Add-on beat frequency oscillator (BFO) for shortwave radio

Expensive shortwave radios include beat frequency oscillators, so why not also add one to an inexpensive radio version?

Broadcast shortwave (SW) radio use may be declining, but there are still lots of interesting signals to listen to between 1.6 and 30 MHz, such as amateur radio (Reference 1), pirate radio stations (Reference 2), aeronautical weather reports (Reference 3) and long-distance aircraft communications (Reference 4). Unfortunately most of these use Single Side Band (SSB) as opposed to the Amplitude Modulation (AM) used by broadcasters, which cannot be received on cheap domestic SW radios.

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AM consists of a radio frequency (RF) carrier wave (in red) which is partially modulated by the information – speech or music – (green), as shown in Figure 1a. The time domain signal in Figure 1a can alternatively be viewed in the frequency domain as in Figure 1b, where the carrier is surrounded by two smaller sidebands containing the information. A diode detector can demodulate this signal. In the time domain, the approach can be regarded as rectification with a low pass filter. In the frequency domain, the non-linear action of the diode mixes the carrier with the sidebands, therefore down-converting it back to an audio signal.

Figure 1 This sequence of images shows AM in the time  (a) and frequency domains (b), along with SSB in the frequency domain (c).

SSB lacks this carrier, with only one sideband as shown in Figure 1c. To demodulate SSB, an external carrier signal must be added; the resulting signal can then be demodulated with a diode detector like AM. This additional carrier is sometimes referred to as a Carrier Insertion Oscillator (CIO) or Beat Frequency Oscillator (BFO). Expensive SW radios include a BFO, so why not try and add one to a cheap one?

SSB demodulation

Many SW radios for SSB published in hobbyist magazines apply a high amplitude BFO signal to the input of the diode detector alongside the intermediate frequency (IF) signal, as shown in Figure 2a. The high BFO amplitude tends to overload any Automatic Gain Control (AGC) loop. Hence, in this type of receiver the AGC loop is disabled and the radio’s dynamic range is alternatively managed by manual RF and/or IF gain control.

Figure 2 These block diagrams show shortwave radios with BFO (a) and low-level BFO injection (b).

Adding a BFO to an existing radio in the way shown in Figure 2a would involve substantial modification to the radio to disable the AGC loop and integrate a manual RF gain control, if not already present. A second disadvantage with this technique is that because the IF signal has a large amplitude at the input of the detector, it can leak into the BFO and modulate it, therefore introducing distortion to the point of making the signal unintelligible.

The Eddystone EB35 radio from the late 1960s (Reference 5) overcame the leakage issue by injecting a low-amplitude BFO signal into the input of the IF amplifier chain. It does however still disable the AGC and instead rely on manual RF gain control.

Proposed here is a method of injecting the BFO at low amplitude into the input of the IF amplifier chain and controlling its level so as to best match the received signal when the existing AGC loop is operational, as shown in Figure 2b. The BFO with level control can be built into an external box with minimum modification, hence is easy to incorporate and then later remove from a valuable vintage radio receiver.

External BFO

The schematic of the BFO is shown in Figure 3. The oscillator is a Hartley type where a tap on transformer T₁ provides the feedback. This is generally at 25% of the winding. T₁ is a recycled IF transformer from an old transistor radio. Most low-cost radios like those shown in Figure 2 use an IF frequency between 455 and 470 kHz.

Figure 3 In this BFO schematic, VR₁ is the pitch and VR₂ is the level.

TR₁ is a BC548 transistor, which although not known for its high frequency performance has sufficient gain at 455 kHz. The IF transformer includes a capacitor to set its operating frequency. To operate effectively as a BFO, the frequency needs are varied over a small range of a few kHz. This is achieved with a varactor diode D₁, which here is half of a 1 A bridge rectifier (Reference 6) and controlled by VR₁.

Level control is provided by VR₂. The level of the carrier is reduced further by selecting a low values for C₅ of 5pF. The values of C₂ and C₃ are not very critical, but past experience has shown that their optimum values are:

where f_R is the operating frequency in MHz and C₂ = C₃ in pF. This is based on the optimum capacitor values for a Colpitts oscillator (Reference 7). The BFO is designed to run off 5V, so a voltage regulator (Reg₁) is included. A 78L05 could be used, but in this application the radio used—a Realistic Patrolman SW60—had a 6 V supply, so a low-drop out K5050 regulator was used instead.

A photograph of the BFO built in a small plastic box and stuck to the back of the SW60 is shown in Figure 4. An additional hole is drilled in the box so that the core of T1 can be tuned. Although the SW60 is based on NPN transistors, it is a positive ground design, so the screen of the coaxial cable is connected to the input of Reg₁ and the purple wire is ground. Another version was built for use with a WWII-era PCR radio where the cables passed through an unused headphone jack socket—requiring no modification to the case.

Figure 4 In this photograph of the BFO attached to the SW60, the top control is BFO level and off, the bottom is BFO pitch, and T₁ is the metal can on the right.

Operation

When first used, the radio should be tuned between stations so only background white noise in heard. The BFO level control should be set to maximum. Doing this may cause a change in the sound of the noise. It is likely that the BFO will be operating outside of the IF bandwidth and will need to be tuning in.

Set the pitch control to approximately midpoint and with a trimmer tool, adjust the core of T₁. As the BFO is tuned across the IF bandwidth, the general tone of the noise will drop and then increase on the other side. Generally, it should be adjusted so that the tone is at its lowest. Turning the pitch control should now cause the tone of the noise to rise on either side. At this point the level control can be backed off as it is probably driving the AGC loop. The optimum level control point is when the background noise is at maximum volume.

Tuning the radio to somewhere in the 7-7.3 MHz range in the late afternoon and evening should pull in stations in the 40-meter amateur band. If the signal sounds distorted, trying increasing the BFO level and the fidelity should improve. Adjusting the pitch control will of course affect the pitch of the received signal. When these two controls are correctly adjusted, the received signal fidelity is very good.

References

1. Amateur Radio bands: https://en.wikipedia.org/wiki/Amateur_radio_frequency_allocations
2. Shortwave Pirate Radio Stations: https://www.hfunderground.com/wiki/List_of_Pirate_Radio_Frequencies
3. VOLMET frequencies: https://dxinfocentre.com/volmet.htm
4. Aeronautical HF Radio: https://swling.com/blog/2015/09/aeronautical-hf-radio-map/
5. Eddystone EC10 Receiver: https://eddystoneusergroup.org.uk/Manuals/Eddystone%20EC10%20Manual.pdf
6. Rectifier diodes as varicaps (varactors): https://www.hanssummers.com/varicap/varicapdiode.html
7. NE602 Application note: https://www.scribd.com/document/46152488/Single-Chip-Frequency-Converter

Gavin Watkins is the founder of GapRF, a producer of online EDA tools focusing on the RF supply chain. When not doing that, he is happiest noodling around in his lab, working on audio electronics and RF projects, and restoring vintage equipment.

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