QCX and IQ SSB reception

QCX analysis

SSB? Well it was a bright idea. 

The QCX+ has pins on the mother board for picking off the Tayloe  detector I & Q audio signals. So how about bring these out via a couple of capacitors to a 3.5mm jack socket ? To which I could connect an audio cable to my PC audio input (Mic). Actually my Mac Mini does not have a mike input (!), so I used a USB external soundcard plug in dongle.

Grey wire is GND

Doing this I had the hope that I could run an SDR program like HDSDR and get 96kHz bandwidth display of frequencies around the centre RX tuning point (700Hz lower than the displayed TX frequency remember).

But it was not what I expected, what I got was this

FT8 signals on 40m band.

And the bandwidth is not +/- 48kHz but more like +/-3kHz!! I guess this is due in part to the low frequency response of the op amp stages after the CMOS Tayloe detector switch and the integration capacitors on the switch. Any way it was a big disappointment. Of course it could still be used for SSB reception by tuning the RX across the band to keep the wanted signal in the centre of the display... but it's not SDR.

Bandwidth

So where is this small bandwidth controlled? In two places:

1. By the input impedance, nominally 50R antenna plus the 10R on resistance of the FST3253 switch = 60R total, on for 1/4 of the time, so actually 240R*, and the 470nF shunt capacitors. 

* As pointed out by W6OQ this makes a filter of 4.8kHz, thanks

2. By the feedback across the op amp of 10k and 10nF, which together make a lowpass filter of 1.6kHz!

Putting this in  LTSpice change components as you wish to simulate... we have

Schematic

and a response of

Response,  Green overall, Blue first filter

So there we are, we cannot get a wide bandwidth SDR display from the I & Q signals from the QCX+.  But it should work OK for a station tuned to the centre frequency where the overall BW is around 2.3kHz at -6dB down.

How to improve for SDR?

We cab change the component values, perhaps like this

New component values for wider BW SDR

and get this

Wide bandwidth possibility

And feed the I&Q signals to a PC based SDR program.

Right now I am reluctant to start tearing out 6 caps and replacing them. We will see later.

How about the QCX's own phase shifters

This is quite a clever piece of circuitry, but with limited performance - OK for the narrow band SDR use, and USB only. Essentially it two of these networks, one with fixed R2, C1 R5, C2 values to give a phase shift across 100-5000Hz. like this

Giving a phase shift (dotted line, ignore the solid) like this

Phase = Dotted line

This gives a reference phase shift of -125 to -320 deg. 100-5000Hz. Then a second network like this

Giving a phase shift as close as possible to 90 deg smaller, like this

Phase = Dotted line

If you take the difference between these you get the grey line in the chart below

Grey = phase difference, Green = reference, Blue = 2nd network

The second network is adjusted during the radio alignment to optimise the differential phase shift to as near as 90deg as possible to detect only USB signals.

But to note is that the 90deg shift is not too bad from 100-2000Hz and so would be OK for USB SSB signals. See the SSB reception below.

Conclusion

QCX+ is an excellent design for its purpose, a good Tayloe detector with limited audio bandwidth, a satisfactory phase network for USB reception and an innovative narrow CW tone filter. All good design.

As many have looked into it can receive SSB in a restricted audio bandwidth, picking off the audio from just before the CW filter, but is not a wide band SDR.

Post Script

I just connected the IQ outputs to my MacBook running CubicSDR program. The QCX is tuned to 7050kHz, this is the IQ spectrum across +/- 48 kHz audio

IQ Spectrum, very narrow. SDR at 7,050,000Hz 

On the left my own strong local CW transmission at 7,020,000Hz

And here's the same CW signal at 7,020,000Hz with the QCX+ tuned to  TX 7,020,000Hz, thus receiving at RX centre of 7019300Hz. You can see how much more gain there is at lower audio frequencies.

CW received on USB in the narrow audio passband, 

TX at 7,020,000Hz

RX & SDR at 7,019, 300Hz USB gives 700Hz tone

Actual SSB reception

This is a test transmission on 7.1MHz LSB made by my ELAD FDM-DUO and received by the QCX+ and decoded using CubicSDR program

Testing with a transmission from my ELAD FDM-DUO

7100kHz LSB

As you can see the QCX+ is capable of narrow band SSB reception when signals are decoded from the  I & Q signals on the main board. These were fed to a StarTech Analog Digital convertor running at 96kHz and then to the CubicSDR program running on my MacBook. They were not readable on the QCX+ itself, due to the CW filter bandwidth muffling them of course.