Monday, March 6, 2023

Digital-only receivers (ie, QDX) and MDS vs. OSS

A recent thread on an FT8-related forum discussed the MDS (minimum discernible signal) of the QDX digital transceiver and how the MDS might be measured for a receiver designed to produce something other than an audio output.

The QDX is designed to produce an input to a computer that will result in an alphanumeric output: an FT8 exchange, a WSPR report, or a JS8Call chat for example.

This got me to thinking about a similarity at my work QTH and how we differentiated MDS from "overall system sensitivity" or OSS.

My work involved radar systems used for air traffic control and weather detection. At regular intervals, we measured the MDS and/or the OSS of around a dozen radars in southeast Texas.

To measure MDS, a signal is injected into the receiver front-end. This signal is reduced in power as we watch the receiver's output (now a detected video signal) decay on a spectrum display. We noted the signal level that resulted in "minimum discernability" and this was the MDS.

Pretty simple, and very similar to how MDS is measured in ham radio receivers.

OSS, though, was a bit different in that it took into account, not just the receiver, but the downstream equipment required to produce a "blip" from an aircraft on an air traffic controller's display, typically many miles (and LAN networks, T1 lines, microwave links, etc) away.

For our secondary radar (Mode S) system, an alphanumeric data block would be attached to the aircraft's representation on the scope. This data included the aircraft ID, altitude and speed info that had been pulse modulated onto the return pulse by the aircraft's transponder. 

The purpose of OSS, as opposed to MDS, was to measure the minimum signal level that would result in the alphanumeric being displayed on an air traffic controller's scope.

To do this, we injected a signal into the receiver. This "synthetic target", as we called it, would be Doppler-shifted (as an aircraft's radar return would be) and delayed in time to represent a given distance from the radar. And it would be repetitive so as to appear at 32 bearings, simultaneously.

If the radar under test was part of a transponder system (secondary radar), the synthetic target would contain fake aircraft ID, altitude and speed data - the info that would be presented to the ATC.

Air traffic controllers would see a circle of identical targets at, for example, 40 miles out. We then dialed back the power level of the injected pulses until half of them disappeared. This value, in dBm, is the OSS.

As a practical example, a radar receiver might have an MDS of -133 dBM whereas a radar/display system might have an OSS of -129 dBm.

The QDX, and the computer used for FT8 and other digital modes, is similar: the QDX is the receiver; the QDX + PC + software (WSJT-X) is the system.

In ham radio, MDS of our equipment is typically measured in CW mode at a bandwidth of 500Hz. Digital-only rigs like the QDX and the QRP Guys AFP-FSK Digital Transceiver can't be selected for CW operation and have a bandwidth of at least 3000 Hz.

IC-705 feeding a variably-attenuated FT8 signal into a QDX

I decided to measure the OSS of my QDX and Dell PC/WSJT-X combo by injecting an FT8 signal into the QDX and then adding attenuation to the RF path until I reached the threshold of detection for this set-up.

This won't give me the MDS of the QDX but it will let me know the overall system sensitivity of the QDX/PC/WSJT-X combo.

This means that I would need a lot of external attenuation since I anticipate the OSS to be somewhere around -120 dBm or better.

  • My injected FT8 test signal was produced by an IC-705 dialed back to 250mW output (-6dBm) measured with a just-calibrated WM-2
  • 73 dB of fixed attenuators took the FT8 signal down to -79 dBm
  • A Heckit step attenuator then provides up to 62 dB of selectable attenuation in 1 dB steps
  • An Elecraft CP1 was initially used to provide 30dB of attenuation. This resulted in strong decodes even at impossible-to-decode signal levels due to stray RF (the CP1 is unhoused). Fixed attenuators and a shielded step attenuator are required for this test.

On 20m, I found the OSS of the QDX+Dell laptop/WSJT-X to vary between -131 and -137 dBm.




  1. "between -131 and -137 dBm" >> is that good/bad/acceptable?
    I'd say: not bad for commercial toys. What's the expert's take?

    BTW thanks for sharing, I love these short T&M posts. It kinda is where ham radio took off for me, working in the Rohde & Schwarz repair and calibration lab. Licensed ham to the left of me, amateur radio operator to the right of me. I was the clown and the joker. Little did I know...

    1. Good morning, Franki,

      I don't know if it"s typical, good or bad - or why the variation from one decode to the next. The recent review of the QDX in QST failed to mention any receiver specs. I don't think there's yet a standard to which digi-only receivers are tested.