Comparison of the sensitivity of SDRPlay’s RSPduo and IC-735

Background

When RSPduo was used as a second receiver in parallel with my IC-735, I discovered that weak CW stations that were difficult to perceive in RSPduo could be clearly readable in the analog receiver.

Since virtually all modern transceivers are SDRs, the question arose as to whether replacing the IC-735 would impair the ability to read weak CW signals. Therefore, in a first step, I performed measurments on RSPduo. Then comes the question of whether there is a general weakness of SDRs or not.

Another problem is strong signals outside the passband that can affect reception. SM5BSZ has measured this. An old FT1000 is better than Perseus SDR, which has 10 bandpass filters on the input.

Method

RSPduo and IC-735 are connected to an antenna via a 3 dB splitter (attenuation 3.5 dB). A MiniVNA is connected to a small transmitter antenna via a switchable attenuator set and the SDR and ICOM are connected to a regular antenna. At 14 to 28 Mhz it is a Yagi at a height of 22 meters and at lower frequencies a dipole at a height of about 20 meters. The VNA is keyed on and off in generator mode. The output power is lowered until the signal is just audible in the IC-735. Then the power is adjusted until there is the same audibility in SDR. The bandwidth is set to 250 Hz. The test was done with three different softwares: SDRuno, HDSDR and SDRConsole. No difference could be noticed.

Frequency: 28.5 MHz

IF AGC in automatic mode. RF Gain = Max

IC-735                                                                      RSPduo

Relative antenna power 0 dB                        + 6dB for the same readability

Noise level without antenna = – 136 dBm

Noise level with antenna (without signal) = – 135 dBm

Frequency: 10.102 MHz in the evening 2100 local time. The band closed.

IF AGC in automatic mode. RF Gain = Max

IC-735                                                                      RSPduo

Relative antenna power 0 dB                        0 dB for the same readability

Noise level without antenna = – 132 dBm

Noise level with antenna (without signal) = – 118 dBm

Frequency: 7.0 MHz with very strong BC stations over 7200 kHz.

IF AGC in automatic mode. RF Gain – 3 steps to avoid “overload” flashing.

IC-735                                                                      RSPduo

Relative antenna power 0 dB                      + 6 dB for the same readability

Noise level without antenna = – 125 dBm

Noise level with antenna (without signal) = – 108 dBm

Frequency: 7.0 MHz with weak BC stations above 7200 kHz.

IF AGC in automatic mode. RF Gain = Max.

IC-735                                                                      RSPduo

Relative antenna power 0 dB                            0 dB for the same readability

Noise level without antenna = – 134 dBm

Noise level with antenna (without signal) = – 113 dBm

Recordings on 20m with IC-735 and RSPduo with HDSDR

ICOM-735 recording

RSPduo recording

The RSPduo has RF gain set to max and IF AGC in Auto.

With the SDR the signal sometimes drops down into the noise.

In the last part of the recordings there are two QRN crashes. The SDR almost mutes while you just hear the crashes in the IC-735.
Maybe the SDR had handled this better with fixed IF gain.