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Review by Allan Linton-Smith 2W 930MHz RF Amplifier + RF Wattmeter You might think that 2W is not much power for an amplifier, but around 65 years ago, the first artificial satellite (Sputnik) was launched with, you guessed it, a 2W RF transmitter onboard. And it generated signals that were heard around the world. So what can you do with 2W? T his handy little RF amplifier module is rated at 2W for VHF applications between 1MHz and 930MHz. It has many applications, including boosting FM radio signals in poor reception areas. The module described here was purchased under the title “RF Broadband Power Amplifier Module for Radio Transmission FM/HF/VHF 1-930MHz 2W (Version L1C)” from eBay for $21.62 (including delivery). However, several competing suppliers are now offering similar devices at even lower prices. It is suitable for all types of radio use, such as shortwave FM radio remote control, FM radio, amateur radio in the 135-175MHz or 380-470MHz bands etc. With the recommended input signal level of 0dBm, the output power is 2.0W up to 500MHz, 1.6W at 512MHz, 1.0W at 930MHz or 0.8W at 1GHz. The input and output connectors are standard SMA female RF sockets, while 12V DC power is supplied via a pair of solder pads. At 2W output, it draws around 400mA for an input power of 4.4W. The preamp stage accounts for 110mA, meaning the power amplifier consumes close to 290mA, making it about 57.5% efficient; around what you’d expect for a linear amplifier at full power. siliconchip.com.au Features The amplifier module comes fitted to quite a good heatsink which will ensure reliability for long-term use at maximum power. During prolonged testing, the measured module temperature never exceeded 40°C and was usually just warm to the touch. As you would expect from the different power figures listed above, the overall gain varies with the signal frequency. It’s around 30-34dB for a 0dBm input up to 350MHz or 20-23dB for frequencies between about 350MHz to 950MHz. It should be noted that these gains are not entirely linear; lower input signals result in higher gain figures. For example, a -46dBm 15MHz input signal gives an output of 0dBm, meaning the actual gain, in this case, is 46dB. At higher output levels, close to 2W, the resulting distortion (THD) figure is up to 20% because it is running into clipping. With that in mind, it would be wise to operate the amplifier at reduced levels to avoid radio interference from the distortion harmonics. The distortion performance before clipping is pretty good at around 1% THD, as shown in Fig.1. Circuit details The circuit of this module is shown in Fig.2. It is pretty straightforward, using an SBB2089Z IC as a preamplifier, powered by a 78L05 5V linear Fig.1: the module’s THD before clipping was measured by feeding a 15MHz signal at -46dBm to the RF input, resulting in a 0dBm output. The first harmonic (at 30MHz) can be seen here at -40dBm, along with some noise from external radio interference. The starting point for the graph is 12MHz, with the end point at 35MHz. Steps are in 10dB for level (vertical) and 2.3MHz for frequency (horizontal). Australia's electronics magazine January 2023  27 Fig.2: both the preamp and power amp chips are three-terminal devices with an input pin, an output pin and ground pin. They are fed with supply current through the output pins, via inductors which present a high impedance at the signal frequency, so they don’t attenuate the signals. regulator. This then feeds a KB042 power amplifier via a 470pF coupling capacitor. The values of both coupling capacitors (both at the input and between the preamp and amplifier) are relatively high. This is so it can accommodate frequencies down to 100kHz. The manufacturer’s recommended value for the SBB2089Z is around 8.2nF for its specified frequency range, from 50MHz to 850MHz. The data sheet for this device indicates that its gain is relatively flat over that frequency range, but I found that the gain was higher from 1MHz to about 50MHz, and lower at 930MHz than I expected. The SBB2089Z draws around 110mA from the 78L05 – more than its recommended maximum, but within its capabilities at any realistic device temperature. There is no data sheet available for the KB042 power amplifier, but we think it operates similarly to the ERA-2SM+. It is powered directly from the 12V supply via another AC-­ blocking inductor. Different versions of this board available online seem to use either 33µH or 68µH inductors in series with SMD ferrite beads. Presumably, those inductance values are not critical to its performance. Note that the +12V supply directly feeds the KB042; therefore, the applied voltage should not exceed 15V; otherwise, the KB042 may blow. It’s best to use a 12V DC regulated supply but you could probably get away with a 12V lead-acid battery. One slightly unusual feature of the circuit is the bias network from the output of the 78L05 to the input of the KB042. The 10kW/5.1kW divider generates a DC bias of about 1.7V, which is applied to the signal via a 100W resistor. Consider that many RF power amplifiers are based on Darlington transistors, and 1.7V is a little more than two base-emitter junction Fig.3: the output response plot for the 2W RF amp over 0-350MHz with a swept input signal at -30dBm. The series of dips are caused by standing waves in the measurement and not the amplifier itself. 28 Silicon Chip The 50W dummy load that was used to test the amplifier module. forward bias voltages. Either this is needed to bias the KB042’s internal transistors into their operating range or (more likely, we think) the intent is to supply additional base current to allow the amplifier to deliver more power before it runs into clipping. The PCB design is pretty much according to the manufacturer’s recommendation for the first IC and the KB042 IC is tacked on to provide the specified output power. Testing I tested this module feeding into a 50W dummy load so that I could make the measurements in dBm. The amplifier has a high internal impedance, so a suitable resistor must be used that can handle the power levels with minimal reactive impedance to maintain a constant resistance at high radio frequencies. I used a specialised resistor (EMC 5307ALN) which can handle 125W from DC-2GHz. I got this from eBay Fig.4: a similar plot to Fig.3 but for a higher frequency range, 240-960MHz. The output level starts to fall off near 400MHz. There are even more dips this time; again, they are artefacts of the measurement system, not the amp. Australia's electronics magazine siliconchip.com.au resulted in somewhat lower output levels as expected. The 2W transmitter “that changed the world” Fig.5: the RF Power Meter is a simple device but very useful nonetheless. Again, we had to trace out the circuit. We couldn’t get to the range switching components to see how they were configured, so they’re not shown here. for $23, including postage, and it performed well, hardly getting warm at 2W. I mounted this resistor inside a 4 x 4cm aluminium housing weighing 56g, although it can be bolted to a larger heatsink or fan for higher power handling. Frequency response Fig.3 shows the amp’s frequency response over 0-350MHz, while Fig.4 is a response plot over 240-960MHz. I did it in two plots since you can see the details better this way. Both were made using a tinySA spectrum analyser with the sweep generator set at -30dBm. This resulted in an output from the amp ranging from -10dBm to +4.2dBm (analyser set to max hold), which is close to the specified performance. The dips in the graph are mainly due to standing waves in the load resistor and the cable to the spectrum analyser. The actual response of the amp would be somewhat flatter than this. Surecom SW-11 RF Wattmeter I also purchased this RF wattmeter to check out the RF amp performance. Measuring just 85x50x55mm, it can handle up to 100W of RF power and has a low range of 10W. Also, it can be switched to SWR (standing wave ratio) mode for analysing antenna characteristics. Its circuit is shown in Fig.5. It is a passive device with fairly straightforward circuitry, which should be capable of accurately measuring RF power up to approximately 400MHz. However, Fig.5 does not show the range switching circuitry built into the unit because it’s virtually impossible to access without destroying the thing. I fed the output of the 2W RF amplifier to the SW-11 together with the EMC 5307ALN 50W RF dummy load. It indicated a maximum output of 2W for an input signal of -30dBm at 20MHz. Higher input levels did not increase power output, and higher frequencies We mentioned the first artificial satellite, Sputnik (1957), in the introduction because it also featured a 2W RF transmitter. For those who are interested, Fig.6 shows its circuit diagram. This was only made public in 2016 by a Russian leaker! It produced a CW signal at 20MHz using a modified Colpitts oscillator and a push-pull output stage. The valves are sub-miniature types, powered by a bank of batteries. A second transmitter was also fitted, which operated at 40MHz. The one-second “beep” was supposedly controlled by an external vibrator (likely via the “controller” input at upper right). Warning We envisage readers possibly using this amplifier module to boost received signal levels within their homes or offices. Radio amateurs could potentially use it as part of a transmitting rig. But keep in mind that unless you have some sort of radio license, transmitting at just about any frequency at 2W is illegal in Australia and New Zealand. It probably isn’t a good idea to connect an antenna to this device’s output unless you know it is legal and safe to do so. We are only aware of the exceptions outside this device’s frequency range, in the 2.4GHz & 5.8GHz bands, and only for frequency-­hopping or digSC itally modulated transmitters. Fig.6: the valve-based 2W 20MHz transmitter that flew on Sputnik, the first artificial satellite, in 1957. Compare this to Fig.2; it’s significantly more complex (although it does incorporate an oscillator) and no doubt would have cost the equivalent of many thousands of today’s dollars (rubles?). Australia's electronics magazine January 2023  29