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| Figure 1: The RX-888 Mk2. |
Comment:
The highs and the lows
Like any receiver, it has limits of its frequency response - both at the upper end where the high-pass filter dominates and at the bottom end where the component selection as well as the design itself will limit low-frequency response.
Let's look at the low end first.
The lows
The low end limit to the frequency response (somewhere below 1 MHz) of the '888 has not previously been well defined. This low frequency response is set by component limitations within the HF signal path, including:
- Coupling capacitors. DC blocking capacitors in series with the signal path will act as high-pass filters, rolling off the low frequencies.
- The Bias-Tee inductor.
The RX-888 has the ability to supply power via the antenna port to an
amplifier. This inductor has a finite inductance and it, too, will
force a high-pass response as well. This inductor's value was measured
as being 10uH (nominal) which presents a reactance of 50 ohms at about 800 kHz
- The coupling transformer. The RX-888 has a transformer that couples the input of the variable gain amplifier (VGA) from the attenuator. As with any transformer, this, too, has defined low-frequency response. This transformer was measured and found to have an inductance of 125uH of its primary (50 ohms at 64 kHz) with the secondary being about 760 uH.
This low-frequency roll-of is not uncommon in broadband receivers: Most amateur transceivers suffer severe performance degradation at LF and VLF frequencies for the simple reason that the designers presume (correctly!) that very few of the users of that gear would ever be interested in that range - and making this assumption simplifies the design somewhat and reduces cost.
Using a signal generator with a constant output, the response of the RX-888 (Mk2) was measured, using the signal strength at 1500 kHz as a reference:
| Frequency (kHz) | Attenuation (db) Unmodified unit | Attenuation (db) Bias-Tee inductor removed |
| 1500 (Reference) | 0 | 0 |
| 1250 | 0.3 | 0 |
| 1000 | 0.4 | -0.1 |
| 750 | 1.0 | -0.2 |
| 500 | 2.6 | -0.3 |
| 475 (630 meters) | 2.9 | -0.3 |
| 400 | 4.5 | -0.2 |
| 300 | 7.4 | -0.2 |
| 250 | 10.9 | -0.2 |
| 200 | 19.9 | -0.2 |
| 150 | 17.9 | 0 |
| 137 (2200 meters) | 14.7 | 0 |
| 100 (Loran C) | 9.7 | 0.3 |
| 75 (DCF77 approx.) | 7.5 | 0.7 |
| 60 (WWVB, JJY) | 6.8 | 1.1 |
| 50 | 6.7 | 1.9 |
| 40 | 7.4 | 5.3 |
| 30 (Submarine comms) | 11.3 | 7.4 |
| 25 (Submarine comms) | 12.8 | 10.5 |
| 20 (Submarine comms) | 17.5 | 15.0 |
| 15 | 22.5 | 22.7 |
| 10 | 30.7 | 32.8 |
| 7.5 | 36.4 | 40.0 |
| 5 | 45.5 | 49.5 |
| 2.5 | 61 | 60 |
| 1 | 80 | 87 |
Table 1: Attenuation measurements at 1.5 MHz and below using both an unmodified RX-888 and the same one after the bias-Tee inductor was removed.
Comments about the frequency response of the unmodified unit:
As can be seen from the table above, the "stock" RX-888 is flat within about 2 dB or so across the AM broadcast band (520-1700 kHz) but it falls off precipitously between 100 and 300 kHz with a bit of "rebound" in the 40-150 kHz area, likely due a very low "Q" resonance of inductance and capacitance of the aforementioned components (inductors, transformers) in the signal path.
In the "VLF" range (30 kHz and below) the unmodified receiver may be somewhat usable when using an active antenna to overcome losses, but at 20 kHz and below the response drops off like a rock and, as the chart shows, it's pretty much unusable below 5-10 kHz.
The factors above conspire to prevent a flat frequency response at lower frequencies - say, those below 1.5 MHz. For the table below, my reference amplitude and frequency is 1.5 MHz as it seemed to be more or less representative of the amplitude response above this, in the HF range - and it seemed to be comfortably above that at which the aforementioned high-pass effects of the components were having a significant effect.
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| Figure 2: The red arrow points to the location of the 10uH bias-Tee inductor. As seen in Table 1, its removal can significantly improve MF and LF performance. Click on the image for a larger version. |
Can anything be done to improve LF/VLF response?
It is possible to modify the RX-888 to improve the low and frequency response by removing the Bias-Tee inductor from the HF port and as can be seen from the above data there is a dramatic difference in usable sensitivity at frequencies below 1 MHz - particularly below 400 kHz.
This is the easiest modification as it entails the removal of a single component and Figure 2 shows the location of this inductor. It may most easily be removed with a hot-air rework tool, but it should be possible to carefully use a solder-wetted iron to heat it and remove with a pair of tweezers (temporarily remove any thermal pad below that portion of the board if it's present) or a very sharp pair of diagonal flush-cut pliers to remove it (perhaps destructively) as well.
There are other ways by which the low frequency response may be improved, including:
- Replacing the coupling transformer. The transformer used in the RX-888 is likely specified for a low-end frequency response of 1 MHz or so, so it's not surprising that this may be the worst offender in low-frequency roll-off. Replacing it with a different unit with larger inductance (a commercial or hand-made unit) would certainly help. It may also be possible to simply replace the transformer with coupling capacitors (say, 0.1uF) - but this would be at the expense of sensitivity and performance across the entire frequency range, something that might be acceptable if one's primary interest was in the MF/LF/VLF spectrum. As the inductance of the transformer's primary is known to be about 125uH, we can see that this is likely the main cause of attenuation below 60 kHz.
- Increasing value of coupling capacitors. The coupling capacitors in series with the signal path are likely not ideal for coupling VLF frequencies. A value such as 0.1 uF or larger would be suggested.
For VLF use (30 kHz and below) if you have interest in this frequency range you may be better off not trying to use the RX-888 - at least directly. Some possibilities include:
- Use a VLF up-converter. Converting the frequencies 0-30 kHz to a higher frequency range will put this spectrum within the useful range of the RX-888 and practically any other modern receiver. There have been a number of VLF up-converter units for sale in the past, but I don't have a specific recommendation. If this up-converter is clocked from the same source as the RX-888's clock (e.g. using its onboard 27 MHz oscillator, or both from a common, external clock) then frequency drift could be minimized.
- Use a sound card. A modest computer sound card capable of 192 kHz sample rate with 16 to 24 bit A/D converter is perfectly capable of ingesting frequencies up through at least 80 kHz and down (nearly) to DC.
Having a receiver capable of VLF (3-30 kHz) or ELF (300-3000 Hz) is one thing, but having an antenna system capable of this is an entirely different matter altogether. There are many available E-field active whips that will work well down into the 10-20 kHz region, but below that frequency you are into the realm of specialized gear - and listening at "audio" radio frequencies in all but the most rural areas devoid of power lines and other forms of civilization can be fraught with frustration and disappointment due to the likely pick-up of mains-related energy and its harmonics.
Here are a few links related to equipment for LF/VLF reception. Note that I have not necessarily built, bought or used the equipment described below, so your mileage may vary.
- The BBB-4 Natural Radio Receiver. This is a simple circuit is intended for "Audible" RF frequency ranges of a few hundred Hz to a few 10s of kHz and is suitable for detecting "spherics" like whistlers and chirps.
- Some thoughts on E-Field Whistler Receiver Design. This article describes design aspects of ELF and VLF receivers
- An up-converter for receiving long and very long waves. This article describes an upconverter that may be built.
- AMRAD low-frequency up-converter. This is another upconverter design for VLF/LF use. The low-end frequency response is limited by T1 in this design.
- Heros VLF-LF-MF up-converter. This is a commercial LF/VLF upconverter.
- The AMRAD Active LF Antenna. This article - from the September, 2001 QST - describes a high-performance antenna capable of reception down to at least 10 kHz. In lieu of the CP666 transistor, one may also use the Russian KП903 (a.k.a. "KP903") power JFET which may be found on EvilBay and other sites.
The effective reception of signals in the LF, VLF and ELF frequency range is highly contingent on having a "quiet" receive site, largely free of local noise sources and also on scrupulous attention to detail when it comes to decoupling the feedline (going to the "noisy" chassis of the receiver) from the antenna to prevent unwanted signals from being conveyed - but that's a topic of its own!
See the article A (semi)-typical suburban E-field whip receive system for the 630 and 2200 meter amateur bands - link. for a few details on how this might be done.
Real-world observations
At the Northern Utah WebSDR - where there are, at the time of writing, full-time WSPR receivers - it so-happens that there are currently some KiwiSDR and RX-888 based receivers sharing the exact, same signal path. The KiwiSDR - which is capacitively coupled (e.g. you can hear "tinny" audio from the receiver tuned to 0 Hz and you apply the source to the antenna connector) has quite good response well into the VLF range.
Compared
to the RX-888, the KiwiSDR performs noticeably better on the 2200 meter
amateur band (137 kHz) in decoding WSPR and FST4W signals in which the '888 is
about 15dB down. As the '888 based system can't hear the 2200 meter
signals as well, this indicates that signal levels feeding the '888 are a
bit too low for it to "hear" the noise floor of the antenna system -
but it also indicates that, perhaps, a few dB of boost in the signal
path may remedy this: This RX-888 has NOT had its bias-Tee inductor removed - but that's on the "to do" list: After the bias-Tee inductor is removed I expect that it will perform comparably to the KiwiSDR at 2200 meters and I'll update this web page after having done so.
As the "LF/VLF" antenna system at the Northern Utah WebSDR is separate from that of the HF signal path - being combined in a special filter/amplifier module - boosting only the LF/VLF path would be the most beneficial as it wouldn't compromise HF reception by potentially overloading the A/D converter as would boosting everything.
The Highs
The RX-888's specifications state that it contains a "60 MHz" low-pass filter - but the precise nature of its response is not noted.
Comment about sample rates and aliasing
The use of the 60 MHz low-pass filter implies that the designers intended an A/D converter sample rate of more than twice that frequency - and since the RX-888 will happily sample at more than 130 MHz, this fits the need. Many users do not operate their RX-888 at 130 MHz, however, as their interest does not extend beyond HF and operate it, instead, at around 65 MHz to reduce CPU and power loading.
A bit of warning here: With a 65 MHz sample rate, the '888 will happily respond to signals above the Nyquist frequency (half of the sample rate, or 32.5 MHz) and these signals - spectrally "inverted" - will naturally appear at lower and lower frequencies as the original source signal's frequency increases. Since the '888s low-pass filter is set at around 60 MHz, it will do nothing to prevent this: The far right column of Table 2, below, shows the aliases of the test frequencies.
What this means is that users of the RX-888 using it at a sample rate lower than 130 MHz should be using an outboard low-pass filter. With a sample rate of 65 MHz, a good-quality 30 MHz Low-Pass filter is appropriate and will suppress aliased signals that would otherwise appear above Nyquist. Such filters may be found online via the usual retailers, but do not overlook an old 30 MHz transmit-type low-pass filter of the sort used to prevent interference to analog TV by an HF transmitter - often found at amateur radio swap meets or on EvilBay for cheap.
The response, relative to 30 MHz, is shown below:
| Frequency (MHz) | Attenuation (dB) | Alias frequency (MHz) @130 MHz sample rate | Alias frequency (MHz) @65 MHz sample rate |
| 30 (Reference) | 0 | - | - |
| 40 | 0.8 | - | 25 |
| 50 | 3.8 | - | 15 |
| 54 | 5.5 | - | 11 |
| 60 | 8.5 | - | 5 |
| 64 | 10.5 | - | 1 |
| 70 | 14.1 | 60 | 5 (double alias) |
| 75 | 17.7 | 55 | 10 (double alias) |
| 80 | 21.7 | 50 | 15 (double alias) |
| 85 | 27.1 | 45 | 20 (double alias) |
| 90 | 32.5 | 40 | 25 (double alias) |
| 95 | 37.8 | 35 | 30 (double alias) |
| 100 | 42.8 | 30 | 30 (triple alias) |
| 105 | 48.0 | 25 | 25 (triple alias) |
| 110 | 52.9 | 20 | 20 (triple alias) |
Table 2: Sensitivity response of the RX-888 relative to 30 MHz
Table 2 shows the amplitude response of the RX-888 (Mk2) relative to 30 MHz and with a sample rate of exactly 130 MHz, meaning that our Nyquist Frequency will be at half this, or 65 MHz. The third column shows the apparent frequency of the "alias" response that results if a signal above Nyquist gets to the A/D converter operating at 130 MHz. (Note: The bias-Tee inductor had negligible impact at the frequencies checked in Figure 2.)
It's worth noting that if you happen to operate the '888 at, say, a 65 MHz sample rate, the Nyquist frequency will be 32.5 MHz which means that if a 50 MHz signal is present, it will show up at (50 - 65 =) -15 MHz, and the appearance of the "negative" frequency means that the sidebands will be inverted (e.g. USB becomes LSB, etc.). What's more is that the amplitude of this "alias" will barely be attenuated - and we know from the chart above that it'll be down around 3.8 dB: This is why a low-pass filter for 30 MHz is strongly recommended if you plan to run the '888 at a sample rate around 65 MHz.
"Could I intentionally use aliases to receive higher frequencies than my sample rate would allow?" After reading this, you might ask yourself "If I operate at a sample rate of 65
MHz, could I intentionally do this to receive spectrally-inverted 6
meter signals between 15 and 11 MHz?" The answer is yes, you could -
and as the chart above shows, they would be only 3.8-5.5dB down from
the "real" signals across that same 15-11 MHz range. Intentionally
allowing aliases to occur is often done to allow the detection of
signals well above the sample rate. The caveat here is that one would want to sharply filter the source of the "above Nyquist" frequencies to limit them to the band of interest as well as prevent noise on the aliased frequency (15-11 MHz in this example) by filtering those frequencies as well. Doing this works just fine as long as
proper filtering and amplification is done to keep out the "unwanted"
signals (at the higher and lower frequencies) and make up for losses. In the example above, the lower part of 6 meters would appear just above the 20 meter band - but if one adjust the sample rate, the alias could be moved farther away from 20 meters and, with proper filtering, one could receive both 6 and 20 meters on the same receiver hardware. |
Table 2 also shows the "double alias" frequency responses (in which the sidebands are inverted twice - or back to normal) that can also occur when signals are above the actual sample rate of 65 MHz, along with the "triple" aliases (inverted again) when signals are above 1.5 times the sample rate.
What the above table above also shows is that the 60 MHz low-pass filter isn't very good: By the time you get to the bottom of the FM broadcast band (88 MHz) we know that the attenuation is only around 32 dB. Here in North America it's common for an FM broadcast station to have many 10s of kilowatts of ERP which means that if you live anywhere near such a station - even if you are using an antenna that wasn't designed to receive FM broadcast frequencies - you may experience some interference around the alias frequencies noted in Table 2.
No matter the sample rate at which you operate your RX-888, it's recommended that you carefully check for aliased responses of FM transmitters. If you find them - and even if you don't - I'd recommend a separate FM broadcast band blocking filter be installed to quash ingress from strong signals: Without it you'll probably get some leakage of moderate-to-strong signals in the 22-42 MHz range (frequency-inverted) if you are running at a 130 MHz sample rate or in the 23-32 MHz range if you are running at a sample rate of 65 MHz.