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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)
  • Quadrature output mode (Clk0, Clk1) for driving Quadrature Sampling Detectors directly - 3.2MHz and above
  • 3.3V p-p squarewave output
  • GPS can optionally be connected to maintain precise frequency
  • 16 configurable frequency presets
  • Control of 6 relays using the relay-switched LPF kit
  • 4 auxiliary outputs for controlling other things (similar to Aux on the Ultimate3S kit)
  • 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 s1.04 firmware operating manual (A4 size)
s1.04 firmware operating manual (Letter paper size for US/Canada)
Si5351A manual Si5351 Assembly: download here!


App Notes

AN001 and AN003 are written for the Ultimate3S transmitter, but they also are relevant to the VFO kit. 

AN Title    
AN001 Frequency drift reduction in the Ultimate3S transmitter A4 format Letter format
AN003 Controlling additional relays using the Ultimate3S “Aux” A4 format Letter format
AN006 HF low distortion sinewave signal generator, 3 to 30MHz A4 format Letter format


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 s1.03 firmware operating manual (A4 size)
s1.03 firmware operating manual (Letter paper size for US/Canada)
s1.02/s1.02a/s1.02b/s1.02c firmware operating manual (A4 size)
s1.02/s1.02a/s1.02b/s1.02c firmware operating manual (Letter paper size for US/Canada)
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
s1.04 22.Mar.2017 - Added quadrature output mode for Clk0/Clk1, operates only at frequencies over 3.2MHz
s1.03 23.Feb.2017 - Increased the number of presets from 10 to 16 (labeled 0..F)
- Added 6 "Band Limit" configurations, for automatic selection of 6 relay-switched LPFs
- Added "Band" and "Aux" settings to the Preset screens
- Ref Freq defaults now to 27.004MHz which is generally closer to reality than 27.000MHz
- New display tags #LP to show the LPF filter in use (0..5), and #AX to show the value of the active Aux parameter
- Bug fix: if you turned the tuning knob too fast, the display lagged behind
- Bug fix: "Diagnostic mode" toggling the LCD data pins, was continuously active, even after leaving Diagnostic mode
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
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
s1.02a 13.Nov.2016 - Bug fix: Negative IF offset did not retain the "-" after editing
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
s1.01a 19.May.2016 - Bug fix: when GPS was enabled, initial display of line 2 was temporarily corrupted ("dddddd" etc.)
s1.01 24.Jan.2016 - Remove 10MHz upper limit on IF Offset frequency setting
s1.00 19.Jan.2016 - Original kit firmware version


3-30MHz Low Distortion Sinewave Signal Generator

The Si5351A Synth in the VFO/SigGen kit produces a squarewave output. Starting in firmware version s1.03 the processor can control the 6 relays in the 6-band Relay-switched LPF kit. This feature can be used to automatically select a LPF depending on the VFO operating frequency, to optimally remove the harmonics and leave only a nice high quality sinewave remaining. If you require a general purpose HF sinewave signal generator then this option could work well for you. The combination of VFO/SigGen kit with relay-switched LPF kit and six LPF kits for 60, 40, 30, 20, 15 and 10m produces a good quality signal generator. This is described in App Note AN006. 

The required kits are also available as a set, see the shop page 

View the embedded image gallery online at:

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. It is available as an optional extra. The 'scope screenshot shows the quadrature output mode using the Clk0 and Clk1 outputs. The graph is measured unwanted sideband suppression in a 40m CW transceiver, using the quadrature mode (RED line) vs using a 4x VFO and 74AC74 divider (BLUE line) - demonstrating that the quadrature output of the Si5351A is accurate. Note that the minimum frequency for quadrature mode is approx 3.2MHz, this is a constraint of the Si5351A chip architecture.

View the embedded image gallery online at:

View the embedded image gallery online at:

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).