View the embedded image gallery online at:

The Si5351A VFO/Signal Generator kit has a rotary-encoder tuned output, and a separate independent fixed (configurable) output. Both outputs have a range of approximately 3.5kHz to 290MHz. A GPS receiver can be connected to maintain precise frequency output.

Shop order

The VFO/Signal Generator kit includes a Si5351A synthesiser module kit and rotary encoder. Optionally it can also be used with the OCXO/Si5351A synthesiser kit. The kit supports the full output range of the Si5351A, which is 3.5kHz to 200MHz according to the datasheet (but appears to work fine up to 292MHz, though who knows with what other consequences). The outputs of the kit are SQUAREWAVE are 3.3V peak-peak. The QRP Labs LPF kit can be fitted optionally to produce a sinewave over a relatively narrow frequency range.

Kit features

  • Rotary-encoder tuned output covering full Si5351A frequency range 3.5kHz to 200MHz
  • Fixed (configurable) independent output, also in the range 3.5kHz to 200MHz
  • Extended range up to 292MHz has been experimentally measured (in breach of the datasheet, of course)
  • IF Offset (positive or negative) to support superhet radios
  • Multiplier to support radios using Quadrature Sampling Detector type mixers (e.g. as in most SDRs)
  • 3.3V p-p squarewave output
  • GPS can optionally be connected to maintain precise frequency
  • 10 configurable frequency presets
  • Selectable tuning rate 1Hz to 10MHz
  • Configurable display format, can show fixed text e.g. your callsign, or GPS data such as date, time, location etc
  • Variable backlight brightness
  • Also supports 4-row LCDs and 16, 20, 24 or 40 characters width

There is PLENTY of program space in the processor and it is expected that functionality will be expanded in future firmware releases. So please email your suggestions!

Instructions - current version

Printed instructions are not supplied with the kit! You need to download them here. You need THREE manuals, and to choose the version appropriate to your PCB and firmware version: the Assembly manual, the Operating manual, the Si5351A manual.

Assembly instructions VFO kit, PCB "QCU Rev 1"
Operating manual s1.02/s1.01a/s1.02b/s1.02c firmware operating manual (A4 size)
s1.01/s1.01a/s1.02b/s1.02c firmware operating manual (Letter paper size for US/Canada)
Si5351A manual Si5351 Assembly: download here!


Instructions - previous versions

Version upgrades can be purchased at QRP Labs or you can upgrade your own firmware using the .hex file from the Yahoo forum.

Operating manual s1.01/s1.01a firmware operating manual
s1.00 firmware operating manual


Kit contents

The kit contains the PCB, back-lit blue LCD, programmed microcontroller and all board-mounted components, rotary encoder, and 4-way ribbon cable. It also includes the separate Si5351A synth kit. Download the instruction manuals using the link above. The PCB is quite compact and care is required in the construction.

Dimensions: the LCD module is 80mm x 36mm. The main board is 80 x 37mm.

Version history

Version Date Features
s1.00 19.Jan.2016 - Original kit firmware version
s1.01 24.Jan.2016 - Remove 10MHz upper limit on IF Offset frequency setting
s1.01a 19.May.2016 - Bug fix: when GPS was enabled, initial display of line 2 was temporarily corrupted ("dddddd" etc.)
s1.02 11.Nov.2016 - Negative IF offset capability
- Added many #-tags to display data from the optional GPS module
- Added multiple scrolling customisable display feature, with configurable pause
- If the #RF tag is used (ref freq) it is continuously updated when GPS disciplined
- Add the #HB tag to display the heartbeat in the display
- Correction threshold - controls when the VFO is updated (when ref freq changes)
- Improvement to the GPS discipline at power-up, so no large frequency jump
- Extended display support to 2- or 4-rows, and 16, 20, 24 or 40 columns
- Bug fix: memory corruption when editing Line1 and Line2 caused unexpected behaviour
- Bug fix: It was impossible to edit display configuration lines longer than 16 characters
s1.02a 13.Nov.2016 - Bug fix: Negative IF offset did not retain the "-" after editing
s1.02b 15.Nov.2016 - Bug fix: Corr. Threshold parameter only accepted 0's and 1's (now full range 000..999)
- Bug fix: #DD, #DM, #DY tags did not work
s1.02c 29.Nov.2016 - Removed the glitch when the VFO frequency changes (audible on a receiver driven by this VFO)
- Slightly improved accuracy of Si5351A configuration, by using integer arithmetic not floating point


Kit photos

Click for larger versions. Note that the rotary encoder IS included with the kit, but the plastic knob pictured is NOT part of the kit.

View the embedded image gallery online at:

Frequency stability

This page relates to the Ultimate3S kit but it is also highly applicable to this VFO/Signal Generator kit; it shows how to obtain 0Hz drift WSPR reports on 10m, using the standard Si5351A Synthesiser.

Concerned about noise?

This nice article by Gwyn G3ZIL compares the performance of a direct conversion WSPR receiver using a QRP Labs Si5351A Synth as LO, to a crystal oscillator. The results are favourable, and also show the importance in some situations of ensuring a good low noise (no mains hum) power supply for the Si5351A.

Case kit

An enclosure kit is available consisting of a black anodised aluminium case, cut, drilled and printed to match the VFO kit. The kit also includes a pack of accessories, containing buttons/switches, connectors and mounting hardware. Please see for details. 

VFO in U3 Box

The U3 Box is compatible with the VFO/SigGen kit. See this nice example by Larry KB3CUF, who uses his VFO as the LO of the QRP Labs receiver kit

VFO kit with the BITX40 radio

The BITX40 radio has an analogue VFO. The desiner of the BITX40, Ashhar Farhan VU2ESE, recommends the QRP Labs VFO kit for use with the BITX40. The BITX40 PCB already has a 2-pin header labelled "DDS1" for use with an external VFO. All that needs to be done is to connect ground and the VFO kit's Clk0 output to this 2-pin header; then cut out L4 from the BITX40 (to disable the BITX40's onboard VFO).