Thank you to Nils Schiffhauer (DK8OK) for submitting an article documenting his tests on the Airspy HF+ Discovery (pdf mirror). Spurred on by discussions on the SDR-Console mailing list, Nils went looking for issues with spurious signals generated by the HF+ itself. In the end he finds some minor spurious signals, but notes that they have absolutely no adverse effects once an antenna is connected.

Youssef Touil (developer of the Airspy HF+ line of products) has also noted that the minimum discernible signal (MDS) of the HF+ is so low that it's become normal for very weak spurs to now be visible without an antenna connected. However, once an antenna is connected the natural atmospheric noise floor which is much higher than the MDS hides these spurs, and so the spurs have no consequence on reception.

In Nils' first test he uses a very high end Winradio W65DDC SDR to detect the spurs coming from an Airspy HF+, Airspy HF+ with preselector retrofit, and the Airspy HF+ Discovery. The results are quoted below:

[With the Airspy HF+] you indeed see a very few spurious signals, all well below -140 dBm/Hz.

With the Airspy HF+ Preselector connected, the number of spurious signal is very much reduced, as is their maximum level.

This picture still further improves with the Airspy HF+ Discovery connected: all visible seven spurious signals are measured to well below -150 dBm/Hz.

The first result is clear:

• Airspy has improved also the spurious signals from model to model, landing at a stunning
  reduction with their matchbox-like Discovery.
• The spurious signals were significantly reduced in both numbers and level.
• Together with sensitivity and dynamic range, the performance of these SDRs is exceptionally
  good. If you see their price tag, they are a real bargain.
• Overall: they deliver professional performance in every important aspect at an incredible low
  price.

In the second test Nils tunes to the center of a spur, then tested with a dummy load and then antenna connected. His results showed that all spurs disappeared once an antenna was connected. He summarizes below:

Yes, there are a very few and low-level spurious signals at all Airspy’s receivers – as they are found [much] worse at some competing SDRs.

• By development, even this has been significantly improved from model to model with the new Discovery leading the gang.
• All spurious signals disappear with an antenna connected.
• There has been found no case where, in practice, any spurious signal even remotely touched or even limited reception of the most miniscule signals.To complain about “spurious signals” simply is “Much Ado about Nothing” in an Ivory Tower, far away from any practical application.