WordCamp LAX 2017

Building an RF Probe for Fun and Measurements

QRP Labs VFO Review [Spoiler: we love it!]

I’ve had the QRP Labs VFO for many many months now and it’s about time I got around to actually writing a review of it. When I originally purchased my BITX40, it did not come with a digital VFO. With the analog VFO, tuning was an exercise in futility. I had never owned an HF radio before, and trying to stay on a station long enough to make a contact was extremely frustrating, and not knowing whether the frequency was within my license was unnerving. I was reduced to listening only.

At the time Farhan suggested the QRP Labs VFO as the digital VFO to buy for the BITX40. It was this recommendation that caused me to purchase the VFO originally. I am so glad that I did!

Getting to Know the VFO

One of the reasons that I held off writing a review for so long is that at first, I did not completely understand how to properly use the VFO. Although the initial setup of the VFO for the BITX40 is fairly straightforward, I have since learned a few things and solved some initial problems. Allow me to explain.

The BITX40 has a 12mhz IF frequency. For Lower Side Band operation such as is common with the 40 meter band plan, the VFO must produce a signal of 5mhz which is then subtracted from the 12mhz IF for an output of 7mhz, lower sideband. To tune up to 7.3mhz, the VFO must output 4.7mhz. 12mhz – 4.7mhz is 7.3mhz. This is called a negative offset. The QRP Labs VFO has the provision for a negative or positive offset. So for use with the BITX40, it is set to -12mhz.

Now this is where it gets a little bit interesting. The IF frequency on the BITX40 is not exactly 12mhz. Neither is the 27mhz Crystal on the QRP Labs VFO exactly 27mhz. Therefore, some compensation must be made in order for the frequency output to match the frequency displayed on the LCD.


I found that the easiest way to calibrate the QRP Labs VFO was to use a recently acquired Icom IC-706 MKIIG which I have already set up for digital modes. I set the offset on the QRP Labs VFO to zero, and then output a signal at 10.5mhz. Then, I tuned the IC 706 to 10mhz. I then adjusted the Reference frequency on the QRP Labs VFO until it matched exactly 10.5mhz output in WSJT-X. Now to properly adjust the offset. I started at -11.9850 and adjusted it until a nearby sideband station came in clearly.

With the QRP Labs VFO calibrated correctly, it now tunes all frequencies correctly. This is vital to digital communications especially. The QRP Labs GPS kit can automate all of this for you, and it’s not expensive. It takes the guess work out of calibration.

Ease of Use

One feature that I really like about the QRP Labs VFO, is that it is very easy to switch to a positive offset, which on the BITX40 gives you Upper Side Band. This has the effect of subtracting the IF from the VFO output instead of adding it. So if the readout says 14mhz, then the QRP Labs VFO is actually outputting 12mhz higher than that (26mhz), and the 12mhz IF is subtracted to get 14mhz. To switch back to Lower Side Band when only has to go back and change the offset to negative and it’s right back where it was. This is extremely convenient!

The other feature I find really nice is that it’s set up for multi-band operation right out of the gate. The latest version of the QRP Labs VFO firmware allows you to assign a relay to a memory. This means that you can use one or more relays to switch different bands directly from one of those 16 memories. I have used this to add 20 meters to my BITX40, while allowing it to switch between 40 and 20 by just selecting which memory I wanted. How cool is that?

In Conclusion

Overall I’m extremely happy with the QRP Labs VFO. I did have one glitch with it however. When I first got it I constructed the kit by following the extensive construction manual. However, it refused to see the SIS 5351 chip, which is on the QRP Labs Synth Kit (part of the VFO kit). Hans at QRP Labs was very cool about it, and sent me another Synth Kit. I constructed it without any issues, and I was very glad to be up and running. QRP Labs customer service is top-notch, there’s no doubt about that.

The other thing I like about the QRP Labs VFO over the raduino (the digital VFO included with the BITX40) is that I can use it for advanced options that one would have to re-program the raduino for. That’s not a horrible thing, but if you just want to mess with some things without having to learn Arduino programming, the QRP Labs VFO has the features that you will want. 

But Wait, There’s More!

Last but not least, the QRP Labs VFO is an excellent introduction into the QRP Labs family of products. It’s not hard to build, with a little bit of patience and understanding it is not hard to calibrate. These boards are used for several of their products. The same board, display and Signal generator are used in their U3S Beacon kit. So if you have built the VFO you can build the U3S quite easily. The VFO can be used to upgrade many transceivers or as the VFO for your home brew projects like the Direct Conversion receiver project. The options are limitless!

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  1. […] make this happen I had to graduate from the QRP Labs VFO and roll my own. What I decided to do was use the QRP Labs Arduino Shield which integrates with […]

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