Some months ago I was perusing the wares of a seller on EvilBay and having decided to purchase one of those all-purpose component testers, I was tempting myself with other items on sale. One of the items that I could not resist was the "RTL.SDR" that was billed as having an onboard frequency upconverter to allow HF operation, plus a more stable reference oscillator.
Stepping back for a moment, the "RTL" SDR dongles have been around for a few years, so-called because they typically contain as one of their active devices the Realtek RTL2832U. These devices - costing roughly $10 (more or less) also contain a device (a Rafael Micro R820T in the case of the one pictured) with an onboard frequency synthesizer, I/Q mixer and (8 bit) A/D converter that allows one to receive signals from 50 MHz (or lower) to well above 700 MHz - sometimes way above 700 MHz, just using a computer with reasonable horsepower and FREE software!
While the thought of an 8 bit A/D converter might make it sound like these devices may be useless when connected to an antenna, the fact that they convert only a limited range of frequencies (approximately +/- 2.4 MHz at most) around the tuned frequency - plus the "oversampling" nature of the raw A/D sampling rate of the 8 bit converter means that relatively narrowband signals can have quite good ultimate quantization range. What can suffer is when, within the sample-rate passband there are signals that range widely in signal strength: Under such conditions it is recommended that one "finesse" the gain controls for optimum input signal levels to the A/D converter.
Perhaps the most popular software to do this is "SDR#" (a.k.a. "SDR-'Sharp'"). Easily found via a web search, this software is compatible with many software defined radio packages and it supports many modes and features, also offering many optional plug-ins.
These low-cost USB dongles were intended to allow the reception of digital TV and FM/Digital Audio broadcast (but not the sort of TV or digital audio broadcast that is used in North America) on a computer so they were designed to tune from something around the low-medium VHF range - say, 20's to 50's of MHz and up, almost completely leaving out the HF frequency bands.
Several schemes have been devised to allow the use of these handy devices to receive HF and these typically involve the use of some sort of upconverter to translate the HF frequencies from as low as the AM broadcast band up to a range that these devices will tune.
So, I soon found myself with the device pictured in Figure 1 in my hands. Getting busy with life and other things, it was a few months before I got around to playing with it and while I quite easily got it working on the "UV" input which, on this particular unit tunes from just below 25 MHz to over 1 GHz (although I'm not sure exactly how far above 1 GHz...) but I was puzzled as to how to configure it operate at HF.
How to make it work on HF:
Back then, a web search didn't help as there was a plethora of hits on "RTL SDR Upconverter" so I removed the unit from its case and found the callsign "BA5SBA" on the board that you see in Figure 2 and got a hit with Google on a German page (this one - LINK - you may need to run it through a translator, so try THIS LINK which may or may not work...) While this web page didn't describe exactly what I needed to do, it did provide enough clues to get it working!
Comment:
As I surmised from what can be seen in Figure 2 - and was verified by the German page listed above - the "HF" converter was simply a diode-ring mixer driven by the already-present 28.3 MHz reference oscillator on board the converter. The output of the diode ring mixer was then connected to one of the inputs of the chip, so all I had to do was to figure out the offset frequency and how to access that input.
Using a signal generator set to 5 MHz I connected it to the "HF" input of the device and, just for kicks, tuned SDR# to 5.000 MHz as well. After a bit of fiddling about, I finally found how to configure the unit to receive at HF.
Once I had done this I saw the 5 MHz signal on the display!
Comments:
Now I am very surprised that I can tune SDR# to "5.0" MHz and see the 5 MHz signal that I am inputting as the chip on this dongle should not function at that frequency! What I expected to have to do was as follows as shown in Figure 3:
Comments on the design of the HF mixer and its limitations:
Sensitivity (or the lack of...)
The very simple mixer on this SDR Dongle has its limitations, the first of which is that being entirely passive, the ultimate sensitivity at HF is not particularly good: Using SSB (2400 Hz) bandwidth, signals down to around -90dBm were audible, but that was about it. While this represents a signal of around 7 microvolts and may seem horrible (and it is, compared to modern HF receivers!) it should be remembered that if this is connected to anything resembling a full-sized HF antenna, it "hears" just fine since the background noise on HF bands - and thus the signals - will typically be above this level - at least on the "lower" bands of 10 MHz and below.
Because it does use a passive diode mixer it can take more signal and this means that if an amplifier is placed in front of it, it is not as likely to be overloaded as other upconverter designs for SDR dongles that use, say, an NE602! This is probably a good thing since the converter chips in these devices are fairly easily overloaded, anyway!
Nyquist rules!
The other problem is that the local oscillator frequency is 28.8 MHz. What this means is that there is a Nyquist limit of 14.4 MHz - and that means that any signal above 14.4 MHz will reappear again as an "image" at a lower frequency, approaching Zero Hz as it approached 28.8 MHz. In other words, if you were tuned to 7.1 MHz in the 40 meter band, you would also hear a signal at 7.1 MHz below 28.8 MHz, or 21.7 MHz! This also means that if you were trying to listen on 21.1 MHz in the 15 meter band, you would hear a signal at its 7.7 MHz image! The situation is worse on 10 meters since the 28.8 MHz local oscillator is nearly in the middle of this band: An FM signal on 29.5 MHz would also appear on 28.1 MHz!
If you look at the circuit board in Figure 2 you'll notice several components in the foreground, namely a number of inductors and surface-mount capacitors, all with the values marked on the silkscreen. Presuming that the values of these components are as-marked, I drew the schematic in LT Spice that is shown in Figure 4 and simulated it with the result in Figure 5. As can be seen, this low-pass filter effectively blocks signals much above 40 MHz or so, offering more than 40 dB of attenuation above 60 MHz.
Other than keeping signals from low-band TV and FM broadcast out of the "HF" input, this filter does not help us at all in our image problem. Because this filter does not block signals immediately above 28.8 MHz, we have another set of images that occur which means that a signal that is at 33.8 MHz - 5 MHz above 28.8 MHz - will also show up at 5 MHz!
If there are no signals (or noise!) on these "image" frequencies, there should be no problem, but this is likely not to be the case during the daytime on a large wire antenna - which means that if you really want to use this dongle for serious HF reception, you need some sort of filter in front of it, either a separate Low Pass (below 14.4 MHz) and High Pass (above 14.4 MHz) to be used individually, or you could use a tunablepreselector to pass only the frequencies of interest. (There would be a problem with 10 and 20 meters as these bands are close to or include the Nyquist frequency!)
Possible modifications:
If you look at Figure 2 you'll note that there are several small "prototype" areas on the circuit board. These are large enough to allow the construction of some simple circuits, namely a bit of additional gain (amplifiers, filters) to improve the sensitivity of the receiver.
As mentioned before another useful modification - one external to this box - would be the addition of a preselector to filter a narrower range of frequencies.
Is this upconverter useful as-is?
In general, yes, with a good antenna. Despite its flaws, when one casually tunes around the HF bands using the built-in upconverter its problems do not immediately jump out, at least on the lower bands.
The biggest advantage of this particular SDR Dongle is that it seems to have quite a stable oscillator on board - much better than the $8 USB "stick" that I have experimented with previously: This one seems to move only a few 100 Hz from cold start even at UHF frequencies whereas the very cheap ones seemed to move all over the place with temperature!
To be sure, one could probably get better HF performance by building (or buying) an upconverter for this sort of SDR that uses a much higher local oscillator frequency - say, 100 MHz - but if one upconverts all of the HF frequencies en-masse, there are a few things that one must remember:
A few brief comments on using it at VHF/UHF:
Remembering to set it to the "Quadrature Sampling" mode when using the "U/V" input, there are a few things to know when using this device - and possibly others like it - with SDR#.
Clicking on the "gear" symbol will open a screen such as that depicted in Figure 6 that allows modification of settings related to the dongle's hardware. In particular, note the check in the box for RTL AGC and the lack of a check in the Tuner AGC box: The RTL AGC box is necessary in both the Quadrature Sampling mode to get the most gain out of the device.
The Tuner AGC box - which is only available when Quadrature Sampling mode is active - if checked, does not seem to work properly in the the version of driver and/or SDR# at the time of initial posting of this blog entry (March, 2015) as it seems to increase the gain too much, causing overload of the A/D converter and actually reducing performance - particularly if a decent antenna is connected!
Figure 7 demonstrates the problem. The top half of the image shows an optimally adjusted amount of gain - a setting of 12.5 dB for this particular antenna (yours will vary!) while the bottom shows a gain setting of 37.0 dB.
Even though more signal is reaching the A/D converter in the bottom example, it is way too much and the converter is being badly overloaded, causing the noise floor to rise from below, submerging weaker signals! If the RF gain is sett too low, degradation can also occur, but this time from too little signal.
The idea is to adjust the RF gain so that there is there is the maximum difference between the highest signal and the noise floor seen between stations - even if the absolute level of the signals may be lower, overall!
[End]
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| Figure 1: The EvilBay "RTL.SDR" with built-in upconverter for HF reception. According to the pictures on EvilBay, the units currently sold host different picture, with a graphical representation of the schematic diagram showing the mixer. Click on the image for a larger version. |
While the thought of an 8 bit A/D converter might make it sound like these devices may be useless when connected to an antenna, the fact that they convert only a limited range of frequencies (approximately +/- 2.4 MHz at most) around the tuned frequency - plus the "oversampling" nature of the raw A/D sampling rate of the 8 bit converter means that relatively narrowband signals can have quite good ultimate quantization range. What can suffer is when, within the sample-rate passband there are signals that range widely in signal strength: Under such conditions it is recommended that one "finesse" the gain controls for optimum input signal levels to the A/D converter.
Perhaps the most popular software to do this is "SDR#" (a.k.a. "SDR-'Sharp'"). Easily found via a web search, this software is compatible with many software defined radio packages and it supports many modes and features, also offering many optional plug-ins.
These low-cost USB dongles were intended to allow the reception of digital TV and FM/Digital Audio broadcast (but not the sort of TV or digital audio broadcast that is used in North America) on a computer so they were designed to tune from something around the low-medium VHF range - say, 20's to 50's of MHz and up, almost completely leaving out the HF frequency bands.
Several schemes have been devised to allow the use of these handy devices to receive HF and these typically involve the use of some sort of upconverter to translate the HF frequencies from as low as the AM broadcast band up to a range that these devices will tune.
So, I soon found myself with the device pictured in Figure 1 in my hands. Getting busy with life and other things, it was a few months before I got around to playing with it and while I quite easily got it working on the "UV" input which, on this particular unit tunes from just below 25 MHz to over 1 GHz (although I'm not sure exactly how far above 1 GHz...) but I was puzzled as to how to configure it operate at HF.
How to make it work on HF:
Back then, a web search didn't help as there was a plethora of hits on "RTL SDR Upconverter" so I removed the unit from its case and found the callsign "BA5SBA" on the board that you see in Figure 2 and got a hit with Google on a German page (this one - LINK - you may need to run it through a translator, so try THIS LINK which may or may not work...) While this web page didn't describe exactly what I needed to do, it did provide enough clues to get it working!
Comment:
- More recently, the some of the sellers on EvilBay are including in their postings some basic instructions on how to get these devices to work on their "HF" ports, but what follows below contains more information - and is hopefully easier to find and more permanent - than an EvilBay posting!
 |
| Figure 2: Inside the BA5SBA "RTL.SDR" with the built-in upconverter - the singly-balanced diode mixer being located under the white blob in the foreground. The jumper in the upper-left corner marked "A" and "P" appears to be intended for putting 5 volts on the "UV" antenna connector to power an external RF amplifier. ("A" = active, "P" = passive). Click on the image for a larger version. |
Using a signal generator set to 5 MHz I connected it to the "HF" input of the device and, just for kicks, tuned SDR# to 5.000 MHz as well. After a bit of fiddling about, I finally found how to configure the unit to receive at HF.
- STOP the SDR receiver first by hitting the square "stop" button at the top.
- Bring up "RTL-SDR Controller" window (accessible by clicking on the "gear" symbol)
- Under "Sampling Mode" set it to "Direct Sampling (Q Branch)". Note: You cannot change the sampling mode without first STOPPING the SDR.
- Restart the SDR by pressing the "start" button (the right-pointing triangle resembling a "PLAY" button)
Once I had done this I saw the 5 MHz signal on the display!
Comments:
- By default, SDR# will set the "RF Gain" setting in this same "RTL-SDR Controller" window(see Figure 6) to minimum gain (all the way to the left.) If you leave it there, the SDR dongle will be EXTREMELY DEAF when it is in the normal "Quadrature Sampling" mode. When you have it in either "Direct Sampling" mode this control is disabled.
- I would suggest checking the box that says "RTL AGC" and moving the RF Gain control in either "Quadrature Sampling" or "Direct Sampling" modes: If you do not do this your receiver may be very deaf!
- I experimented with using this device with the "HDSDR" program and found that at HF, its sensitivity was noticeably worse than that obtained when using it with SDR#. I'm not sure why this might be, but it seems to have something to do with the way the "ExtIO" driver - or HDSDR itself - handled the internal gain settings of the dongle.
 |
| Figure 3: A screenshot of SDR# showing the configured "Shift" for the mixer. Click on the image for a larger version. |
Now I am very surprised that I can tune SDR# to "5.0" MHz and see the 5 MHz signal that I am inputting as the chip on this dongle should not function at that frequency! What I expected to have to do was as follows as shown in Figure 3:
- Knowing that the onboard mixer uses the 28.8 reference oscillator, check the "Shift" button on SDR#.
- Enter a frequency of "-28800000"(yes, negative 28.8 million) into the box as shown in Figure 3.
- Note: There is a bug in some versions of SDR# that, if you are already tuned to a frequency lower than the "shift" frequency, the tuning may not work properly until one sets a much higher frequency with the 100 MHz digit and tunes back down.
- Having done this, one need not mentally add the desired HF receive frequency to 28.8 MHz: The receive frequency is simply input to the main frequency tuning as one would any other frequency!
- If the 28.8 MHz frequency reference oscillator is somewhat off frequency one must first set the frequency using the "normal" VHF/UHF mode and calibrate it in the "RTL-SDR Controller" window and THEN, after switching to HF mode as described above, one can input a frequency that is slightly different from 28.8 MHz as needed to correct the offset. I did not have to make any correction at all!
- The 28.8 MHz reference oscillator supplied with this unit appears to be quite a good, stable TCXO and is only off a few PPM!
- If you want to use the VHF/UHF input again, remember to reset the "Sampling Mode" back to "Quadrature Sampling" and UN-CHECK the "Shift" box that is visible in Figure 3.
Comments on the design of the HF mixer and its limitations:
Sensitivity (or the lack of...)
 |
| Figure 4: The low-pass filter in front of the mixer. Click on the image for a larger version. |
Because it does use a passive diode mixer it can take more signal and this means that if an amplifier is placed in front of it, it is not as likely to be overloaded as other upconverter designs for SDR dongles that use, say, an NE602! This is probably a good thing since the converter chips in these devices are fairly easily overloaded, anyway!
Nyquist rules!
The other problem is that the local oscillator frequency is 28.8 MHz. What this means is that there is a Nyquist limit of 14.4 MHz - and that means that any signal above 14.4 MHz will reappear again as an "image" at a lower frequency, approaching Zero Hz as it approached 28.8 MHz. In other words, if you were tuned to 7.1 MHz in the 40 meter band, you would also hear a signal at 7.1 MHz below 28.8 MHz, or 21.7 MHz! This also means that if you were trying to listen on 21.1 MHz in the 15 meter band, you would hear a signal at its 7.7 MHz image! The situation is worse on 10 meters since the 28.8 MHz local oscillator is nearly in the middle of this band: An FM signal on 29.5 MHz would also appear on 28.1 MHz!
 |
| Figure 5: The response of the low-pass filter depicted in Figure 4. The bandpass response is the solid line. Click on the image for a larger version. |
If you look at the circuit board in Figure 2 you'll notice several components in the foreground, namely a number of inductors and surface-mount capacitors, all with the values marked on the silkscreen. Presuming that the values of these components are as-marked, I drew the schematic in LT Spice that is shown in Figure 4 and simulated it with the result in Figure 5. As can be seen, this low-pass filter effectively blocks signals much above 40 MHz or so, offering more than 40 dB of attenuation above 60 MHz.
Other than keeping signals from low-band TV and FM broadcast out of the "HF" input, this filter does not help us at all in our image problem. Because this filter does not block signals immediately above 28.8 MHz, we have another set of images that occur which means that a signal that is at 33.8 MHz - 5 MHz above 28.8 MHz - will also show up at 5 MHz!
If there are no signals (or noise!) on these "image" frequencies, there should be no problem, but this is likely not to be the case during the daytime on a large wire antenna - which means that if you really want to use this dongle for serious HF reception, you need some sort of filter in front of it, either a separate Low Pass (below 14.4 MHz) and High Pass (above 14.4 MHz) to be used individually, or you could use a tunablepreselector to pass only the frequencies of interest. (There would be a problem with 10 and 20 meters as these bands are close to or include the Nyquist frequency!)
Possible modifications:
If you look at Figure 2 you'll note that there are several small "prototype" areas on the circuit board. These are large enough to allow the construction of some simple circuits, namely a bit of additional gain (amplifiers, filters) to improve the sensitivity of the receiver.
As mentioned before another useful modification - one external to this box - would be the addition of a preselector to filter a narrower range of frequencies.
Is this upconverter useful as-is?
In general, yes, with a good antenna. Despite its flaws, when one casually tunes around the HF bands using the built-in upconverter its problems do not immediately jump out, at least on the lower bands.
The biggest advantage of this particular SDR Dongle is that it seems to have quite a stable oscillator on board - much better than the $8 USB "stick" that I have experimented with previously: This one seems to move only a few 100 Hz from cold start even at UHF frequencies whereas the very cheap ones seemed to move all over the place with temperature!
To be sure, one could probably get better HF performance by building (or buying) an upconverter for this sort of SDR that uses a much higher local oscillator frequency - say, 100 MHz - but if one upconverts all of the HF frequencies en-masse, there are a few things that one must remember:
- If you use a chip like the NE602, it may simply overload when connected to a full sized HF antenna unless you are prepared to add switchable attenuation to it and/or a preselector!
- On a full-sized antenna, intercepting and converting the entire HF spectrum can represent quite a bit of signal energy. If the HF upconverter that you are using does not, itself, overload, there is a good change the the converter chip in the RTL-SDR itself will overload with so much energy from such a wide variety of frequencies! Remember: The receiver chips in these dongles were intended to work with the relatively weak and sparse broadcast signals intercepted on small, indoor antennas, not the entirety of the HF spectrum when the bands are open!
 |
| Figure 6: The screen for configuring the dongle's hardware. Click on the image for a larger version. |
A few brief comments on using it at VHF/UHF:
Remembering to set it to the "Quadrature Sampling" mode when using the "U/V" input, there are a few things to know when using this device - and possibly others like it - with SDR#.
Clicking on the "gear" symbol will open a screen such as that depicted in Figure 6 that allows modification of settings related to the dongle's hardware. In particular, note the check in the box for RTL AGC and the lack of a check in the Tuner AGC box: The RTL AGC box is necessary in both the Quadrature Sampling mode to get the most gain out of the device.
The Tuner AGC box - which is only available when Quadrature Sampling mode is active - if checked, does not seem to work properly in the the version of driver and/or SDR# at the time of initial posting of this blog entry (March, 2015) as it seems to increase the gain too much, causing overload of the A/D converter and actually reducing performance - particularly if a decent antenna is connected!
 |
| Figure 7: Top: Properly adjusted RF gain setting Bottom: The degradation from too much RF gain! Click on the image for a larger version. |
Figure 7 demonstrates the problem. The top half of the image shows an optimally adjusted amount of gain - a setting of 12.5 dB for this particular antenna (yours will vary!) while the bottom shows a gain setting of 37.0 dB.
Even though more signal is reaching the A/D converter in the bottom example, it is way too much and the converter is being badly overloaded, causing the noise floor to rise from below, submerging weaker signals! If the RF gain is sett too low, degradation can also occur, but this time from too little signal.
The idea is to adjust the RF gain so that there is there is the maximum difference between the highest signal and the noise floor seen between stations - even if the absolute level of the signals may be lower, overall!
[End]