SDRplay TECHNICAL SPECIFICATIONS AND INTERMODULATION INTERFERENCE
This post is concerned with IIP3 (Third order Input Intermodulation) Technical Specifications and in this respect the quality and detail of those provided by SDRplay in their site under the heading “DETAILED TECHNICAL INFORMATION” are, in my opinion, among the very best I have ever seen in my professional life.
https://www.sdrplay.com/wpcontent/uplo ... nR1P1.pdf
SDRplay TECHNICAL SPECIFICATIONS AND INTERMODULATION INTERFERENCE
SDRplay TECHNICAL SPECIFICATIONS AND INTERMODULATION INTERFERENCE
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Re: SDRplay TECHNICAL SPECIFICATIONS AND INTERMODULATION INTERFERENCE
SDRplay TECHNICAL SPECIFICATIONS AND INTERMODULATION INTERFERENCE
This is especially true regarding the Input 3rd Order Intercept Point or IIP3, which is given for every product, for each frequency and for all sensitivity levels. The meaning of IIP3 is generally known and in any case descriptions and explanations are easily found on Internet. For our analysis it is enough to understand that IIP3 is a Figure of Merit which defines the input overload power level (in dBm) at which our receivers will start producing spurious signals within their passband, or in other words, their own interference.
By using the wealth of data provided, we can easily have a quantitative idea of our receiver’s intermodulation performance across its entire frequency range. Try to obtain this info for other apparently similar products!
As an example let us consider a practical case in the 40 m band. The 8 MHz IIP3 specification for the RSP1A is: IIP3 = 4.24 dBm at the maximum sensitivity of 0.3 uV, which corresponds to a received power of 117 dBm / 50 Ohm.
The 40 m band extends from 7.0 to 7.3 MHz. Through the day, but at night especially, at F1 =7.3 MHz operates a very strong broadcast station, sometimes coming in at – 40 dBm. At F2 = 7.25 MHz another broadcaster comes in at an average power of 70 dBm. These two stations can potentially produce two inband intermodulation products as follows:
IIP3(1) = (2*F1) – F2 = 14.6 – 7.25 = 7.35 MHz
IIP3(2) = (2*F2) – F1 = 14.5 – 7.30 = 7.20 MHz
This second product then could appear smack into the 40 m. band as a self generated spurious signal and interfere with an on air signal at that frequency. But what would the level of this spur be? A practical rule of thumb states that the power level Ps of the spur will be three times the difference Pd between the strength of the strongest of the two incoming and the rated IIP3 value of the receiver. In our case (see SDRplay specs) IIP3 =  4.24 dBm, thus:
Pd = 40 – 4.24 = 35.76 dB
Ps = 35.76 * 3 = 107.26 dBm
But the rated sensitivity of the RSP1A is 117 dBm, so that this spur would be received as a real signal with a level 10 dB higher than a true weak signal at the same frequency!
In reality we must consider the noise floor and look at the real sensitivity of our receiving system, receiver + antenna. Let us look at Table 3 of this thread:
viewtopic.php?f=5&t=3685
which I have again uploaded below. If we were in a QUIET RURAL area, our most likely noise floor would be 112 dBm, so our receiver’s sensitivity would exceed the noise by: 117 – 112 = 5 dB. If we now used a 5 db attenuator between antenna and receiver, our useful sensitivity would still be the best possible, but both F1 and F2 signal power would decrease by 5 dB with a very large effect, considering the times 3 factor:
Pd = 40 + 5 – 4.24 = 40.76 dB
Ps = 40.76 * 3 = 122.28 dBm
Thus the spur could not give us any problem because it had dropped below the level of the noise floor!
It can be argued that the same result could be obtained reducing the RSP1A gain by 7 dB by placing the gain slider position to 1: our IIP3 would increase to +1.94 dBm and we would have:
Pd = 40 + 1.94 = 41.94 dB
Ps = 41.94 * 3 = 125.8 dBm
Again placing the spur below the noise floor.
But we must remember that reducing the RF gain can nonproportionally degrade the noise figure, so the most efficient solution is to use an external attenuator. For example, looking again at the specification, going from position zero to position 4 for the RF gain, NF goes from 18 to 50.5 dB (32.5 dB worse) while IIP3 goes from – 4.24 to +13.68, or 18 dB better.
This is especially true regarding the Input 3rd Order Intercept Point or IIP3, which is given for every product, for each frequency and for all sensitivity levels. The meaning of IIP3 is generally known and in any case descriptions and explanations are easily found on Internet. For our analysis it is enough to understand that IIP3 is a Figure of Merit which defines the input overload power level (in dBm) at which our receivers will start producing spurious signals within their passband, or in other words, their own interference.
By using the wealth of data provided, we can easily have a quantitative idea of our receiver’s intermodulation performance across its entire frequency range. Try to obtain this info for other apparently similar products!
As an example let us consider a practical case in the 40 m band. The 8 MHz IIP3 specification for the RSP1A is: IIP3 = 4.24 dBm at the maximum sensitivity of 0.3 uV, which corresponds to a received power of 117 dBm / 50 Ohm.
The 40 m band extends from 7.0 to 7.3 MHz. Through the day, but at night especially, at F1 =7.3 MHz operates a very strong broadcast station, sometimes coming in at – 40 dBm. At F2 = 7.25 MHz another broadcaster comes in at an average power of 70 dBm. These two stations can potentially produce two inband intermodulation products as follows:
IIP3(1) = (2*F1) – F2 = 14.6 – 7.25 = 7.35 MHz
IIP3(2) = (2*F2) – F1 = 14.5 – 7.30 = 7.20 MHz
This second product then could appear smack into the 40 m. band as a self generated spurious signal and interfere with an on air signal at that frequency. But what would the level of this spur be? A practical rule of thumb states that the power level Ps of the spur will be three times the difference Pd between the strength of the strongest of the two incoming and the rated IIP3 value of the receiver. In our case (see SDRplay specs) IIP3 =  4.24 dBm, thus:
Pd = 40 – 4.24 = 35.76 dB
Ps = 35.76 * 3 = 107.26 dBm
But the rated sensitivity of the RSP1A is 117 dBm, so that this spur would be received as a real signal with a level 10 dB higher than a true weak signal at the same frequency!
In reality we must consider the noise floor and look at the real sensitivity of our receiving system, receiver + antenna. Let us look at Table 3 of this thread:
viewtopic.php?f=5&t=3685
which I have again uploaded below. If we were in a QUIET RURAL area, our most likely noise floor would be 112 dBm, so our receiver’s sensitivity would exceed the noise by: 117 – 112 = 5 dB. If we now used a 5 db attenuator between antenna and receiver, our useful sensitivity would still be the best possible, but both F1 and F2 signal power would decrease by 5 dB with a very large effect, considering the times 3 factor:
Pd = 40 + 5 – 4.24 = 40.76 dB
Ps = 40.76 * 3 = 122.28 dBm
Thus the spur could not give us any problem because it had dropped below the level of the noise floor!
It can be argued that the same result could be obtained reducing the RSP1A gain by 7 dB by placing the gain slider position to 1: our IIP3 would increase to +1.94 dBm and we would have:
Pd = 40 + 1.94 = 41.94 dB
Ps = 41.94 * 3 = 125.8 dBm
Again placing the spur below the noise floor.
But we must remember that reducing the RF gain can nonproportionally degrade the noise figure, so the most efficient solution is to use an external attenuator. For example, looking again at the specification, going from position zero to position 4 for the RF gain, NF goes from 18 to 50.5 dB (32.5 dB worse) while IIP3 goes from – 4.24 to +13.68, or 18 dB better.
 Attachments

 TABLE 3
 TABLE 3  PREDICTED & CLASSED NOISE DATA.png (35.71 KiB) Viewed 2689 times
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Re: SDRplay TECHNICAL SPECIFICATIONS AND INTERMODULATION INTERFERENCE
SDRplay TECHNICAL SPECIFICATIONS AND INTERMODULATION INTERFERENCE
Uploaded below a worked out IIP3 example with actual signals received by the RSPduo on 11/07/2018. See comments here:
viewtopic.php?f=5&t=3436
Uploaded below a worked out IIP3 example with actual signals received by the RSPduo on 11/07/2018. See comments here:
viewtopic.php?f=5&t=3436
 Attachments

 Example with the RSPduo
 Example of IIP3.jpg (284.64 KiB) Viewed 2610 times
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Re: SDRplay TECHNICAL SPECIFICATIONS AND INTERMODULATION INTERFERENCE
SDRplay TECHNICAL SPECIFICATIONS AND INTERMODULATION INTERFERENCE
Uploaded below are two tables showing IIP3 levels for the RSP1A calculated for a strong interfering signal F1 at 40 and 50 dBm on most HF frequency bands and for the QUIET RURAL and for the RURAL situations. Based on data provided by the SDRplay detailed specifications, we can conclude that the critical IIP3 input level for the RSP1A lies somewhere between 40 and 50 dBm. In fact at 50 dBm there is no danger of IIP3 at any HF frequency. The figures in red in the 40 dBm table show the cases where interference would be above the likely Noise Floor, requiring either an internal RF gain reduction or (better) an external attenuator with a value given by column E  EXCESS SENSITIVITY. The two figures in red in the 50 dBm table merely show only what would happen if we tried to improve the sensitivity (excess sensitivity is negative with respect to Noise Floor) by adding an external low noise amplifier. See also on the external attenuation question:
viewtopic.php?f=5&t=3723
Since, from a practical standpoint, it is extremely unlikely to experience on air signals with levels above 50 dBm and extremely likely to have a higher Noise Floor, we can conclude that the RSP1A and its companions of the RSP class, represent an extremely good and practical compromise among available maximum bandwith (10 MHz) sensitivity and resistance to inband overload.
Out of band overload is wisely dealt with by means of the array of builtin low pass and notch filters (also extremely well specified by SDRplay) so, both in theory and in practice, it is very difficult to find fault with the RSP class receivers and probably impossible to find similar units so well specified.
Uploaded below are two tables showing IIP3 levels for the RSP1A calculated for a strong interfering signal F1 at 40 and 50 dBm on most HF frequency bands and for the QUIET RURAL and for the RURAL situations. Based on data provided by the SDRplay detailed specifications, we can conclude that the critical IIP3 input level for the RSP1A lies somewhere between 40 and 50 dBm. In fact at 50 dBm there is no danger of IIP3 at any HF frequency. The figures in red in the 40 dBm table show the cases where interference would be above the likely Noise Floor, requiring either an internal RF gain reduction or (better) an external attenuator with a value given by column E  EXCESS SENSITIVITY. The two figures in red in the 50 dBm table merely show only what would happen if we tried to improve the sensitivity (excess sensitivity is negative with respect to Noise Floor) by adding an external low noise amplifier. See also on the external attenuation question:
viewtopic.php?f=5&t=3723
Since, from a practical standpoint, it is extremely unlikely to experience on air signals with levels above 50 dBm and extremely likely to have a higher Noise Floor, we can conclude that the RSP1A and its companions of the RSP class, represent an extremely good and practical compromise among available maximum bandwith (10 MHz) sensitivity and resistance to inband overload.
Out of band overload is wisely dealt with by means of the array of builtin low pass and notch filters (also extremely well specified by SDRplay) so, both in theory and in practice, it is very difficult to find fault with the RSP class receivers and probably impossible to find similar units so well specified.
 Attachments

 Overall IIP3 analysis on all HF bands at 40 dBm
 IIP3 EFFECTS RSP1A (40 dBm).png (48.49 KiB) Viewed 2518 times

 Overall IIP3 analysis on all HF bands at 50 dBm
 IIP3 EFFECTS RSP1A (50 dBm).png (44.64 KiB) Viewed 2518 times
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Re: SDRplay TECHNICAL SPECIFICATIONS AND INTERMODULATION INTERFERENCE
LOOKING AT THE RSP2/RSP2PRO
The specifications for the RSP2 are those published by QST magazine and are based on measurements done in the ARRL labs. For the IIP3 specification, ARRL gives the following measured data taken at 14 MHz:
Ps = 97 dBm with F1 at a power of 44 dBm
Note that 44 dBm is half way between 40 and 50 dBm, as deterrmined in the previous posts for the RSP1A & RSPduo. Using the previously described rule of thumb, we can find the value of IIP3 as follows:
97 = 3*(44  IIP3)
97 = 132 + 3*IIP3
97 = 132  3*IIP3
IIP3 = (13297)/3 or IIP3 = 11.66 dBm
PROOF: Ps = 3*(4411.66) = 97 dBm
This result is also confirmed by the ARRL measurement of the second order intercept point IIP2. The rule being that the second order intercept point is located at 9.6 dB below IIP3. In our case:
IIP2 = 11.66  9.6 = 21.66 dBm
The IIP2 measured by ARRL is: IIP2 = +21 dBm measured at zero gain, out of a total gain of 40 dB so this measurement must be corrected by 40 dB:
IIP2 (ARRL) = +21  40 = 19 dBm which is very near our value of 21.66 dBm.
We can conclude that, considering IIP3, the RSP1A and the RSPduo have a better IIP3 performance than the RSP2/RSP2PRO.
The specifications for the RSP2 are those published by QST magazine and are based on measurements done in the ARRL labs. For the IIP3 specification, ARRL gives the following measured data taken at 14 MHz:
Ps = 97 dBm with F1 at a power of 44 dBm
Note that 44 dBm is half way between 40 and 50 dBm, as deterrmined in the previous posts for the RSP1A & RSPduo. Using the previously described rule of thumb, we can find the value of IIP3 as follows:
97 = 3*(44  IIP3)
97 = 132 + 3*IIP3
97 = 132  3*IIP3
IIP3 = (13297)/3 or IIP3 = 11.66 dBm
PROOF: Ps = 3*(4411.66) = 97 dBm
This result is also confirmed by the ARRL measurement of the second order intercept point IIP2. The rule being that the second order intercept point is located at 9.6 dB below IIP3. In our case:
IIP2 = 11.66  9.6 = 21.66 dBm
The IIP2 measured by ARRL is: IIP2 = +21 dBm measured at zero gain, out of a total gain of 40 dB so this measurement must be corrected by 40 dB:
IIP2 (ARRL) = +21  40 = 19 dBm which is very near our value of 21.66 dBm.
We can conclude that, considering IIP3, the RSP1A and the RSPduo have a better IIP3 performance than the RSP2/RSP2PRO.
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