Spectrum display Magnify mode = Zoom FFT

There was a longstanding wish of many mcHF users to be able to see the signals in the spectrum or waterfall display with ore resolution. Clint has provided us with a 2x Magnification mode that doubles the resolution, but in reality the same resolution is used, but the points in the display are interpolated. The problem remained that it needs more processor power to calculate higher resolution FFTs in order to obtain more resolution. The solution is to use a technique that is described by Lyons (2011) as the Zoom FFT.

The Zoom FFT is used when fine-grained spectral resolution is needed within a small portion of a signal´s overall frequency bandwidth range (Lyons 2011). That is exactly the situation we face when we want to see a small part of the spectrum display of the mcHF, but with high resolution. The Zoom FFT helps us to perform this without having to calculate high order FFTs. It consists of the following components:

1. complex frequency conversion to baseband
2. lowpass filtering of the I & Q path
3. downsampling of I & Q
4. complex FFT of I & Q

But how can we save processor cycles, if we perform all of these steps instead of just calculating one higher order FFT?

Well, step 1 has already been performed in the mcHF, because we need the frequency conversion for the filtering and demodulation, so this step has already been performed and does not take any additional processor power. For step 2, the decimation lowpass filtering, we use highly efficient cascaded IIR biquad filters that take low processor power. The coefficients have been precalculated, we use four stages with elliptic response for steepest filter response and highest stopband. The following cutoff frequencies are used: 1x magnification = 48kHz spectrum = 24kHz decimation filter [hardware! no software filter used!] 2x magnification = 24kHz spectrum = 12kHz biquad decimation filter 4x magnification = 12kHz spectrum = 6kHz biquad decimation filter 8x magnification = 6kHz spectrum = 3kHz biquad decimation filter 16x magnification = 3kHz spectrum = 1.5kHz biquad decimation filter 32x magnification = 1.5kHz spectrum = 750Hz biquad decimation filter