Update: Same board, brand new enclosure! Now available in limited quantities.

The SDR front-end filter-preamp is now available. This design takes the previous passive Shelf+Low-Pass filter design and adds a preamp at the output. This will improve the performance of most wideband SDR receivers.

Specifications: Filter-Preamp-v1-rev-2.pdf

To order: filter-preamp

I was recently asked about the filter-preamp, and wrote this in response:

The filter is useful when using a wide-band SDR such as the RX-888, since there is always the challenge of managing dynamic range in an SDR.  The RX-888 has a 16-bit Analog to Digital Converter at the input (and some other SDRs use 12 or even 8 bits).  A 16-bit ADC provides a dynamic range of about 96dB, and the signal-strength levels at the antenna can often exceed this range.  A little bit of overload isn’t fatal, but too much will result is significant distortion and reduce the receiver performance.  If you reduce the input gain to minimize the overloading signals then you will not be able to receive the weaker signals.

This dynamic range problem tends to have a frequency dimension as well.  In the USA, we have the AM broadcast band (540 – 1700 KHz), and these are often the strongest signals seen at the receiver input.  In addition, the regular atmospheric noise is stronger at lower frequencies, so receiver gain is usually not helpful at the lower end of the HF spectrum.

In Europe, the AM broadcast stations are spread across the entire HF spectrum, and mitigating the overload problems caused by these is a much tougher problem!

Note that traditional receivers, having band-filters and preselectors at the input are less prone to this overload, as they only have to work with a relatively narrow slice of the spectrum at any given time.  The RX-888 is continuously receiving the full spectrum, from the KHz region up to 30 MHz (or 60 MHz).  Also, with the wideband SDR we usually use a wideband (or multi-band) antenna, which doesn’t help the overload situation.

Also, the SDR, being a sampling receiver has an issue with “aliasing”, where signals higher in frequency than 1/2 the sample-clock frequency will be aliased down in frequency and appear as an interfering signal in the receiver range of interest (see “Nyquist Frequency” for details.)  Most SDRs have an input filter that attenuates these frequencies above the Nyquist rate, but with the RX-888 we usually run the sample clock at about 66 MHz (half the maximum)  The RS-888 internal filter is designed for the faster sample-clock, and so provides no attenuation for signals in the 30-60 MHz range.

So, what to do?  The filter I provide has two sections:  The low-frequency “shelf” filter, and a high-frequency anit-alias filter.  The shelf filter provides a gradual and limited attenuation increase through the HF band (more attenuation at low frequencies), which compensates for atmospheric noise levels and provides significant attenuation of the AM broadcast band.  The shelf filter has two identical sections — usually we want both of them enabled, but in a quiet-RF location we can disable one of them for less low-frequency attenuation.

The anti-alias filter provides a sharp cutoff above 30 MHz, with over 50 dB ultimate attenuation.  This greatly reduces the aliasing problem.

These filters do add loss, and the RX-888 isn’t particularly sensitive as it has a high input noise figure.  The low-noise / high dynamic-range preamp section of the filter-preamp compensates for the filter-loss, and adds about 9dB of additional gain which improves the RX-888 small-signal sensitivity.  A few more dB gain would be nice, but that would be a different design.  The filter-preamp I have now does provide a noticeable performance improvement in most cases. 

Near Friday Harbor (home of Turn Island Systems) we have a RX-888 receiver in a fairly quiet location. Before the filter-preamp was added the AM broadcast stations were the dominant signals and we had to reduce receiver gain to avoid overload.  The filter did a good job of equalizing (average) signal strengths across the full HF range.

The RX-888 adaptor kit is now available directly from TAPR:

RX888 Clock Kit and Thermal Pad

If you are not familiar with TAPR, this brief introduction comes from their website:

TAPR is a non-profit 501(c)(3) organization of amateur radio (“ham”) operators who are interested in advancing the state of the radio art.  The initials stand for “Tucson Amateur Packet Radio” but today the organization is much broader than that: we long ago became an international organization, and while we still support packet radio our areas of interest have expanded to include software defined radio,  advanced digital modulation methods, and precise time and frequency measurement.

Turn Island Systems is proud to be associated with the good people of TAPR.

It’s now available and ready to ship!

Details

Purchase

As you may know, the powerful RX888 SDR receiver is truly a game-changer — especially when used in conjunction with ka9q-radio and wsprdaemon software. This combination provides full HF-band coverage and analysis — much more than I can go into here.

But to obtain the desired frequency accuracy and stability necessary for serious propagation analysis the RX888 does need an external 27 MHz reference clock. Fortunately, the receiver provides a small “U.FL” connector on the board, allowing easy interface to the clock circuitry. However, it’s not as simple as connecting the output of a GPSDO (typically a Bodnar, or the upcoming TAPR reference). The RX888 clock input does not provide a 50 Ohm termination, and requires an AC-coupled reference of around 1V P-P. What is needed at this interface is a DC-block, an attenuator, and a termination.

Rather than string together a bunch of little adaptors, we (Turn Island Systems and TAPR) have come up a single-board solution, as well as a replacement back-panel (so you don’t have to drill a hole in the existing panel for an SMA jack), and a short U.FL jumper cable.

That SMA plug on the right-hand side of the adaptor is not installed for the “Inside the RX888” configuration, but the footprint is there so this board can be used as an external adaptor.

As you can see, the circuit is quite simple. Here’s the schematic:

And it’s pretty easy to cobble together an interface board yourself. Here’s my first attempt, which has been in operation for about a month now. This one isn’t exactly the same (it’s missing one capacitor and the attenuator values are slightly different):

I usually do my prototyping with surface-mount components, but for old-times sake I decided to use my old stash of through-hole parts on this. I’m waiting for delivery of the assembled boards, but if all goes as planned these kits should be available by the end of April.

If you want to use off-the-shelf components to interface between a Bodnar GPSDO and the RX888, I recommend inserting a DC-block at the RX888 external clock jack, followed by a 10 dB attenuator (the links are to probably-OK parts, I haven’t tried these particular ones myself). The attenuator reduces the output of the Bodnar to a more appropriate level, and provides a “good enough” termination at the RX888. Since the board-end of the internal jumper cable is unterminated the 10dB attenuator only provides about 8 dB of attenuation, and this is fine. The attenuator also provides a compromise termination to both the cable from the GPSDO and to the internal jumper.

Here is a good description of the do-it-yourself external clock modification to the RX888: http://www.sonic.net/~n6gn/ExtClockMod.pdf

Note that since Glenn wrote this we have discovered that the coupling / DC-block capacitor is desirable. Also, Glenn has included a switch that allows you to enable/disable the RX888 internal clock. Many of us are not bothering with the switch, but just pulling the internal “enable” jumper clip.

And here is another excellent post from KA7OEI that discusses the RX888 External Clock issue in good detail: mk2.htmlhttps://ka7oei.blogspot.com/2024/03/using-external-clock-with-rx-888-mk2.html