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SynthUSBII USB RF Signal Generator & CPDETLS-4000 RF Power Detector

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TomC:
My old Heathkit RF SG tops at 110 MHz and for a while I've been wanting to upgrade so I can experiment with higher frequencies, but my wife would have my head if I invested on a top of the line HP for my hobby! :phew: Even an older used model from eBay! 

So when I saw this Windfreak SynthUSBII RF SG at Saelig for $199 during Cyber week I decided to take a chance and see what it could do!

Attachment #1: This is a picture of the SynthUSBII obtained from the link below.

https://windfreaktech.com/product/usb-rf-signal-generator/

The unit is very small, about 5 x 2.5 cm and is controlled by the included software. One caveat is that although you can set the power output to one of four levels, there is no way to know with reasonable accuracy what those levels are.

Attachment #2: According to the manufacturer the SynthUSBII frequency response should be similar to this. I would like to verify that and see how close my unit is to the published response curve. I suspect it will vary from unit to unit, but I'll post whatever results I come up with for my unit. I would also like to know what the response curve for the other power levels looks like.

Not that knowing the output power at the SMA connector would be that helpful in every case, since at the higher frequencies the power arriving at the DUT may be a different level due to losses loading etc.! So it would be better, in my opinion, to measure the power level as close to the DUT as possible.

After playing with the unit for a while I decided I also needed a way to check the power output. My OWON SDS7102 DSO can do a reasonable job to around 100MHz, but beyond that readings to the maximum usable frequency of about 350 MHz are of uncertain accuracy, and what about the frequencies to the SynthUSBII limit of 4.4 GHz. Using an RF probe didn't seem to be a reasonable alternative, I own a couple of those and they did a dismal job! And buying a calibrated power meter for that frequency range will get me back to that very uncomfortable head chopping situation! :palm:

So when I saw the CPDETLS-4000 RF Power Detector at Digi-Key for $31 I decided to give it a try!

http://www.digikey.com/product-search/en?WT.z_cid=sp_744_0310_buynow&site=us&lang=en&mpart=CPDETLS-4000

Attachment #3: This is a picture of the CPDETLS-4000 obtained from the manufacturer's website.

This device covers a frequency range of 10MHz to 4 GHz, so it seemed to me that it would be a perfect, inexpensive companion, for the SynthUSBII. One caveat is that the manufacturer's datasheet doesn't mention the accuracy or tolerance of the given tabulated values. However, since they used 3 decimal places I'm going to embrace a little wishful thinking and hope that they are extremely accurate!

Attachment #4: These are the tabulated values for Frequency & Power in versus DC output that appear in the datasheet

As I experiment with these devices I plan to post my findings in this thread.

saelig:

--- Quote from: TomC on February 03, 2016, 04:53:01 am ---My old Heathkit RF SG tops at 110 MHz and for a while I've been wanting to upgrade so I can experiment with higher frequencies, but my wife would have my head if I invested on a top of the line HP for my hobby! :phew: Even an older used model from eBay! 

So when I saw this Windfreak SynthUSBII RF SG at Saelig for $199 during Cyber week I decided to take a chance and see what it could do!

Attachment #1: This is a picture of the SynthUSBII obtained from the link below.

https://windfreaktech.com/product/usb-rf-signal-generator/

The unit is very small, about 5 x 2.5 cm and is controlled by the included software. One caveat is that although you can set the power output to one of four levels, there is no way to know with reasonable accuracy what those levels are.

Attachment #2: According to the manufacturer the SynthUSBII frequency response should be similar to this. I would like to verify that and see how close my unit is to the published response curve. I suspect it will vary from unit to unit, but I'll post whatever results I come up with for my unit. I would also like to know what the response curve for the other power levels looks like.

Not that knowing the output power at the SMA connector would be that helpful in every case, since at the higher frequencies the power arriving at the DUT may be a different level due to losses loading etc.! So it would be better, in my opinion, to measure the power level as close to the DUT as possible.

After playing with the unit for a while I decided I also needed a way to check the power output. My OWON SDS7102 DSO can do a reasonable job to around 100MHz, but beyond that readings to the maximum usable frequency of about 350 MHz are of uncertain accuracy, and what about the frequencies to the SynthUSBII limit of 4.4 GHz. Using an RF probe didn't seem to be a reasonable alternative, I own a couple of those and they did a dismal job! And buying a calibrated power meter for that frequency range will get me back to that very uncomfortable head chopping situation! :palm:

So when I saw the CPDETLS-4000 RF Power Detector at Digi-Key for $31 I decided to give it a try!

http://www.digikey.com/product-search/en?WT.z_cid=sp_744_0310_buynow&site=us&lang=en&mpart=CPDETLS-4000

Attachment #3: This is a picture of the CPDETLS-4000 obtained from the manufacturer's website.

This device covers a frequency range of 10MHz to 4 GHz, so it seemed to me that it would be a perfect, inexpensive companion, for the SynthUSBII. One caveat is that the manufacturer's datasheet doesn't mention the accuracy or tolerance of the given tabulated values. However, since they used 3 decimal places I'm going to embrace a little wishful thinking and hope that they are extremely accurate!

Attachment #4: These are the tabulated values for Frequency & Power in versus DC output that appear in the datasheet

As I experiment with these devices I plan to post my findings in this thread.

--- End quote ---

Hi There!

Thanks for buying the amazing SynthUSBII (http://www.saelig.com/windfreak/synthusbII.htm) in our Saelig CyberWeek Sale!  I contacted David Goins (Windfreak Technology CTO) and he makes this comment:  "Those detectors are sensitive to output loading.  In my experience they are not just sensitive to the load resistance, but can also vary just based on the length of the output cables that are measuring that voltage.  It would be nice if the datasheet mentioned what measurement device was used – was it a 1Mohm input impedance volt meter??

That’s been my experience using them, but maybe this one is better.  But there is no active device / buffer to factor out the RF impedance of the diode inside that device from the load impedance, which makes the system a voltage divider…  If you could get the exact setup from the manufacturer then you'll probably get accurate enough results..""

Hope that helps!  I'll be interested to see your results posted here!

Alan Lowne  CEO Saelig Co. Inc.

TomC:

--- Quote from: saelig on February 03, 2016, 04:57:20 pm ---
Hi There!

Thanks for buying the amazing SynthUSBII (http://www.saelig.com/windfreak/synthusbII.htm) in our Saelig CyberWeek Sale!  I contacted David Goins (Windfreak Technology CTO) and he makes this comment:  "Those detectors are sensitive to output loading.  In my experience they are not just sensitive to the load resistance, but can also vary just based on the length of the output cables that are measuring that voltage.  It would be nice if the datasheet mentioned what measurement device was used – was it a 1Mohm input impedance volt meter??

That’s been my experience using them, but maybe this one is better.  But there is no active device / buffer to factor out the RF impedance of the diode inside that device from the load impedance, which makes the system a voltage divider…  If you could get the exact setup from the manufacturer then you'll probably get accurate enough results..""

Hope that helps!  I'll be interested to see your results posted here!

Alan Lowne  CEO Saelig Co. Inc.

--- End quote ---

Hi Alan,

Thanks for the info!
Sorry I didn't respond sooner, I got sick with the flu and just started feeling better!

My preliminary testing indicates that this detector also exhibits some sensitivity to output loading, so far it seems to be limited to about a 2% change. I've been using a DMM with a 2.5G ohm input impedance, but haven't decided yet if I want to stick with it or use a different instrument for the final measurements.

Anyway, I plan to post pictures of the final setup. Also, I'm working on a spreadsheet based on the datasheet's tabulated values that uses interpolation to provide values in between the given ones (see attachment #1).

As far as the detector's input interface, I believe, since the manufacturer specifies it as 50 ohm, that it must contain some type of input matching network (see attachment #2). However, when directly connected to the output of the SynthUSBII, my preliminary testing indicates that it doesn't quite behave the same as say, for example, a resistive 50 ohm terminator.

By the way, I also bought an Owon AG1012F during Saelig's 2014 CyberWeek sale! In case you are interested, here is a link to the thread where I describe my experience and document many tests. Warning, this is not as rosy as the reviews posted on your site!

https://www.eevblog.com/forum/testgear/owon-ag1012f-arbitray-waveform-generator/

TomC:
On this post I'd like to explore the possibility of using the SynthUSBII & CPDETLS-4000 to get a better idea of the frequency response of a DSO beyond the manufacturer's specified bandwidth. Hopefully, this knowledge can help the user determine the amplitude of signals that exceed the DSO's specified bandwidth and thus extend the usefulness of inexpensive lower bandwidth DSOs. As an example I'll be using my Owon SDS7102 DSO for the following tests.

For this experiment as well as for future experiments I'm relying on the CPDETLS-4000 datasheet's tabulated values (see the first post). To get values in between the published values I created a spreadsheet that uses linear interpolation. Frequency is tabulated at 5MHz intervals from 10MHz to 1GHz and at 10MHz intervals from 1GHz to 4GHz. Power is tabulated at 0.01 dBm intervals from -10 dBm to 10 dBm and there is an extra column with the corresponding mVpp values. The spreadsheet is in Open Office .ods format and is too large to attach to this post, so I uploaded the two versions to OneDrive for anyone that may want a copy:

CPDETLS-4000_new.ods - This is the full version that includes the interpolation formulas.

https://onedrive.live.com/redir?resid=967A90CA47FD025B!204&authkey=!AB5PBzQdKrjbsuM&ithint=file%2ctxt

CPDETLS-4000 compact plus.ods - This version has the same functionality but instead of the formulas it only has the values. The advantage is that it's more compact and loads faster.

https://onedrive.live.com/redir?resid=967A90CA47FD025B!203&authkey=!ALlQueFdX8yHsGY&ithint=file%2ctxt


Attachment #1 - This shows how the SynthUSBII & CPDETLS-4000 were connected to the DSO for this experiment. After trying a number of different adapters and cables it became evident that the implementation of this connection has a significant impact on the readings. I opted for the adapters that provided the shortest possible signal path between the elements. In my view this setup resulted in more credible results in that there was a closer match between the DSO's and CPDETLS-4000's readings in the 35MHz to 100MHz frequency range. Although, in my opinion, connecting these 3 devices together without a splitter results in an impedance mismatch. I believe that keeping the signal path as short as possible minimizes the impact of signal reflections on the readings.


Attachment #2 - This shows how the DMM & CPDETLS-4000 were connected for this experiment. The implementation of this connection didn't seem to be as critical although during earlier experiments I though I had detected some output loading sensitivity. As it turns out, I now believe that what I saw was due to slight differences in output power as the SyntUSBII warms up. Later observations confirmed that the SynthUSBII output power slightly increases as it warms up, however, after about one hour it seems to become nearly stable. The DMM reading shown on this picture (47.67mV) was taken during the 250MHz test. The reading jumps around slightly, so for more stable readings I used the DMM's MAXMIN feature to pick the Maximum value.


Attachment #3 - This shows the SynthUSBII PC software setup for the 250MHz test. For this experiment I started with a 35MHz setting and then increased the frequency in 5MHz steps until reaching 350MHz. I chose this limit because the SDS7102 DSO seems to be able to display frequencies of up to around 350MHz without noticeable evidence of aliasing.


Attachment #4 - This shows the SDS7102 DSO's screen during the 250MHz test. The SynthUSBII output tends to jump around slightly, so I decided to set the DSO to Average 16 for more stable readings. If the Vp measurement (Vp means Vpp in Owon lingo) still varied I picked the lowest reading. In this case the DSO reports that this signal is only 302mVpp, but as will see the CPDETLS-4000 reports a higher value.


Attachment #5 - This shows the area of the spreadsheet that allowed me to determine the dBm and mVpp values reported by the CPDETLS-4000 during the 250MHz test. I picked the value closest to the DMM's reading, in this case, since the DMM read 47.67mV I picked the 47.656 value on the 250MHz column. So in this case the CPDETLS-4000 reports a 485.9mVpp signal (-2.29dBm) as opposed to the 302mVpp reported by the DSO. From these figures we can calculate a % Error (percentage error) that could be used later to deduce the amplitude of 250MHz signals observed with the DSO. In this case the % Error is:

      (485.9 - 302)/302 x 100 = 60.894%

What this means is that the amplitude of 250MHz signal readings obtained with the DSO is actually 60.894% higher than what the DSO reports. For example, if the DSO reading is 302mVpp then the actual signal amplitude is:

      302 x 0.60894 + 302 = 485.9mVpp

And if the DSO's reading is 400mVpp then the actual signal amplitude is:

      400 x 0.60894 + 400 = 643.576mVpp

The spreadsheet includes a facility that allows the user to calculate % Error or signal amplitude by just entering the known values.


Attachment #6 - This is a line chart of the SDS7102 DSO (blue) and CPDETLS-4000 (red) readings I obtained during this experiment. The line chart also includes the % Error for each pair of readings (green). Note that the Y scale for the DSO & CPDETLS-4000 readings is on the left and the Y scale for the % Error is on the right.

Looking at the % Error curve we can see that up to about 200MHz the readings reported by the SDS7102 DSO & the CPDETLS-4000 are within plus or minus 10% of each other. In my opinion this is probably within the tolerance range of the readings considering the way the test was implemented. So I wouldn't know which reading from each pair should be picked as more accurate.

However, from 200MHz to 350MHz it seems evident to me that the DSO's response is on a fairly steep roll-off and the CPDETLS-4000 readings are the most accurate within this range. So I think that in this range it would be appropriate to use the % Error and the associated SDS7102 DSO reading to deduce values closer to the actual signal amplitude. Still, keep in mind that there is a tolerance associated with the deduced amplitude, probably in the range of plus or minus 10%.


Attachment #7 - This is the spreadsheet used to plot the line chart. In addition to the line chart it contains all the tabulated readings obtained during this experiment.


Some background on why sometimes it's possible to extend the usefulness of DSOs (the way I see it)
--------------------------------------------------------------------------------------------------

Like most modern DSO's, the SDS7102 uses sin(x)/x interpolation to reconstruct the input signal. Since the single channel sampling rate is up to 1GS/s, the Nyquist frequency is 1G/2 = 500MHz. So in theory, provided a rectangular filter that totally rejects all frequencies above 500MHz is used, it should be able to faithfully reconstruct input frequencies of up to 500MHz.

Unfortunately close to perfect filters are expensive and difficult to attain, so to cut cost manufacturers may rely on a number of compromises to get the most cost effective results. For example, instead of a rectangle the filter may roll-off slowly and as the frequency of the input signal increases the signal amplitude displayed by the DSO will be proportionally lower. In addition, the filter may not be able to totally reject everything above the Nyquist frequency. As a result, as the input frequency approaches the Nyquist frequency some higher frequency components will also make it through and cause aliasing.

To compensate for these shortcomings the manufacturer may use a sampling rate that exceeds the Nyquist criteria by a wide margin and specify the DSO's bandwidth well below the Nyquist frequency. Consider for example the SDS7102, the specified bandwidth is 100MHz but the Nyquist frequency is 500MHz. So at the bandwidth's highest frequency (100MHz) we get 10 samples per period instead of the Nyquist minimum requirement of 2 samples per period.

Would it be advantageous to know the filter's response curve?
-------------------------------------------------------------

I think so, specially on DSO's like the SDS7102 where the Nyquist frequency is much greater than the specified bandwidth. This may mean that the DSO's front end will pass frequencies above the specified bandwidth. Although at some point the filter's roll-off will cause attenuation distortion, if the user knows the details of this response curve it should be possible to deduce the amplitude of the displayed signal. This may not be perfectly accurate but still a lot closer to reality than what's displayed by the DSO.



   ---------------------------------------------------------------



Next I plan to repeat this experiment using the X10 scope probe

TomC:
This post is basically a repeat of the previous experiment. The one notable difference is that instead of connecting directly to the DSO's BNC I'm using one of the stock X1 / X10 probes that came with the DSO. My SDS7102 came with Owon T5100 probes. According to the manual the system bandwidth when these probes are used is 6MHz on X1 and 100MHz on X10. For this experiment the probe will be set to X10. As in the previous experiment, amplitude values for input frequencies from 35MHz to 350MHz in 5MHz steps will be tabulated.


Attachment #1 - This shows how the DMM, SynthUSBII, CPDETLS-4000, and the T5100 probe were connected for this experiment. The implementation of this connection doesn't have nearly as much impact on the readings as the configuration used in the previous experiment. I believe this is due to the probe's 10 M ohm input resistance. In my opinion this higher value results in a less severe impedance mismatch when the 3 devices are connected together. However, I still opted for the adapters that provided the shortest possible signal path between the elements. In my opinion, as the frequency increases the probe's input capacitance (14.5pF - 17.5pF) will have a larger influence on the probe's input impedance resulting in a larger impedance mismatch. So, as in the previous experiment, I believe that keeping the signal path as short as possible minimizes the impact of signal reflections on the readings.


Attachment #2 - This is a line chart of the DSO & T5100 probe combination (blue) and CPDETLS-4000 (red) readings I obtained during this experiment. As in the previous experiment, the line chart also includes the % Error for each pair of readings (green). Note that the Y scale for the DSO & T5100 probe combination / CPDETLS-4000 readings is on the left and the Y scale for the % Error is on the right.

Looking at the % Error curve we can see that up to about 190MHz the readings reported by the DSO & T5100 probe combination and the CPDETLS-4000 are within plus or minus 10% of each other. In my opinion this is probably within the tolerance range of the readings considering the way the test was implemented. So I wouldn't know which reading from each pair should be picked as more accurate.

However, from 190MHz to 350MHz it seems evident to me that the DSO & T5100 probe combination's response is on a fairly steep roll-off and the CPDETLS-4000 readings are the most accurate within this range. So I think that in this range it would be appropriate to use the % Error and the associated DSO & T5100 probe combination reading to deduce values closer to the actual signal amplitude. Still, keep in mind that there is a tolerance associated with the deduced amplitude, probably in the range of plus or minus 10%.


Attachment #3 - This is the spreadsheet used to plot the line chart. In addition to the line chart it contains all the tabulated readings obtained during this experiment.



   ---------------------------------------------------------------



Next I plan to repeat this experiment using other X10 scope probes that are rated by the manufacturers as 200MHz or more.

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