Project Introduction


I Didn't Build This!

Builder beware! These build notes are based upon the author's experiences with predecessor kits, rather than being based upon the author's build (NOT!) of the actual Ensemble RX VHF kit. Thus, there are no photos of the "completed" board, usually found at the end of each stage. In addition, many of the test results are theoretical, not supported by actual measurements on the actual kit abuilding.

I didn't do the photos, Either!

Many thanks and kudos to Nils Brolund for the fine photography on the completed kit.


This kit, based upon a design by Jan G0BBL, and implemented and distributed by Tony KB9YIG, extends the Ensemble II RX kit , replacing the front-end with a circuit that down-converts VHF signals in the 2m, 4m or 6m bands to HF signals that can be received in the RX's "back-end" (which is essentially the Ensemble II RX).

The parts provided in this kit permit the builder to select which of the major VHF bands to implement; all the parts for all the options are included in the kit. Band-specific parts are identified throughout the documentation; the builder makes his/her choice at build-time.

The builder will find invaluable information at the website of Fred PE0FKO, especially the setup of the software and firmware, as documented in the excellent users manual from Bob G8VOI.

Theory of Operation

Block Diagram

6m/4m/2m Converter Block Diagram

Relationship to the Ensemble RX II HF/LF Kit

This kit reuses a large percentage of the design of the Ensemble II RX (HF/LF) kit. Refer to that kit's home page for a theory discussion on the radio. This kit reuses the following stages, basically untouched, from the Ensemble II RX:

The main difference is in the RF and COntrol stage, which is based upon an earlier design for a VHF add-on board for the Softrock V9.0 RX. The theory of the Software Designed Radio can be gleaned from the Ensemble II RX documentation. Here, we will concentrate on the differences (where the RF/Auto BPF functionality is replaced with the stage described in the following paragraphs.

RF Input.

This discussion will use, as an example, a 144 MHZ signal that will be down-converted to an IF in the apporpriate HF frequency range for detection in the "standard SDR" stages.

The incoming signal from the Antenna terminal (or, via a jumper wire from J1/P100 Pin 1) , e.g., 144MHz, is filtered, then pre-amplified by Q1 and fed to a double-balanced mixer, U7.


U7 also receives a Local Oscillator signal via from the Si570's (U3's) LO Out. This local oscillator signal must be at a frequency (in our example, 115.2 MHz) which, when mixed with the incoming 144 MHz signal, will produce both a difference (144 - 115.2 = 28.8MHz) and the sum (144 + 115.2MHz = 259.2MHz). Fred PE0FKO's Firmware handles this translation. See notes below on how to program the AVR (U1) to result in an IF frequency that is equal to the Ensemble's quadrature clock frequency.

Intermediate Frequency (IF)

The output of the mixer at U7's pin 2 is filtered to remove the sum (image frequency). The (difference) IF is transformed into two anti-phase signals by T2 and fed to the Ensemble's Quadrature Sampling Detector via T2's secondaries and the coupling resistors R16 and R17. At this point, the radio operates pretty much the same way that the HF/LF versions of the Ensemble II RX operate.


Tuning the band is then done in the Ensemble VHF RX on signals centering around the quadrature clock frequency (converted in software to the corresponding VHF frequency display values).

Calculating the LO Frequencies

Care must be taken to calculate the LO frequencies so as to arrive at an IF that is exactly equal to the quadrature clock frequency. This is done using a multiplier factor that is specific to the band. This multiplier for 2m is 0.8; for 4m and 6m it is (4/3).

Calculation of LO frequency for 2m band

To determine the correct frequency for the SI570 for the 2m band, multiply the Desired VHF frequency by 0.8 - see following table:

VHFLO=0.8*VHFQuad = LO/4IF=VHF-(4*quad) Shortcut = 0.2*VHF
144.00115.20 28.8028.8028.80
Calculation of LO frequency for 6m band

To determine the correct frequency for the SI570 for the 6m band, multiply the Desired VHF frequency by (4/3)- see following table:

VHFLO=(4/3)*VHFQuad = LO/4IF = VHF - (4*quad) Shortcut = (1/3)*VHF
Calculation of LO frequency for (EU) 4m band

To determine the correct frequency for the SI570 for the (EU) 4m band, multiply the Desired VHF frequency by (4/3)- see following table:

VHFLO=(4/3)*VHFQuad = LO/4 IF = VHF - (4*quad)Shortcut = (1/3)*VHF
I/Q Swapping

For the bands below 2m (i.e., 6m and, in the EU, 4m) the LO frequency will be higher than the desired VHF frequency. This calls for swapping the I and Q leads from their normal ring/tip orientation (this can be done by appropriately installing the jumper wires in the operational amplifier stage.

(go directly to build notes)

Project Schematic

(Resistor testpoints (hairpin, top, or left-hand lead), as physically installed on the board, are marked in the schematic with red dots)

Main Circuit Schematic(s)

(above schematic has clickable areas that can be used for navigation)

(go directly to build notes)

Project Bill of Materials

See Project Bill of Materials

Project Expert's (terse) Build Notes

Board Top

Board Bottom


Latest schematic sheets are at:

Build Procedure

Project Detailed Build Notes

For the non-expert builders among us, this site takes you through a stage-by-stage build of the kit. Each stage is self-contained and outlines the steps to build and test the stage. This ensures that you will have a much better chance of success once you reach the last step, since you will have successfully built and tested each preceding stage before moving on to the next stage.

Each stage is listed below, in build order, and you can link to it by clicking on its name below (or in the header and/or footer of each web page).

Background Info


Winding Inductors

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


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

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


View the above in full-screen mode on Youtube.

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.

Work Area

Misc Tools

Project Completed Stage

Top of the Board

View of Completed Top

Bottom of the Board

View of Completed Bottom

Project Testing

Each stage will have a "Testing" Section, outlining one or more tests that, when successfully completed, provide you with the confidence and assurance that you are heading in the right direction towards a fully tested and built transceiver.  

When you perform a test, you should always record the results of the test where indicated in the Testing section. This will make troubleshooting via the reflector much easier, since you will be communicating with the experts using a standard testing and measurement regime.

When comparing measurements to those published in these notes, the builder should be aware that actual and expected values could vary by as much as +/- 10%. The idea behind furnishing "expected/nominal" measurement values is to provide the builder with a good, "ballpark" number to determine whether or not the test has been successful. If the builder has concerns about his measurements, he should by all means pose those concerns as a query in the Softrock reflector so the experts can provide assistance.

It goes without saying that you should ALWAYS precede any tests with a very careful, minute inspection (using the best light and magnification available to you) to be sure all solder joints are clean and there are no solder bridges or cold joints.

This kit can be built and reliably tested using nothing more than a common multimeter. Tests assume that the builder has a decent digital multimeter of sufficiently high input impedance as to minimize circuit loading issues.  Measurements will be taken of current draws, test point voltages, and resistances.

Most stages will have a current draw test, in which the builder tests the stage's current draw in two different ways:

  • First, testing the draw through a current-limiting resistor
  • Then, when that test is OK, removing the current-limiting resistor and measuring the real current draw.
Some tests will require you to use your ham radio to receive or generate a signal of a specified frequency in order to test transmitters, oscillators, dividers, and/or receivers.
Optional testing. If the builder has (access to) a dual channel oscilloscope, along with an audio signal generator and an RF signal generator, and feels the need to perform tests beyond the basic DMM tests, certain stages will include in their testing section some optional tests involving this advanced equipment.

The IQGen or DQ-Gen programs available free from Michael Keller, DL6IAK, can be used in a pinch to get the sound card to produce audio tones for injection into the circuit.

You can always use Rocky to generate I and Q signals for tests requiring these audio signals (this is the author's preferred way)

Power Supply USB Power Supply Local Oscillator Quadrature Clock Generator RF Front End Quadrature Sampling Detector Operational Amplifiers