Ensemble RX III Home -
Band: HF


General Info

The Ensemble RX III kit and its 4.5: X 2" board representing the culmination of a long line of Softrock SDR receivers from Tony Parks, KB9YIG. The line began with the Softrock 40 RX and went through several iterations of receivers. The most recent receiver was the Ensemble RX II with an automatically switched bandpass filter. This current kit is a refinement on that kit and further streamlines the kitting process for Tony Parks, the designer and supplier.  

The differences between the Ensemble RX II and RX III are:

  1. The RX III is HF only (it does not offer LF and HF options), and
  2. The RX III provides 9.5dB of pre-amplification on the highest band group (Band 3: 15m/12m/10m).

Tony's schematics can be downloaded via the following links:

Band Coverage

The Ensemble RX III kit provides coverage of HF ham bands from 160-10m, in four different optional "super bands" (each with underlap and overlap within the parameters of the associated bandpass filter):

  1. Band group 0: 160m - Continuous coverage from 1.8 to 4.0 MHz (attenuated)
  2. Band group 1: 80m and 40m - Continuous coverage from 4.0 to 8.0 MHz (attenuated)
  3. Band group 2: 30m, 20m, and 17m - Continuous coverage from 8.0 to 16 MHz
  4. Band group 3: 15m, 12m, and 10m - Continuous coverage from 16 to 30 MHz (preAmplified)

The band coverage is via 4 switchable "band groups" ("superbands"). Band switching is performed under program control, in conjunction with programmatic control of the receive frequency. This control is provided by an Atmel ATTiny85 micro-controller, acting as a USB device to control the Si570 programmable oscillator and automatically switch to the appropriate band group (0-3) as the frequency changes.

As a welcome improvement over earlier models, the Ensemble line of kits provide pcb-right-angle jacks for all external connections: Antenna, USB from the PC, I/Q output to the PC, and Power to the Board. Thus, once built, the kit can be placed in a suitable enclosure and handled thereafter as a "blackbox peripheral" to the PC.

The design of the Ensemble RX is very similar to the receiver design of its sibling Ensemble RXTX. The major difference is the greater band coverage of the Ensemble RX kit (roughly 4 "superbands" on the RX vs. 1 "superband" on the RXTX) and the addition of the 9.5 dB preamp for the high band group..

This kit is an excellent value for both the licensed amateur and the SWL who is comfortable with building electronics kits. The skill level and experience requirements are medium-level because of the small size of the components, the requirement to be able to solder SMT parts, and the requirement to wind and install inductors. Thousands of builders have proven this is not an insurmountable set of requirements. If you are new at this, you should try one of Tony's sub $20 monobander RX kits as a "starter/learner" kit.

WB8YQJ Build Notes

Check out this web page from Don, WB8YQJ,  for some great insights into and hints for the build of this kit

Recommended Enclosure

Tom KM5H offers a very nice enclosure for the Ensemble RX line.

Build Stages

This kit is built in succeeding stages. Each stage is testable upon completion.

The stages in the build are, in build order:

Theory of Operaton

Basic Theory

For a very readable (if somewhat dated) presentation of the fundamentals of SDR receivers, see the presentation by Bob, G8VOI..

Also, dont forget to check out this excellent video review of the Ensemble RX II by Alan Wolke, W2AEW.  While it addresses the RX II, it is applicable to this RX III receiver as well.  Just ignore the references to the RX II's LF option and recognize that the RX III has the pre-amp added to the band 3 filter chain:

Alan has also produced an excellent tutorial on how I and Q signals can be used to provide any modulation/demodulation scheme you would want.  It is short but very educational:


Block Diagram

block diagram of ensemble rx

This receiver implements a quadrature sampling detector to produce low frequency I and Q signals for input to the stereo line in inputs of a PC sound card. The I and Q signals are the product of the quadrature sampling detector (QSD) stage, in which bandpass filtered "chunks" of RF are mixed with quadrature clock signals to produce the down-converted I and Q signals. These products (the I and Q pairs) are identical to each other in all respects except phase, where they are 90 degrees apart.

The I/Q products of the QSD ("mixer"), when input to the appropriate SDR program through the PC's STEREO line-in soundcard input, result in a spectrum display on the PC which will show signals arrayed around a "center frequency". This "center frequency" is the frequency of the I/Q outputs from the Quadrature Clock Generators. The bandwidth of the signals either side of the center frequency will be approximately equal to the sampling rate of the PC's sound card. Thus, if the local oscillator is tuned to produce 28.4 MHz to the Quadrature Clock Generators, they will output two signals (I and Q clocks) at 7.1MHz (the "center frequency"). If the PC's sound card has a 48 kHz sampling rate, then the SDR program can translate the QSD's I/Q outputs into a chunk of spectrum that is 24 kHz either side of the center frequency of 7.1 MHz: i.e, 7.076 - 7.124 MHz.

As the user tunes the receiver, varying the frequency of the local oscillator, the micro-controller tracks the frequency and switches the appropriate bandpass filter into the RF chain. The SDR program's display will update to show the new center frequency and adjust the scale to reflect the current +/- bandwidth around that center frequency. At all times, the operator can see all signals that are within this movable "window" (whose total width is 48, 96, or 192 kHz, depending upon the sampling rate of the PC's soundcard).

The receiver is controlled via a USB connection from the PC. This USB connection provides a "USB 5V" bus for the local oscillator and micro-controller. A separate 3.3 V voltage regulator on the 5 USB 5 volt bus provides power to the programmable oscillator, the Si570.

The RX has an Atmel ATTiny85 micro-controller unit which, acting as a USB device, and on the "USB 5V" rail, controls the frequency output of the programmable local oscillator (Si570) and provides two switching signals which can be used to select one of four filter banks in the band pass filter

The output of the local oscillator is at a frequency which is 4 times the desired center frequency of the receiver and is consumed in the Quadrature Clock Generators.

The Quadrature Clock Generators divides the local oscillator frequency by 4 to produce two clock signals - QSE Clk 0 and QSE Clk 1 - which will be used to clock the QSD stage. These I and Q clock signals are identical in all respects but phase (they are in quadrature - 90 degrees phase separation).

Rf at the antenna jack is filtered through the Bandpass Filter Stage, where one of four "chunks" of the HF band is selected by the micro-controller, based upon the tuning of the programmable Local Oscillator. The signal is attenuated for the two lower bands and pre-amplified for the high band.  The filtered (and/or attenuated/amplified) RF is passed as input to the QSD Stage.

The Quadrature Sampling Detector (QSD) Stage acts very similar to a mixer. It incorporates a high-speed switch that is clocked by the two QSD clock signals from the Quadrature Clock Generators and switches the incoming RF into a RC sampling network. The result is two outputs at low frequency and also in quadrature, which are the down-converted, baseband analogs of the incoming RF signals.

The outputs of the QSD stage are then fed into a pair of high gain Operational Amplifiers to produce the I and Q baseband signals which will be input to the PC soundcard's stereo Line In.


main schematic

Board Layouts


topside layout


undersideside layout

Trouble-Shooting Layout

Here is also a "see through" bottom view that shows both sides of the board, including the topside silk screening (reversed, of course) to aid in trouble-shooting from the bottom of the board.


Completed Images


completed topside


completed underside Go to Power Supply Stage Go to USB Poweer Supply Stage Go to Local Oscillator and Control Stage Go to Quadrature Clock Generator Stage Go to Automatic Band-Pass Filter Stage Go to Quadrature Sampling Detector Stage Go to Operational Amplifier Stage Go to Operational Amplifier Stage Go to Power Supply Stage Go to USB Power Supply Stage Go to Local Oscillator and Control Stage Go to Quadrature Clock Generator Stage Go to Automatic BandPass Filter Stage Go to Quadrature Sampling Detector Stage Go to Operational Amplifier Stage

Background Info


Winding Inductors

This page has the following inductors:

To learn how to wind coils and transformers, please read the

Component Identification

After soldering problems, the most common cause of trouble in kit building is the installation of the incorrect component. Most often this is the case with resistors (hint - if voltage or current draw tests are way out of whack, suspect resistors or solder bridges). Invest in a cheap multimeter and MEASURE the resistance. As Pete, M0FMT, said: " ... then you don't have to worry about this modern trend of poor identification of components. I doubt you will find an issue with Tony's kits but measuring is best. A cheap OVA meter costs 2 or 3 bucks and a simple cap bridge is easy to build. (you can search E-Bay for "L/C/F Inductance Capacitance High Precision Meter" and will find several digital L/C meters for $20-$30) These two bits of kit are worth there wieght in gold. Oh and most meters have diode and transistor testers build in... remarkable when you think back to the darkages when I were a lad."

It cannot be overstressed: do not rely solely on the color codes for identification of the correct resistor; ALWAYS measure with your ohmmeter as a double check. Measure twice, cut once.


If you are not experienced at soldering (and even if you are somewhat experienced at soldering), refer to excellent tutorial on basic soldering techniques by Tom Hammond N0SS (SK).

This video provides some excellent hints at soldering (and de-soldering) SMT components:

The video below describes techniques for soldering SOIC 14 (and 16 and 8) SMDs


View the above in full-screen mode on Youtube.

You may also want to review the information from the HamNation series on George, W5JDX, and his build of the Softrock Ensemble RXTX. He uses an inexpensive heat gun and Solder paste to install SMT capacitors and ICs. The Session where he introduces the process is in Episode #70 at 36 minutes, 30 seconds into the podcast.

For the more adventurous, there is a process using solder paste and an electric oven called the reflow process, which can be used to install all the SMT chips to one side of the PC Board. This is documented by Guenael Jouchet in the following Youtube segment:

ESD Protection

You may wish to review the message topic beginning at Message 43554 for a common-sense discussion on ESD.

  • Avoid carpets in cool, dry areas.
  • Leave PC cards and memory modules in their anti-static packaging until ready to be installed.
  • Dissipate static electricity before handling any system components (PC cards, memory modules) by touching a grounded metal object, such as the system unit unpainted metal chassis.
  • If possible, use antistatic devices, such as wrist straps and antistatic mats (see Radio Shack's Set for $25 or the JameCo AntiStatic mat for $15)).
  • Always hold a PC card or memory module by its edges. Avoid touching the contacts and components on the memory module.
  • Before removing chips from insulator, put on the wrist strap connected to the ESD mat. All work with CMOS chips should be done with the wrist strap on.
  • As an added precaution before first touching a chip, you should touch a finger to a grounded metal surface.
  • If using a DMM, its outside should be in contact with the ground of the ESD mat, and both leads shorted to this ground before use.
  • See the review of ESD Precautions at this link.

Work Area

  • You will need a well-lit work area and a minimum of 3X magnification (the author uses a cheap magnifying fluorescent light with a 3X lens. This is supplemented by a hand-held 10 X loupe - with light - for close-in inspection of solder joints and SMT installation.
  • You should use a cookie sheet or baking pan (with four sides raised approximately a half an inch) for your actual work space. It is highly recommended for building on top of in order to catch stray parts, especially the tiny SMT chips which, once they are launched by an errant tweezer squeeze, are nigh on impossible to find if they are not caught on the cookie sheet.

Misc Tools

  • It is most important to solidly clamp the PCB in a holder when soldering. A "third-hand" (e.g., Panavise or the Hendricks kits PCB Vise) can hold your board while soldering.   In a pinch, you can get by with a simple third-hand, alligator clip vise. Jan G0BBL suggests "A very cheap way is to screw a Large Document Clip to a woodblock which will clamp the side of a PCB."
  • Magnifying Head Strap
  • Tweezers (bent tip is preferable).
  • A toothpick and some beeswax - these can be used to pickup SMT devices and hold them steady while soldering.
  • Diagonal side cutters.
  • Small, rounded jaw needle-nose pliers.
  • Set of jewelers' screwdrivers
  • An Exacto knife.
  • Fine-grit emery paper.