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Using Cheap Asian Electronic Modules Part 18: by Jim Rowe 500MHz frequency counter and a wideband preamp This month we look at two more low-cost RF/UHF modules. One is a tiny digital counter module which can operate up to 500MHz with a resolution of 0.1kHz. The other is a low-noise wideband amplifier module. The two modules can be combined to make a very sensitive frequency counter. F irst, let’s look at the 500MHz frequency counter module. It’s pretty small, with the PCB measuring only 58 x 32mm; exactly the same size as the backlit 8x2 LCD display board it’s mounted behind. The whole assembly measures only 58 x 40 x 28mm, including the SMA input connector mounted on the underside of the counter PCB. A subminiature on/off slider switch is fitted on the right-hand end of the same PCB, with the ends of a standard 9V battery clip lead attached nearby. Before we go any further, I should note that the slider switch in the counter module pictured turned out to be very flimsy, with the actuator falling out after being used only a couple of times. Rather than try and fix it, I removed the rest of the switch (which is why it’s missing in the pictures) and used a small toggle switch off the PCB to perform the same function. 82 Silicon Chip Fig.1 shows the full circuit and IC1, an ATmega48PA microcontroller, does most of the work. As well as doing the frequency counting, it also displays the result on the LCD module. The other IC to its left (IC2) is obviously a prescaler but I can’t find any real information on it; 5064N06 is what is marked on its IC package (it looks to be pin-compatible with the MB506 prescaler IC). By measuring its input and output frequencies, I determined that it is a 64:1 prescaler, so IC1 only needs to measure frequencies up to 7.8125MHz (500MHz ÷ 64), which is well within its capabilities. IC1 uses a 20MHz crystal (X1) for both its master clock and its counting timebase. To allow adjustment of the exact frequency for calibration of the counter, the module’s designers have provided a 5-40pF trimmer cap to “pull” its frequency. Australia’s electronics magazine At first, it appears that the 8x2 LCD module has no connections to its builtin LED back-lighting but these are presumably made inside the module. There’s no trimpot to adjust the LCD contrast but the default contrast seems to be fine. There’s provision on the counter PCB for a 6-pin header (shown at lower left in Fig.1) with the same connections as used for the ICSP connector on most Arduino MCUs. This would allow you to reprogram IC1 if you wish. There’s also provision on the counter PCB for three 2-pin headers for jumper shunts (J1-J3) but I haven’t been able to find any information on their function. All of the counter circuitry runs from 5V DC, derived from the 9V battery via REG1, a 78L05 regulator. The total current drain measured 57mA, much of which would be for the LCD backlight. Therefore it would be a siliconchip.com.au Fig.1: IC2 is marked 5064N06 and is most likely a variant of the MB506 prescaler IC. The MB506 can divide its input frequency by 64, 128 or 256, and is set to a division ratio of 64:1 by connecting SW & SW2 to Vdd. The 6-pin header is not fitted, but can be installed if IC1 needs to be reprogrammed. good idea to power it from a 9V alkaline battery or a 9V DC plugpack. You could even use a 5V USB charger with its output wired directly to the output of REG1. Trying out the counter I powered the module using a 9V alkaline battery and connected its input to the 10.000000MHz output from a GPS-disciplined Rubidium vapour frequency standard. Then I adjusted the frequency reading using the 5-40pF trimcap on the counter PCB. The adjustment was fairly critical and the closest reading I was able to achieve was 10.0002MHz, ie, 20ppm or 200Hz high. That’s quite reasonable. I then checked its operation over the full range of frequencies it claims to handle, using my Gratten GA1484B signal generator. With the generator’s output set to 0dBm (224mV RMS), there were no problems measuring frequencies from 500MHz down to about 8MHz. Below 8MHz, I found that the siliconchip.com.au Fig.2: input sensitivity for the 500MHz frequency counter module over its full range. Below 1MHz no reading was recorded with an output level of +13dBm. Australia’s electronics magazine July 2018  83 Fig.3: complete circuit for the low-noise wideband amplifier module. It’s a simple design incorporating a single IC (N02), which is very similar to an ERA-2SM+, in a 4-pin Micro-X package. signal level had to be increased somewhat to get a correct reading. In fact, for frequencies below 3MHz I needed to crank up the generator’s output to its maximum level of +13dBm (1.00V RMS); even so, I got no reading below 1MHz. I then measured the input sensitivity for reliable readings over the range from 1MHz to 500MHz and the resulting plot is shown in Fig.2. The effective input sensitivity is below -15dBm (40mV) for all frequencies above 25MHz, falling to around -19dBm (25mV) at 500MHz. But it rises fairly steeply at lower frequencies to reach 0dBm (224mV) at around 7.5MHz and climbs further to +13dBm (1.00V) at 3MHz. So although the mini 500MHz counter module is claimed to be able to operate down to 100kHz, its useful range is really from 1MHz to 500MHz. At this point, I decided to try fitting three pin headers to the pads marked JP1-JP3. Shorting JP1 did not appear to have any effect on the readings. JP2 caused the readout to only display 0MHz. This might be some sort of disabling or gating function for the counter. Fitting JP3 causes the value displayed to be about 95.45% of the actual value. Overall these jumpers may be for a feature that didn’t make it into the final product. Mounting it in a case Since its performance is quite good, I decided to build it into a UB3 Jiffy box, which is large enough to also house the 9V battery, making it a selfcontained portable instrument. I mounted the module itself behind the box lid/front panel using 9mm long untapped spacers and 15mm long M3 screws, replacing the original four very short screws on the top of the module. I cut a 38 x 18mm rectangular window in the lid for the LCD and mounted a small toggle switch below it for 84 Silicon Chip on/off switching. I then drilled a 10mm diameter hole at the back for access to the module’s SMA input connector. A strip of sturdy gaffer tape was also used to hold the battery securely in one end of the box. You could build the module into an even smaller UB5 Jiffy box (83 x 54 x 31mm) if you don’t need to include the 9V battery for fully portable operation. Despite its flimsy on-board on/off switch, the 500MHz frequency counter has the potential to be quite useful for many applications. They’re priced at $19 from Banggood (siliconchip. com.au/link/aak3). You can also find them on eBay or AliExpress for around $15 or less. Low-noise preamplifier Next up is a low-noise amplifier module. Its PCB measures 32.5 x 24.5mm, with SMA input and output connectors at each end and pads for a mini 2-way terminal block for power along one side. The circuit for the module is shown in Fig.3. The amplification work is done by the “NO2” IC, which is similar to the Mini-Circuits ERA-2SM+ device used in our recent UHF Prescaler (siliconchip.com.au/Article/10643; May 2017) and 6GHz Frequency Counter (siliconchip.com.au/Series/319; October-December 2017) projects. It’s in the same kind of 4-pin MicroX package and the circuit of Fig.3 is virtually identical to the recommended circuit for the ERA-2SM+. To check out the module’s performance, I connected it to a 9V regulated supply (it draws around 40mA) and linked its RF output to an Agilent V3500A RF power meter. Then to check its noise performance I terminated its input with 50W and measured its output over the module’s claimed range of 0.1MHz-2GHz and beyond (up to 4GHz, in fact). The results of this first test are shown in the blue curve of Fig.4, with Australia’s electronics magazine the noise level axis on the right. The module’s noise level is close to -50dBm across the entire range so it qualifies as a low-noise amplifier or “LNA”. For the frequency response, I drove the input with my Gratten GA1484B signal generator, using a short SMA cable and a T-connector at the input with a 50W terminator. I ran the signal generator from 0.1kHz to 4GHz with its output level set to -30dBm. I ran the same test with just the test cable and subtracted the cable loss from the earlier results, giving the red curve in Fig.4, which corresponds to the gain axis. This shows a gain figure of around 30dB up to 1GHz, dropping to 24dB at 2GHz, then to 16dB at 3GHz and a whisker less than 12dB at 4GHz. So the module provides a useful amount of gain up to 2GHz. Finally, I did some measurements to see the input signal levels that the module could handle before compression took place. I actually used a second module for this testing, and the second module turned out to have lower gain than the first, by about 3dB. That’s why the levels shown in Fig.5 are all a little lower than in Fig.4. At just about all frequencies, the maximum input level without compression is close to -20dBm, or 22.4mV across 50W. Above this, gain falls away. So it’s better to think of it as a lownoise preamplifier rather than a power amplifier. They are available from Banggood (siliconchip.com.au/link/aak4) for around $10 each, or even less on eBay. You’ll pay more for a pair of edgemount SMA connectors! Teaming it up with the frequency counter module Since both modules can be run from 9V DC, you could power them from the same supply, although the combined current draw of nearly 100mA is on the high side for a 9V battery. siliconchip.com.au Silicon Chip Binders NOW PRICED AT $19.50 * PLUS P & P Fig.4: the blue curve represents the noise output of the preamplifier module when terminated with 50W, from 10MHz to 4GHz. The red curve shows the gain of the preamplifier over the same range. Are your copies of SILICON CHIP getting damaged just lying around in a cupboard or on a shelf? Can you quickly find a particular issue that you need to refer to? Keep your copies safe, secure and always available with these handy binders These binders will protect your copies of SILICON CHIP. They feature heavy-board covers, hold 12 issues & will look great on your bookshelf. H 80mm internal width Fig.5: shows the input signal levels the module could handle before compression took place. Note that a second module was used with these tests, one which had a gain about 3dB lower than the module used for the tests in Fig.4. You would definitely need to use an alkaline 9V battery if you don’t want to power them from a plugpack. It’s simply a matter of using a short SMA patch cable to wire the output of the LNA to the input of the frequency counter and you will have a counter with a sensitivity of around -40dBm from 20MHz to 500MHz, falling to -30dBm at 10MHz, -20dBm at 6MHz and around -10dBm at 3MHz. This would mean, for example, that you could connect a whip antenna to siliconchip.com.au the LNA input and “sniff” the transmission from an RF transmitter which operates in the 10-500MHz range by simply bringing the two antennas close SC together. The 8x2 LCD on the 500MHz frequency counter module will display up to 4 digits of precision. Australia’s electronics magazine H SILICON CHIP logo printed in gold-coloured lettering on spine & cover Silicon Chip Publications PO Box 139 Collaroy Beach 2097 Order online from www. siliconchip.com.au/Shop/4 or call (02) 9939 3295 and quote your credit card number. *See website for overseas prices. July 2018  85