I went back to compare standard vs. sync detection to refresh my memory for this review after using sync detection almost exclusively for the past two months. There is a definite difference—the “High Fidelity†label on the front panel of the unit is not an exaggeration. The SE-3 also seems to help in rendering weaker signals more intelligible, although it is hard to describe exactly what the effect is—there seems to be less background hiss and sizzle, regardless of how the treble control is set. The signal seems to get “lifted†out of the background noise. I was already sold on the advantageous of synchronous detectors, but the SE-3 is tops.
I also purchased a four port SE-6 in order to switch between the receivers that I have that have an IF output. Installing the SE-6 involves hooking the receiver with the highest IF output level to the “A†channel—other receivers are connected to the other RCA jacks and internal trimmer potentiometers are adjusted so that the IF output levels are approximately equal. The SE-6 has an RCA jack output for the connection to the SE-3. Sherwood recommends turning off the receivers that are not in use, since the isolation between ports on the SE-6 is relatively low and there can be interaction between receivers if they are powered up.
The Sherwood website http://www.sherweng.com has detailed information on interfacing various receivers to the detector and also has MP3 and RealAudio files comparing signals using a standard receiver and a receiver connected to a SE-3. The recording with the SE-3 takes advantage of both sync detection and passband shift to increase the treble response in the signal, so the SE-3 signal sounds much brighter than the standard AM detection signal.
Sherwood Engineering will also provide modifications for radios without an IF output jack, so they can be used with the SE-3. See their website for information on which radios can be modified and whether the mods can be performed by the owner or require that the receiver be sent to Sherwood. Sherwood Engineering’s postal address is: 1268 South Ogden St., Denver, CO, 80210; phone 303-722-2257, Fax 303-744-8876. They also offer filter and performance enhancements for several receivers, particularly Drake and JRC models.