I recently received my assembled QMX+. It is a very neat transceiver,
well designed and nicely built.
I have played with the USB, reading the IQ samples from the 20-bit,
48-kHz ADC. The QMX+ is in CW mode and IQ is enabled. I cannot find
information on the architecture, but I guess it is like follows.
The antenna signal passes through the bandpass filters to the Tayloe
detector. The frequency to the mixer is the dialed frequency minus the
IF frequency of 12000 Hz minus the side tone frequencey of 700 Hz. If I
dial 7020 kHz the mixer will use (7020000-12000-700) Hz = 7007300 Hz.
The resulting product passes through an amplifier with a lowpass filter
in front of the ADC, about 24-kHz bandwidth. The sampled signal has two
channels, I and Q, real and imaginary part. These signals, which have a
bandwidth of ±24 kHz, are available at the USB port. If I listen to a CW
signal tuned to 7020 kHz, it appears as a +12700-Hz signal on the USB
port.
The CPU in the QMX processes the signal by mixing it with 12 kHz
(complex sine, exp(jwt)). The resulting signal, I/real part only,
(appears at +700 Hz), is then passed through the CW bandpass filter,
nominally adjusted around 700 Hz. I have attached a PDF sketch of my
understanding.
Is this description correct?
I experiment with a configuration where the CW filter is a (IQ/complex,
two equal channels) Gaussian low-pass filter around zero frequency with
a bandwidth of 10-50 Hz. This corresponds to 20-100 Hz bandwith in a
bandpass configuration. Consequently, I mix the USB signal with IF+ST,
before feeding it to the filter. The LP output is then mixed with the ST
frequency to the speaker/earphone. (Note that zero beat is not useful in
the analog implementation due to noise and bias.)
Best regards
Anders, SM5KAE
PS I get some tiny spikes 1 kHz apart in the FFT of the USB data. Are
these generated in my setup? DS
