A Review of the SDRplay RSP2 by DB Gain

Over on the Utility DXers file section at udxf.nl/ute-info.html, Mr. D.B. Gain has uploaded his latest review of the SDRplay RSP2 (pdf). The SDRplay RSP2 is the successor to the RSP1, and is a 12-bit SDR with tuning range from 10 kHz – 2 GHz. It currently costs $169.95 USD.

DB Gain’s review first covers the features of the RSP2, and some basic SDR vs Analogue theory. He talks a bit about what criteria makes a good SDR and discusses why SDRs are so good for digital work. The review then goes on to talk about the SDRuno software, sensitivity settings, and voice mode work. The review mostly concerns the RSP2’s use on HF, and in this respect DB Gain appears appears to be extremely impressed with the results that the RSP2 gives him.

Previously DB Gain has also reviewed our RTL-SDR V3 dongle (pdf).

The first page of DB Gain's SDRplay RSP2 Review
The first page of DB Gain’s SDRplay RSP2 Review
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I think most of the negative comments re SDRUno come from users who have not familiarised themselves with it. Out of the box, it is indeed an awkward interface and I almost gave up myself. But as it is the software which SDR Play recommends for use with the RSP’s, I kept on plugging away and finally got to the stage where I always use it. The comments about ‘windows that pop up and have to be re-started every time’ indicate that the user has not bothered setting up and saving a workspace. Once this has been done, the whole shebang can be opened and closed with one standard Windows key-press. All the windows appear where you specified, and all will close down together.
Also, the LNA is variable in gain and is most certainly not an on/off choice.

Martin Ehrenfried - G8JNJ

Hi Andy,

Understood about saving the workspace, I must admit I never got that far.

I did find that in general use I had to leave a lot of separate windows open in order to be able to access all the controls quickly. I one stage I noticed that I had to have seven separate windows open in order to be able to vary the RF gain, change modes and bandwidths and adjust the waterfall settings.

With regard to the RF gain. From memory (as I no longer have the RSP) when adjusting the RF gain on the HF bands, the gain distribution is varied throughout the receiver processsing chain. At some point the gain reduction algorithm switches the pre-amp on and off . When the RF pre-amp was active it degraded the dynamic range.

However SDR Play may have modified the way the gain adjustment operates since the time I owned the RSP1.

Please feel free to send me an email if you wish to discuss this further.


Martin – G8JNJ

D. B. Gain

In that respect, reading the SDRuno Cookbook, as linked to in the review and below, will go a long ways toward removing some of the “cryptic” nuances of the application.

Martin Ehrenfried - G8JNJ

I’m a bit suspicious of this review, which doesn’t seem to have anything negative to say about the RSP2.

I found plenty of problems with the RSP1, which I’m fairly sure, despite mainly cosmetic changes will still be present in the RSP2.

In particular:-

SDRUno is not the most user friendly piece of software, yet the reviewer has chosen not to repeat the shortcomings that he noted in a previous RTL SDR V3 review.

Under certain circumstances an 8 bit RTL dongle using 2 Meg sampling can out perform a 12 bit RSP using 65 Meg sampling as there are fewer strong signals within the sampled chunk of spectrum.

The RTL 820T2 has a tracking filter whereas the RSP uses switched band pass filters.

For example I found that at my location an 8 bit RTL dongle out performed the RSP1 on the VHF airband as strong paging signals at around 140MHz were within the RSP1 filter passband but not the RTL dongle.

In addition the SRP2 filter ranges are not ideal, especially the lowest 12MHz Low Pass filter range which passes the MF, 6,7,9 &11 Broadcast bands, all of which produce very strong signals especially at night. So even if you enable the new MW filter, the SW broadcast stations are still present within the filter pass band.

Any strong signal within this pass band will limit the dynamic range. SDRPLAY have tried to resolve this problem with a broadband AGC system, which reduces the overall gain if a strong signal gets close to overdriving the ADC. However in practice this results in the signal levels and noise floor bouncing up and down as the interfering signal (or signals) vary in strength.

If you turn the broadband AGC off and use manual control, you may have to reduce the gain so much to achieve adequate dynamic range, that the receiver sensitivity suffers as a result.

The ARRL touch on this issue in their review of the RSP.

I could go on, but I think you get the idea. None of these points were covered in the review, but I’m fairly sure that most folks would notice them, even if they were not sure of the exact cause.

The reviewer implies that the folks who have found problems with the RSP don’t know what they are doing. Yet he mentions the need to finely tweak settings in order to get the best out of the RSP. Yet the same is true of the RTL dongle and almost any other SDR radio.

A lot of folks compare the RSP to more expensive receivers and state that by comparison it’s very good value for money. I just wish they would use the value for money argument against something like the RTL SDR V3 instead, as I think this is a much more representative comparison.

I don’t think the RSP is a particularly bad radio (although it’s one of the very few radios’s I’ve owned that I’ve actually sent back for a refund). But don’t believe for one minute that it’s entirely problem free, or a major upgrade from a 8 Bit dongle.

Just about everything I could hear with the RSP I could hear just as well with an 8 Bit RTL (even in direct sampling mode) after a bit of tweaking.

D. B. Gain

On the complexities of SDRuno, yes it’s complex, but I find it a wonderful piece of software, especially considering what you pay for it. The software performs as well for the V3 and other RTL dongles as it does with the RSP2, but I prefer HDSDR with the V3 because that combination results in barely any cpu use for what I do with it.

I have and had no intention of “implying the folks who have found problems with the RSP don’t know what they are doing”, just that in many cases they might have found acceptable performance if they had adjusted the IF AGC and Gain Reduction controls. And as you noted, the auto IF AGC function jumps the noise floor around a good deal, which is why I shut that feature off and use manual IF AGC, and oddly enough I do so for the V3 as well.

I also have no intention of demeaning the V3, you get far more than your monies worth with it, and I feel the same for the RSP2, just that with the RSP2 I’m much more confident that the HF signal I’m hearing above 14MHz is actually on the frequency tuned rather than an alias. I wonder what the dynamic range of a 12bit sdr at 2MHz sample rate is? That’s my usual setting, with some decimation thrown in. Thanks for reading the review!

Martin Ehrenfried - G8JNJ


Thanks for the response.

The main issue with SDRUno is not really the complexity, it’s the way the GUI is constructed with illogical groupings of controls on several different windows that pop up on top of each other and which have to be individually reopened if you start another application.

One other issue with the RSP is the way in which the pre-amp operates on the bands below 30MHz. It’s not easy to vary it’s gain, so basically it’s either on or off.

You don’t really need it on the bands below about 10MHz, but it is required above 10MHz if you wish to be able to make use of the RSP’s full sensitivity. However the pre-amp has a poorer intermodulation performance than the ADC. If you apply a two tone test you can see the 3rd order IMD products due to the pre-amp start to rise above the noise floor at a level of about 10dB lower than the point at which the ADC starts to clip and produce further IMD products. So on the HF bands you can either have good sensitivity and moderate dynamic range, or poor sensitivity and something near the dynamic range a 12 bit ADC should be capable of.

This is particularly problematic if you are quickly tuning between, or monitoring different frequencies that are spaced more than a few MHz apart, you have to constantly juggle the pre-amp and gain controls in order to optimise the sensitivity / dynamic range at any moment in time.

I tried the RSP at different sampling rates and with decimation, but I still couldn’t get the desired performance. I think the problem is that the ADC is still exposed to a very wide chunk of spectrum regardless of whatever sampling rate is in use.

I also run a 14 Bit KiWi SDR covering 0-30MHz which is connected to a good broadband vertical antenna http://southwest.ddns.net:8073/ and it’s an order of magnitude better in this respect, so I know what to expect from a 12 bit SDR

When reading other reviews I have noticed that several people who have given the RSP a good rating have used resonant antennas or have added external band pass filters or pre-selectors in order to overcome some of these issues. Especially when DXing on the Medium Wave broadcast band.

The RTLSDR V3 does suffer from a problem with image signals centered on 14.4MHz (1/2 of the 28.8MHz Nyquist sampling rate). However in most cases these are not problematic due to the relationship of the active and inactive segments of the sampled, real and image spectrum.

A simple external low or high pass filter (two components) can usually help minimise such issues.

I tried extremely hard to like the RSP1, and spent quite a lot of time building up external filters and equalisers in order to try and improve it’s performance to a level I would have found acceptable and I had quite a number of email exchanges with SDR Play’s technical support, who were very helpful in trying to resolve the issues I flagged up.

But in the end I decided it just wasn’t for me and sent it back.

However your milage may vary etc……


Martin – G8JNJ

Useful Gain

“AMBC notch … I found that employing it in the AMBC band when strong signals were present the receiver overloaded’
=> Then what is it useful for?

“accounts of users decrying poor performance from the RSP due to IMD, overload, etc. etc. I’m certain that if a bit of adjustment had been made to the IF AGC and RF Gain Reduction controls those users would have found, in most cases but the direst, the RSP2 perfectly suited to the radio environment of their location.”
=> Translates to “It’s crap, but most beginners won’t notice”

D. B. Gain

Oops, sorry I didn’t make clear that if I employed the ambc notch without adjusting the IF AGC and Gain Reduction controls, imd spurs would result – at my station. The reason one might not want to employ the ambc notch during ambc listening is it greatly attenuates the entire ambc band, spurs or no. The notch is useful for cases where one desires ambc attenuation while listening to the ambc band, and to keep ambc energies from impinging upon the RSP2 during use outside the ambc band. The IF AGC and Gain Reduction controls are your friend in any case. I hope that makes things a bit more concise.

On the comment regarding “beginners won’t notice”, imd is hard to ignore in any case, beginner or veteran, hence why I mentioned the means to avoid it by adjusting the IF AGC and Gain Reduction controls. By the way, thanks for reading the review!