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| SynthUSBII USB RF Signal Generator & CPDETLS-4000 RF Power Detector |
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| TomC:
Hi G0HZU, Thanks for your insightful response, you are obviously well versed on RF technology! I'm just starting to get my feet wet on this fascinating field! --- Quote from: G0HZU on March 27, 2016, 12:50:32 pm ---The detector looks like it is a 50R load with a simple Schottky diode and it probably has a small amount of capacitance at the output. --- End quote --- According to the manufacturer this detector contains a zero bias Schottky and a 100pF output capacitor (which they call video capacitance). The data sheet also identifies the RF input as 50 ohms, but there are no other details as to how this RF load is implemented. My LC meter reads about 6.5nF and my DMM reads open circuit in the 20M range. So if there is a 50 ohm resistor in there it must be connected in series with the capacitor. Maybe something like the circuit on attachment #1? --- Quote from: G0HZU on March 27, 2016, 12:50:32 pm ---The USB powered signal source looks like it is based on the ADF4351 chip and this will produce an output rich in harmonics. You could easily be looking at harmonics only 10-15dB below the main signal. All of your plots appear to be using the detector as a linear detector because of the response curves provided with it. So I think you are going to get confusing results when you connect the two devices together. The high harmonic content can produce high levels of uncertainty in the measurement (with this type of RF detector) and a lot depends on the phase relationship between the signals and also on how hard you drive the detector up in this linear region. The harmonics can make the detector read lower or higher than you might expect. This all assumes that the RF detector is indeed a linear diode detector and (in theory at least) a linear diode detector (used as a power meter) can have approx +/- 20% of uncertainty if you have just one harmonic at -20dBc, or +/- 6% uncertainty if the harmonic is at -30dBc. --- End quote --- --- Quote from: G0HZU on March 27, 2016, 01:04:01 pm ---You can reduce the effect of all this by operating the RF detector at lower power levels such that it operates well below the linear mode. But you would ideally need to be feeding it with RF signals that are equal or smaller to the thermal voltage Vt and this is 25mV which is about -20dBm. The detector will then detect the influence of harmonics 'correctly'. However, down at these power levels the detector will be very sensitive to temperature changes and this would make the detector fairly impractical unless you corrected for temperature somehow. You can often extend this low level (or 'square')detection range by playing with the load resistance at the output of the detector but probably only by 10-15dB. --- End quote --- As you surmised the SynthUSBII is based on the ADF4351 chip, see attachment #2. As you know this chip produces square waves and in the SynthUSBII they are basically applied to the output unfiltered, so the odd harmonics are very prominent (See attachments #3 & #4 for an example at 100MHz). Although I was already aware of the harmonics, I hadn't considered the fact that their effect could be different depending on the instrument used to measure the signal's amplitude. Before you brought this up I was thinking that since the harmonic content was present when the response curve given by the manufacturer was obtained, then, for comparison purposes, it shouldn't matter if it was present during my experiment. As it turns out, the CPDETLS-4000 input to output ratio is frequency dependent. When a particular Vpp level is present at its RF input its DC output increases as the frequency increases for part of its frequency range, then it starts decreasing towards the end of the range. Depending on the harmonic content, as you predicted, this behavior will tend to overstate/understate the true signal level. With this new insight I now see that this behavior is at least partly responsible for the differences between the response curve given by the manufacturer and the response curves obtained during my experiment. On the region of operation of the zero bias Schottky within the CPDETLS-4000 the only clue I have is the datasheet's Output Voltage vs Input Power table (see Attachment #5). Based on this I suspect that it operates on both the square law and linear portions as well as on the transition region in between (readers that may be wondering where these regions lie please see attachment #6). As I understand it, a detector's region of operation can be identified by its output voltage. For a typical zero bias Schottky detector 10mVs or less indicates that it's operating in the square law region, from 10mVs to 100mVs the transition region, and 100mVs or more the linear region (see attachment #7). With this in mind it seems that I may be able to stay in the square law region throughout the entire frequency range by attenuating the SynthUSBII output so that it doesn't exceed about -8dBm (252mVpp). From #5 I can see that at this input power level the highest CPDETLS-4000 DC output is not far from 10mVs. So I plan to try to implement a test using this criterion to see how the shape of the response curve looks like. Perhaps, since the uncertainty introduced by the harmonics will be reduced as you explained, the response plot will be a closer match to the response curve given by the manufacturer. --- Quote from: G0HZU on March 27, 2016, 12:50:32 pm ---Because the ADF4351 output spectrum is so rich in harmonics I think you will have to filter the output before connecting the diode detector to it or the results you get for 'power' will have too much uncertainty to be of any practical use. You really need to use clean sine wave signals with this type of RF detector at the power levels you are using. --- End quote --- Since the response curve supplied by the manufacturer includes the harmonic content, I don't think that filtering the harmonics would result in a closer match. However, for other applications, like checking the response of probes (previous posts) or filters, I totally agree that using clean sine waves would produce results with much less uncertainty. Now I need to start dreaming of a way to implement a tunable low pass filter that can be used to cover the entire frequency range! Attachment #1 - This is a picture of an unrelated evaluation board. I'm using it to illustrate a 50 ohm impedance matching network, this particular implementation seems to consist of a microstrip a coupling capacitor and a resistor. Attachment #2 - This is a picture of the SynthUSBII PCB. Attachment #3 - Here the SynthUSBII is connected to the DSO's CH1 BNC via a 50 ohm feed-through terminator. The SynthUSBII is set to high power at 100MHz. The DSO is set to view the time domain. Attachment #4 - Same setup as #3 but the DSO is set to FFT to view the frequency domain. Attachment #5 - CPDETLS-4000 datasheet page showing the Output Voltage vs Input Power graph and table. Attachment #6 - Illustration of a diode's VI curve where the square law and linear regions of operation are identified. Attachment #7 - Illustration of a typical zero bias schottky diode's input power vs output voltage plot where the square law, transition, and linear regions are identified. |
| G0HZU:
--- Quote ---Since the response curve supplied by the manufacturer includes the harmonic content, I don't think that filtering the harmonics would result in a closer match. However, for other applications, like checking the response of probes (previous posts) or filters, I totally agree that using clean sine waves would produce results with much less uncertainty. --- End quote --- The problem is that the uncertainty from the harmonics is phase dependent and so a harmonic at -10 to -15dBc could cause quite a large window of uncertainty if you operate up in the linear region. For example, if you were to deliberately change the phase of the -10dBc harmonic across a 360degree range then the detector voltage would change quite a bit. Possibly by >2dB in terms of indicated 'power' although it depends on how linear the detector is. So even though the average power of the signals isn't changing in this test and 'only' the relative phase of the signals is changing, the detector will misreport a significant change in power as the phase is changed. As the phase could be fixed anywhere in this range on a 'real' test then you end up with huge amounts of uncertainty in the system if you don't know the phase relationship. If you operate it down into the square law region things will get a whole lot better but then you may run into temperature issues because the detector response will shift wrt temperature quite a bit down in the square law region. You could introduce some passive compensation for this (and recalibrate) but this is still going to be a bit of a fudge. |
| TomC:
--- Quote from: G0HZU on March 31, 2016, 08:24:17 pm ---The problem is that the uncertainty from the harmonics is phase dependent and so a harmonic at -10 to -15dBc could cause quite a large window of uncertainty if you operate up in the linear region. For example, if you were to deliberately change the phase of the -10dBc harmonic across a 360degree range then the detector voltage would change quite a bit. Possibly by >2dB in terms of indicated 'power' although it depends on how linear the detector is. So even though the average power of the signals isn't changing in this test and 'only' the relative phase of the signals is changing, the detector will misreport a significant change in power as the phase is changed. As the phase could be fixed anywhere in this range on a 'real' test then you end up with huge amounts of uncertainty in the system if you don't know the phase relationship. If you operate it down into the square law region things will get a whole lot better but then you may run into temperature issues because the detector response will shift wrt temperature quite a bit down in the square law region. You could introduce some passive compensation for this (and recalibrate) but this is still going to be a bit of a fudge. --- End quote --- Thanks for your detailed explanation! It inspired me to do some more research on the subject and to conduct some experiments. I plan to post the results as soon as I'm done. |
| TomC:
On this post I describe a couple of recent experiments involving the SynthUSBII and CPDETLS-4000. First I re-plotted the SynthUSBII response curve with a 4dB attenuator connected to it's output. The intent was to cause the CPDETLS-4000 to operate in the square law region. Admittedly, the attenuator I had on hand is not rated for GHz frequencies, but even in the low VHF region the results were dismal, a 3dB attenuator didn't do much better. The readings I obtained were erratic and quite a bit different than the levels reported by my DSO (see attachment #0). Next I wanted to verify the accuracy of some of the calibration points given on the CPDETLS-4000 datasheet. The only signal source that I own that is suitable for this task is a 10MHz Dual Channel AWG, so I could only check the 10MHz calibration points. The results seem to indicate that the accuracy of the CPDETLS-4000 starts to degrade when the input power drops below 0dBm. By the time the input power drops to -10dBm the readings can be up to 25% off. Perhaps this has to do with drift caused by temperature changes when operating in the square law region as G0HZU suggested. However, the manufacturer claims an operating temperature range of -20 to 70 degrees Celsius and states that the instrument is suitable for "General Lab Use" (see attachment #1). So I think it would be reasonable to assume that some type of compensation was incorporated. Another contributing factor may be the accuracy of my DMM, the manufacturer claims ±(0.05%+5) on the 200mV range, but I think readings of a few millivolts or fractions of a millivolt are probably not quite that accurate. Attachment #2 shows the results of this experiment. Attachment #0 - SynthUSBII response curves. Orange, SynthUSBII set to power level 3 with a 3dB attenuator connected to its output. Red, SynthUSBII set to power level 3 with a 4dB attenuator connected to its output. I didn't check the full range of frequencies, it seemed pointless given the initial results. Attachment #1 - Page 1 of the CPDETLS-4000 datasheet. Shows the device's features and applications as described by the manufacturer. Attachment #2 - Illustration of the spreadsheet I used to collect the data intended to characterize the CPDETLS-4000 at 10MHz. It contains the tabulated values obtained during the experiment as well as several calculated values. The notes at the bottom explain how the tabulated values were obtained and show the formulas used for the calculated values. The reason why I chose to obtain the tabulated values in the manner indicated by the notes is as follows: First, in my opinion, the calibration points given by the CPDETLS-4000 datasheet represent the values that one would see in a true 50 ohm system. To help me figure out the proper signal amplitude required to check a particular calibration point I used an SMA T to connect CH1 of my AWG to the CPDETLS-4000 and to CH1 of the DSO (see attachment #3). Then I varied the AWG's CH1 amplitude until the CPDETLS-4000 DC output matched the DC value given in the datasheet for that particular calibration point. However, the CPDETLS-4000 doesn't have a consistent 50 ohm impedance, so the DSO's CH1 mVpp readings are not suitable for checking the accuracy of the calibration points. To get around this, once the AWG's CH1 was set to the proper amplitude I copied its settings to CH2. I then connected the AWG's CH2 to the DSO's CH2 via a 50 ohm feed-through terminator (see attachment #3). The terminator provides a consistent 50 ohm impedance, so the DSO's CH2 mVpp readings are suitable for checking the accuracy of the calibration points. Attachment #3 - CPDETLS-4000, DSO, AWG, and DMM connections. |
| G0HZU:
Dunno if this helps but I have a HP 8473C detector here that works up to 26GHz. It looks very similar to the one you have and it uses a simple Schottky diode. However, it is a very expensive device because the diode is packaged to work well up to 26GHz. But apart from this I think the technology is similar to the detector you are using. http://cp.literature.agilent.com/litweb/pdf/5952-8299.pdf If you look at the spec for input VSWR and frequency response this detector is very flat indeed up to many GHz. I also have a couple of Agilent ESGD 4433 (4000) sig gens and the typical flatness spec to 4GHz is very impressive. See below for the datasheet spec for a typical generator of this type. If I connect the 8473C to the sig gen at 100MHz and set the level down at -20dBm this gets me close to square law operation. If I do this the 8473C detector output is about 6.6mVDC. If I turn up the level 1dB it goes to about 8.1mV and if I turn it down 1dB it goes down to 5.25mV DC. If I then sweep across the full 4GHz range of the sig gen I see very little change in the detector voltage it stays at 6.6mV and probably only changes by +/- 0.1mV. So the detector flatness is very good indeed. However, if I cool or heat the 8473C then the detector voltage changes quite a bit. i.e. it's only flat and consistent at a fixed temperature when used down at these tiny signal levels. So it's great for checking flatness but not so great at measuring absolute power levels. Your detector should be similar although the performance in terms of flatness and input VSWR etc will be dictated by the diode package and also by how well it is fitted inside the body of the detector in terms of stray inductance etc. If this is done with a diode package that has significant stray inductance then you might only get good flatness performance up to 500MHz or maybe 1GHz. |
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