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Review By Allan Linton-Smith TOUCHSCREEN 54MHz to 13.6GHz Signal Generator Here's an excellent example of the march of technology. It wasn't that long ago that low-cost ADF4351-based signal generator modules became available. These could generate signal frequencies up to an amazing 4.4GHz. But in a little under two years, the new ADF5355-based modules go up to a whopping 13.6GHz. All this for around and $250, with touchscreen control! T hese modules need no modifications or additional circuitry; just plug in a 5V DC supply or USB cable, attach your cables and go! In this review, we'll describe the ADF5355-based unit and show you some practical applications. This device is based around the Analog Devices ADF5355 Microwave Wideband Synthesizer (with Integrated VCO). It's 84 Silicon Chip paired with an SM32 colour touchscreen for control. This chip has many similarities to the ADF4351 which was reviewed by Jim Rowe in the May 2018 issue, starting on page 82 (siliconchip.com.au/Series/306). However, the module described back then (which cost about $30) was 'bare bones' and had no user interface; you had to build one. That isn't the case here, as Australia’s electronics magazine this unit comes fully assembled and ready to use, as a complete (if basic) test instrument. The ADF5355 module we obtained has four outputs. The middle SMA connector on the right-hand side delivers signals from 54MHz to 6.8GHz, while the bottom connector provides identical signals but 180° out of phase. The top right-hand output (connected to the spectrum analyser in the accompanysiliconchip.com.au ing photo) has a signal which is multiplied by two compared to the other two. The output from this terminal can sweep from 13.4GHz to 13.6GHz in one millisecond, in 100kHz steps (as shown on the screen; remember, the frequencies from this output are doubled). I find that amazing! The ADF5355 achieves this by having an integrated VCO with a fundamental output frequency of 3400-6800MHz. Its output goes to divide by 1, 2, 4, 8, 16, 32, or 64 circuits that allow the user to generate RF output frequencies as low as 54MHz (ie, 3.4GHz ÷ 64). For applications that require isolation, the RF output stage can be muted, with the mute controllable via the touchscreen. Note that you can also purchase similar touchscreen-controlled modules with the lower-spec ADF4351 chip that we mentioned above. These are available from various internet sellers for around $78 including delivery, and are a very convenient option for those who don't need to go above 4.4GHz and just want to plug in a power supply and go! Specifications (as per manufacturer's data sheet) • • • • • • • • Frequency range: 53.125MHz to 6.8GHz (outputs A) and 106.25MHz to 13.6GHz (output B) Accurate from -40°C to +85°C Low phase noise: typically -103dBc/Hz (13.6GHz, 100kHz offset) RF output power: 8dBm at 1GHz, -3dBm at 6.8GHz Programmable output power level: +5dBm to -4dBm Power supply: 5V DC Internal VCO frequency range: 3.4GHz to 6.8GHz Harmonic content: -22dBc (2nd harmonic), -20dBc (3rd harmonic) of phase), it also multiplies the VCO frequencies by two, to create signals from 106MHz to 13.8GHz. These go to RFoutB; see the red box in Fig.1, the block diagram from the device's data sheet. Outside this red box, the block diagram is very similar to that of the ADF4351. We described its operation in detail in the aforementioned May 2018 article, so if you want a more complete description of its operation, please refer to that article. Note that there is no RFoutB- output, possibly because the IC is already overcrowded and the designers were seeking to achieve the maximum frequency for the minimum price. Interestingly, the transistor count for this IC has increased from 36,955 (ADF4351) to 103,665 plus 3,214 bipolar semiconductors. That's 2.8x more transistors – so perhaps there is not much spare room! Block diagram Internally, the ADF5355 is almost identical to the ADF4351, except that the VCO (voltage-controlled oscillator) core operates at higher frequencies, from 53MHz to 6.8GHz. As well as dividing the VCO output frequencies by up to 64 and then sending them to RFoutA+ and RFoutA- (180° out FUNCTIONAL BLOCK DIAGRAM REFIN A REFIN B 10-BIT R COUNTER ×2 DOUBLER CLK DATA LE AV DD AV DD CE DVDD VP RSET We paid $248 including postage for our module – but bearing in mind that even old benchtop GHz generators can cost thousands, that's peanuts! We spotted a 20-year-old 20GHz Anritsu generator on eBay for $16,000. This module is well and truly affordable by comparison! We connected the module's output to a spectrum analyser to check it out (see Fig.2). Sweeping over the 13.6-13.8GHz range, the output level was reasonably flat. We only measured an output level of -5.12dBm (124mV into 50Ω), but that is nevertheless very useful. Note that the spectrum analyser trace is set on "max hold" during the sweep, to give a graph without dips. Resolution at this frequency is 1MHz, so you won't see the 100kHz troughs. The output power from the manufacturer's data sheet, shown in Fig.3, indicates how the output level decreases as VVCO VRF MULTIPLEXER ÷2 DIVIDER MUXOUT CREG 1 LOCK DETECT DATA REGISTER FUNCTION LATCH CREG 2 CHARGE PUMP CPOUT PHASE COMPARATOR VTUNE VREF INTEGER REG FRACTION REG VCO CORE MODULUS REG VBIAS ×2 VREGVCO OUTPUT STAGE THIRD-ORDER FRACTIONAL INTERPOLATOR RFOUTB PDBRF ÷ 1/2/4/8/ 16/32/64 N COUNTER OUTPUT STAGE CPGND AGNDRF SDGND RFOUTA– ADF5355 MULTIPLEXER AGND RFOUTA+ AGNDVCO Fig.1: the ADF5355 block diagram, taken from its data sheet. Aside from the frequency doubler and extra output in the red box, it is similar to that of the ADF4351 chip described in the May 2018 issue. siliconchip.com.au Australia’s electronics magazine May 2020 85 lead and four standoffs for mounting. We highly recommended that you mount the module in a Jiffy box or similar, because all the soldered connections are left bare on the PCB, and these can easily short against a metal object, which may spell D-O-O-M for your $250 signal generator. We were very impressed with the output of this little unit; it very nearly achieves the levels specified in the Analog Devices data sheet. At these high frequencies, even a simple SMA to BNC adaptor can change the signal characteristics, either through power loss or standing waves. This may result in a sweep which is not flat and also create analytical errors when testing devices using various adaptors and connectors. The ADF5355 module we obtained has four outputs, two which go from 53.125MHz to 6.8GHz and are 180° out of phase from each other, and one which goes from 106.25MHz to 13.6GHz in 100kHz steps. All outputs can be swept across a user-defined frequency range over a period of 1ms or more. Usability OUTPUT POWER (dBm) The touchscreen is easy to use but it, and the buttons, are quite small so big fat fingers may upset the settings. The solution is to use some sort of pointer – a pencil will work. It then performs very smartly. No written instructions accompanied our module, but we were able to extract them from a link provided by the seller. However, because they were originally written in Chinese, the English is a bit “lost in Google translation”. While it is quite intuitive to use, the learning curve is still a bit steep. We will give a few hints later. If you purchase one of these, press your seller to include a printed manual, because a lot of the feedback online is complaints about the lack of a manual. It did, however, come with a USB power Fig.2: a spectral analysis of the module's output level when sweeping from 13.4GHz to 13.6GHz. The result is commendably flat, although the overall level is a little low at -5.12dBm (or 124mV into 50Ω Ω). The analysis resolution is 1MHz, so you won't see the 100kHz troughs caused by the stepped sweep. (The test setup used to capture this is shown on page 84). 86 Silicon Chip The manufacturer suggests that this chip could be used for wireless infrastructure, microwave links, satellite comms, clock generation, test equipment and instrumentation. As an example of the latter, we used it to test for cable losses, by connecting the cables between the generator's output and the input of a spectrum analyser, and sweeping up to 2.5GHz. Two three-metre cables were tested, one made of standard coax with BNC connectors and the second, a low-loss 3mm cable with SMA connectors. The results are shown in Fig.5. You can see that the coax cable with BNC connectors loses 12dB more signal than the low-loss cable at 2.5GHz, and this demonstrates that very good cables are required at high frequencies. This module would also be convenient 10 9 8 7 6 5 4 3 2 1 0 –1 –2 –3 –4 –5 –6 –7 –8 –9 –10 –40°C +25°C +85°C 1 2 3 4 5 6 7 FREQUENCY (GHz) Figure 19. Output Power vs. Frequency, RFOUTA+/RFOUTA− Fig.3: graph10pF (from theCapacitors, manufacturer's data sheet) (7.5nH a Inductors, Bypass Board Losses De-Embedded) showing the chip's output power. Note how the output level decreases as the frequency is increased. Australia’s electronics magazine siliconchip.com.au 12714-016 frequency increases. In Fig.3, we've again connected the module to a spectrum analyser (this time a more modern device) and set it to sweep from 100MHz to 6.5GHz. The markers show the amplitude at a few different frequencies over this range; again, the trace was set on "maximum hold" to give a usable graph. Our module was pretty well in line with the specifications, but there are some additional losses at multi-GHz frequencies due to PCB layout, adaptors, connectors and cables. At these dizzy frequencies, you have to be careful to keep signal paths short! Practical applications Fig.5: one practical use for this module (among many) is testing cable losses in combination with a spectrum analyser that lacks a tracking generator. Here you can see that low-loss coax (in blue) loses, err, less signal compared to the box standard coax (mauve). Fig.4: we set up the module to sweep from 100MHz to 6.5GHz and plotted the resulting output level on a slightly more modern spectrum analyser. It was set on "maximum hold" to give a better result. These results are not far off what Analog Devices specifies for the chip, although some small losses in the board are apparent. for testing frequency counters and similar devices. Naturally, there are many other ways to use this module, and we leave that up to your imagination! The future! If progress continues at this pace, in two years we should have an affordable 50GHz signal generator IC. Already as we write this, there is a Texas Instruments signal generator IC available which has a higher output power and better stability than the ADF5355. It is the LMX2594 15GHz Wideband PLLatinum™ RF Synthesizer with phase synchronisation and JESD204B support. No doubt there will be bigger and better to come! SC Some useful tips • Our supplier advised us not to remove the screen cover, as the screen surface is easily scratched. • Mount the module in a box of some sort to protect it. • Always stop the sweep before changing any values. • Always set the start freque ncy before the stop frequency. • After using the keyboard, press Enter, then Back, and keep pre ssing Back until the keyboard disapp ears. • You cannot set the resolu tion to less than 100kHz. • You can increase the sw eep time, but the minimum is 1ms. The ADF5355-based touchscreen module at right, and the cheaper ADF4351-based touchscreen module at left. You can see that the module at right has more output connectors and a slightly different user interface. siliconchip.com.au Australia’s electronics magazine May 2020 87