Tagged: intermodulation

Optimizing the RSP1A at LF/MW/HF by Understanding Intermodulation

SDRplay have recently published an informative white paper that explains what intermodulation and higher order mixing effects are, and how they can affect reception on an SDR such as the SDRplay. This paper could be a useful introduction to understanding how to optimize reception of weak signals when they are in the presence of strong signals. While written for the SDRplay, the same knowledge and tips could be applied to any similar SDR.

Later in the paper they also show how to eliminate intermodulation effects by enabling the MW/AM notch filters on the SDRplay RSP1A unit, and by carefully choosing the LO frequency.

The RSP1A covers the spectrum from 1kHz to 2GHz, and phantom signals can be a menace for all wideband SDR receivers. More and more is being published about the most obvious culprit which is inter-modulation caused by very strong interferers such as MW/FM broadcast transmitters – indeed, all the current SDRplay RSPs have built in filters to help reduce the problems caused by that.

But the reality is, particularly at HF and below, that a phantom signal may occur for other reasons such as higher order mixing effects. Sometimes, it can be difficult to know what is the cause of the phantom signal. If you can understand the cause, there are additional steps you can take to overcome it.

We’ve just published this white paper to explain the difference between intermodulation and higher-order mixing effects, and what practical steps you can take to reduce the latter in particular. Our example uses an RSP1A operating at frequencies below 60MHz.

First pager of the SDRplay whitepaper on intermodulation effects.
First pager of the SDRplay whitepaper on intermodulation effects.

ADALM-Pluto 2-tone Test at 144 MHz

Over on his YouTube channel Adam 9A4QV has been testing his ADALM-PLUTO SDR in the 2M ham band at around 144 MHz. In one of his videos he shows a 2-tone test. A 2-tone test is used to determine how well an SDR can handle two strong narrowband signals at once, without causing intermodulation and imaging problems. The two tones in his video occur with real world signals on the 2M band when two amateur radio operators are transmitting strong signals at the same time.

The video shows that the Pluto SDR has some intermodulation problems occurring when the two strong signals transmit at once. No problems are noticed when only one signal transmits.

Problems like this with the PlutoSDR may be expected as it was never designed to be a high performance receiver, but rather a tool for learning and experimentation. But it is still possible to use it as a more general purpose receiver if you are aware of the limitations.

ADALM-PLUTO 2-tone test 144MHz

Measurements on RTL-SDR E4000 and R820T DVB-T Dongles: Image Rejection, Internal Signals, Sensitivity, Overload, 1dB Compression, Intermodulation

Over on the SDRSharp Yahoo group, HB9AJG has posted an interesting report in a PDF file containing some measurements (Note you will need to be a member of the group to download the file titled "Some Measurements on E4000 and R820 Tuners.pdf". Here is a Direct Mirror of the file.) quantifying the performance of both the E4000 and R820T RTL-SDR DVB-T dongles. See the discussion in the Yahoo group here.

These results confirm the feeling that many RTL-SDR users have had: that the E4000 is more sensitive in the lower frequencies, and that the R820T is more sensitive in the higher frequencies, which is why it is recommended for ADS-B. The results also show that the R820T is better in terms of Image Rejection and Internal Signal Birdies.

He comprehensively summarizes his results in the following

Image Rejection
Because the E4000 is a Direct Conversion Receiver, it has an Image Rejection problem. By switching on Correct IQ in SDR# a more or less acceptable 50dBs are reached. For the same reason, a "hump" shows in the center of the spectrum display. By using a well filtered external power supply (not from the USB connector) the hump might be reduced.
Internal signals
The E4000 shows many signals actually not present at its input ("birdies"). Birdies are easy to recognize: most of them (except the harmonics of the clock) vary their frequency when moving the spectrum window in frequency. Many of them even move up if you move the window down in frequency.

The R820 is much cleaner in this respect: besides the harmonics of the clock (28.8MHz) only few birdies show up.

Both dongles have a very high sensitivity. Between about 50 and 450MHz the E4000 is about 5dB better than the R820 (-139dBm vs -134dBm). At 1000MHz the E4000 is about 8dB less sensitive (-129dBm vs -137dBm). No measurements could be made above 1040MHz.

Overload and 1dB Compression
If a signal is strong enough, it may cause overload, i.e. many (unwanted) signals show up on the spectrum display that are not present at the antenna input. Also, if we listen to a desired signal, another signal (if strong enough) may cause a reduction of the S/N ratio of the desired signal.

Both dongles have a digitally tuned RF filter after the preamplifier that (together with the following digital signal
processing) improves the overload/1dB compression limit considerably.

- The filter of the E4000 is about +/-0.8MHz wide, but less steep than the filter of the R820.
- The filter of the R820 is about +/-3MHz wide, but steeper than the filterof the E4000.

For the E4000 the overload/1dB compression limit is not linearly dependent of the gain set in Configuration of SDR#:
if the gain is reduced by 13/20/30dB, the overload limit is improved by only 7/14/25dB (measured on 145MHz only).

For the R820 the overload/1dB compression limit is quite linearly dependent of the gain set in Configuration of SDR#: if the gain is reduced by 11/20/30dB, the overload limit is improved by 12/20/30dB (measured at 145MHz only).

For both dongles it seems there is nothing to be gained from activating RTL AGC or Tuner AGC.

Intermodulation products in general show up close to the overload/1dB compression limits. However, if the strong
signal is on the roll off of the filter, they appear well before this limit.

Aliasing always occurs if an insufficiently band limited signal is sampled, i.e. if the signal to be sampled contains frequencies above half the sampling frequency. Thus, aliasing is an effect showing up in many SDRs, not only in these dongles. In both types of dongles there is not much space for brick wall filters. Therefore, aliasing effects are well visible with both dongles.

What do we learn from these tests?

- Both types of dongles are very sensitive. The choice depends on which frequency range you are most interested in.

- Considering internal signals and image rejection, the R820 is much cleaner than the E4000.

- Set the spectrum display of SDRSharp to show a range of not more than 60db above the noise floor. If a signal is close to the top, you know you are close to overload.

- Both types of dongles are prone to overload by strong signals within their filter bandwidth: +/-0.8MHz for the E4000, +/-3MHz for the R820. Therefore, keep signals within this bandwidth to not more than about 60dB above the noise floor
by reducing the gain. If increasing the gain does not audibly increase the signal to noise ratio of the desired signal any more, reduce the gain by one step. Do not switch on RTL AGC or Tuner AGC, as it seems there is nothing to be gained.

- Outside their filter bandwidth both types of dongles can live with much higher signals without showing serious degradation. Use the gain control as explained above to check a possible reduction of signal to noise ratio of the desired signal or the appearance of "new" signals not present at the antenna

- Intermodulation occurs if several strong signals are present within the bandwidth of the dongle. Their individual power adds up (add 3dB per equally strong signal). Therefore, in frequency bands with many strong signals, e.g. broadcast bands, the gain must be reduced even further. Watch for "new" signals appearing when increasing the gain, and then reduce the gain by one step.

- If very strong signals are present at the antenna input >-40dBm), they should be attenuated by bandstop or notch filters.