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Dec 29

BITX40 VFO Alternatives (and more!)

The BITX40 is a great radio but before they started shipping with a digital VFO, the BITX40 VFO was a big hurdle to getting on the air. The VFO is what tunes the radio, and the BITX40 VFO isn’t very stable. This can make it hard to stay on a frequency once you tune to it. The VFO uses a Voltage Controlled Oscillator, so depending on what voltage it’s given, it changes the frequency. The 10K potentiometer serves the purpose of varying the voltage.

The supplied [pot]entiometer is only a 3/4 turn pot. So the entire 200khz tuning range happens in less than a full turn. Tests with a volt meter show that only about half a turn shows any voltage change, so really you get about 1/2 turn. That’s an extremely coarse adjustment, and because of drift, you have to make a lot of adjustments. The other issue is that there is no frequency readout, so you don’t really know where you’re tuning unless you can calibrate it somehow.

Tuning Alternative 1: DDS

The option many are going with is a digital VFO using a microcontroller (commonly the Arduino Nano) and the SIS5351A DDS chip. It gives precise tuning and control and is usually built to include a LCD frequency display. Farhan is now including the Raduino digital VFO with the new BITX40. This brought a price increase, and it’s now $59 instead of the previous $45. All the same, it’s an amazing deal and will help a lot of people have an easier time with the kit. Another alternative for folks like me who didn’t get it with the Raduino is the QRP Labs VFO, which is a beautiful kit works well.

Tuning Alternative 2: Crystal or Ceramic Oscillator

A more limiting option is to replace L4 with a crystal or Ceramic Oscillator that is on a frequency that will land your radio in a range acceptable to you. For example, a 4.75mhz crystal or ceramic oscillator will put the radio at 7.25mhz (12mhz IF minus Crystal/Ceramic Oscillator frequency) and then that can be bent around a little bit using a variable capacitor. This modification was developed by Peter Parker, VK3YE, and his Video On YouTube explains it fully. It’s lower power draw than a DDS, and far more stable than the built in BITX40 VFO.

Tuning Alternative 3: Stabilize the BITX40 VFO

A lesser traveled road has been to stabilize the BITX40 VFO. Experiments have been done with using highly stable components to eliminate drift, and results have been good. The BITX20 group on Yahoo Groups is the place to go if you’re interested in that. Farhan also included 220K resistors in the kits and shows what to do with them on the official BITX Hacks blog. He also recommends gluing down the L4 inductor since it will change the frequency if it moves at all.

Lets assume that you’ve done those things. Now you still have that 3/4 turn pot, getting about 1/2 turn of adjustment out of it. What can be done to get finer tuning control? The easy solution is to simply buy a 10 turn potentiometer. They are available for $3-4 on Ebay if you don’t mind waiting several weeks for shipping. They even have an indicator on them which is quite handy and could even be helpful as a frequency indicator by way of a small lookup chart. But what if you don’t want to do that?

Like many hams and electronic hobbyists, I like to salvage old electronics. I had a Canon Laser Fax/Printer collecting dust, so I tore every part out of it that looked remotely useful. In it were some nylon gears as part of a reduction drive. I grabbed those gears and also grabbed the plastic shafts they were on, by breaking them off with pliers. The largest drive gear was keyed and exactly the same shaft size as the 10K pot that came with my BITX40. I cut some holes in an Altoids tin and mounted the potentiometer and worked on getting the gears set up. It took some doing, but I now have a 20:1 reduction drive! It really helps to tune in signals, and the little bit of backlash it has is not horrible. But how do I know what frequency it’s on?

By measuring the voltage output by the pot, and calibrating it against another receiver (I used an online SDR) it’s possible to correlate a frequency with a voltage. Finally, a use for that extra Harbor Freight multimeter!

There are many mechanical alternatives that are worth exploring. Many old home stereo tuners used elegant twine and pulley arrangements that were a pleasure to operate, and a similar setup is possible with the BITX40. Your imagination is the limit.

I hope you’ve found this helpful. Post your hacks and interesting links below. 73!