TelephoneBill
Well-known member
There are some impressive PCBs on the market these days. Various low cost “copy” Evaluation Boards in particular offer tremendous functionality. I was therefore delighted recently to stumble across the ADF4351 board on eBay, which is a signal generator PCB capable of generating all frequencies from 34 MHz to 4.4 GHz. I always wanted to play with GHz, just for the sheer fun, and at under $20 (including postage) this was too good to miss.
This evaluation board gives direct access to registers inside the ADF4351 chip. So using a Teensy 3.1 as the master controller, I hooked these two boards together via the SPI bus, and that gave me a very wide ranging signal source that was immediately under my own fine program control.
However, one problem immediately arises when you begin to play around with GHz type frequencies – how can you examine what is actually happening inside an experiment in real time? Such extreme frequencies quickly get beyond the power of ordinary 100 MHz scopes to display events. So you need some alternative method of signal monitoring. That is where another evaluation board – the AD8317 – comes in. This little $13 beauty will accept suitable input signals across the range 1 MHz to 10 GHz via an SMA connector and coax cable, and will output a d.c. voltage proportional to the amplitude of the input signal. The output can either be read directly on a simple multimeter, or some other low frequency display if you prefer.
The AD8317 is a very sensitive logarithmic signal detector. It can detect signals down to almost millivolt amplitudes peak to peak (-55 dBm) or up as high as hundreds of millivolts (0dBm). Also, the d.c. output from it is relatively independent of the input signal frequency, so has a nearly flat response over a very wide range. It can therefore easily measure the highest GHz frequency available from the ADF4351 and across a wide range of amplitudes.
These two boards working together form a great GHz experimental pair and they both accept +5 volts as a power supply. I use the Teensy 3.1 USB supply for the ADF4351 and a 12 volt lead acid battery into a 7805 regulator for the AD8317.
Experiment (1) – Test the true bandwidth of a Rigol DS1104 100 MHz scope:
In the first picture below you see the ADF4351 mounted on plywood, together with a Teensy 3.1 controller. Three orange wires implement the SPI bus connection. There are two output SMA connectors. I have one SMA (Out+) coupled via thin RG316 50 ohm cable to the AD8317 and the other (Out-) coupled via black RG58 to a 50 ohm termination resistor soldered to the cable inner and outer ends. Across the resistor is the scope lead with its probe set to X10. These two outputs have identical amplitude signals but the second is 180 deg inverted wrt to the first. The frequency was slowly increased in 1 MHz increments above 100 MHz until the signal on display started to peak and then drop rapidly. It was then backed off to the point where it peaked at 139 MHz and the snapshot taken.
With the AD8317 detector also connected, I was able to monitor the signal level from the ADF4351 generator independently at the same time by observing the output d.c. voltage. You can see this in the picture on the multimeter as 0.523 volts. Even when the frequency increased beyond 139 MHz and the displayed amplitude was dropping, the d.c. voltage measured remained constant. This gave me confidence that the reduction was down to the scope and its probe and not due to any significant change in the generator output. The X10 probe is quoted as 7pF input capacitance, which at 139 MHz equals a reactive impedance of 163 ohms. This will affect the presented 50 ohm resistive load to some extent but not explain the sudden rapid drop seen at 141 MHz and beyond (which was dramatic).
I conclude therefore that the Rigol DS1104 has a true bandwidth of 140 MHz using my cheap and cheerful probes which came with it. The probes spec itself puts them at 150 MHz.
I was also keen to know that the AD8317 readings could be trusted. So I separately plotted a graph of signal level against d.c. output voltage at a much lower frequency of 1 MHz (and other frequencies up to 25 MHz). This graph is shown below. The remarkable thing was that a d.c. voltage of 0.523 corresponds on my graph to about -7 dBm. Looking up the peak to peak voltage on a table for -7dBm it gives 282 mV pk-pk. The scope display showed 290 mV pk-pk at 139 MHz.
This is obviously “beginner’s luck” to get such close agreement (at 1 MHz and 139 MHz), but it is reassuring to know that the AD8317 is not in significant disagreement with the scope display below the frequency 3 dB point.
In another post, when I have tidy-ed it up a bit, I will show my Teensy code for directly controlling the ADF4351. Not only can you set the frequency, but you can alter the output power levels too – in four discrete jumps. I’m using the lowest power setting in the above experiment. I’ll also include another interesting experiment. This time at 1 GHz with some amplitude modulation.
This evaluation board gives direct access to registers inside the ADF4351 chip. So using a Teensy 3.1 as the master controller, I hooked these two boards together via the SPI bus, and that gave me a very wide ranging signal source that was immediately under my own fine program control.
However, one problem immediately arises when you begin to play around with GHz type frequencies – how can you examine what is actually happening inside an experiment in real time? Such extreme frequencies quickly get beyond the power of ordinary 100 MHz scopes to display events. So you need some alternative method of signal monitoring. That is where another evaluation board – the AD8317 – comes in. This little $13 beauty will accept suitable input signals across the range 1 MHz to 10 GHz via an SMA connector and coax cable, and will output a d.c. voltage proportional to the amplitude of the input signal. The output can either be read directly on a simple multimeter, or some other low frequency display if you prefer.
The AD8317 is a very sensitive logarithmic signal detector. It can detect signals down to almost millivolt amplitudes peak to peak (-55 dBm) or up as high as hundreds of millivolts (0dBm). Also, the d.c. output from it is relatively independent of the input signal frequency, so has a nearly flat response over a very wide range. It can therefore easily measure the highest GHz frequency available from the ADF4351 and across a wide range of amplitudes.
These two boards working together form a great GHz experimental pair and they both accept +5 volts as a power supply. I use the Teensy 3.1 USB supply for the ADF4351 and a 12 volt lead acid battery into a 7805 regulator for the AD8317.
Experiment (1) – Test the true bandwidth of a Rigol DS1104 100 MHz scope:
In the first picture below you see the ADF4351 mounted on plywood, together with a Teensy 3.1 controller. Three orange wires implement the SPI bus connection. There are two output SMA connectors. I have one SMA (Out+) coupled via thin RG316 50 ohm cable to the AD8317 and the other (Out-) coupled via black RG58 to a 50 ohm termination resistor soldered to the cable inner and outer ends. Across the resistor is the scope lead with its probe set to X10. These two outputs have identical amplitude signals but the second is 180 deg inverted wrt to the first. The frequency was slowly increased in 1 MHz increments above 100 MHz until the signal on display started to peak and then drop rapidly. It was then backed off to the point where it peaked at 139 MHz and the snapshot taken.
With the AD8317 detector also connected, I was able to monitor the signal level from the ADF4351 generator independently at the same time by observing the output d.c. voltage. You can see this in the picture on the multimeter as 0.523 volts. Even when the frequency increased beyond 139 MHz and the displayed amplitude was dropping, the d.c. voltage measured remained constant. This gave me confidence that the reduction was down to the scope and its probe and not due to any significant change in the generator output. The X10 probe is quoted as 7pF input capacitance, which at 139 MHz equals a reactive impedance of 163 ohms. This will affect the presented 50 ohm resistive load to some extent but not explain the sudden rapid drop seen at 141 MHz and beyond (which was dramatic).
I conclude therefore that the Rigol DS1104 has a true bandwidth of 140 MHz using my cheap and cheerful probes which came with it. The probes spec itself puts them at 150 MHz.
I was also keen to know that the AD8317 readings could be trusted. So I separately plotted a graph of signal level against d.c. output voltage at a much lower frequency of 1 MHz (and other frequencies up to 25 MHz). This graph is shown below. The remarkable thing was that a d.c. voltage of 0.523 corresponds on my graph to about -7 dBm. Looking up the peak to peak voltage on a table for -7dBm it gives 282 mV pk-pk. The scope display showed 290 mV pk-pk at 139 MHz.
This is obviously “beginner’s luck” to get such close agreement (at 1 MHz and 139 MHz), but it is reassuring to know that the AD8317 is not in significant disagreement with the scope display below the frequency 3 dB point.
In another post, when I have tidy-ed it up a bit, I will show my Teensy code for directly controlling the ADF4351. Not only can you set the frequency, but you can alter the output power levels too – in four discrete jumps. I’m using the lowest power setting in the above experiment. I’ll also include another interesting experiment. This time at 1 GHz with some amplitude modulation.