Project Introduction



This kit, designed by Jan G0BBL and implemented and distributed by Tony KB9YIG, is a plug-in board for the Softrock Lite + Xtall RX V9.0 which down-converts VHF signals in the 2m and 6m bands to HF signals that can be received on the V9.0 RX. Also documented here is a 4m version (four metres, for our EU friends), courtesy of Bob G8VOI. The parts for the 4m version are included in the kit, with the exception of two capacitors (2.2 pF and 120 pF ceramic) which the builder must obtain. In addition, the schematic calls for band-specific parts C15 and L5; however, these were subsequencly dropped from the final design as unnecessary.

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/2m Converter Block Diagram

Relationship to Lite + Xtall RX V9.0

The converter plugs into the Softrock Lite + Xtall V9.0 RX at the on-board jacks J1 and J2. Power comes from the V9.0 RXs 12 Vdc bus (on pin 3 of J2). The down-converted VHF signals are fed to the V9.0 RX QSD circuit via pins 7-9 of J1/P100.


The incoming signal from the Antenna terminal (or, via a jumper wire from J1/P100 Pin 1) , e.g., 144MHz, is pre-amplified by Q1 and fed to a double-balanced mixer, U1.


U1 also receives a Local Oscillator signal via pin 4 of V9.0 RXs J1. This local oscillator signal must be at a frequency (e.g., 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 Local Oscillator to result in an IF frequency that is equal to the V9.0 RXs quadrature clock frequency.

Intermediate Frequency (IF)

The output of the mixer at U1s pin 2 is filtered to remove the sum (image frequency). The (difference) IF is transformed into two anti-phase signals by T1 and fed to the V9.0 RXs Quadrature Sampling Detector via pins 7-9 of J1 (in lieu of the usual BPF output for normal HF RX).


Tuning the band is then done in the V9.0 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.

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)

Project Bill of Materials

See Project Bill of Materials

Project Expert's (terse) Build Notes

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

Work Area

Misc Tools

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)