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QUESTION 1: Why does the ALT-512 have higher receiver performance than most other SDR transceivers, including many costing several times as much money?
ANSWER 1: There are several factors contributing to this:
Most SDR transceivers were designed to be a 'Jack of all Trades' (which unfortunately means 'Master of None'). SDR radios may be used for many things (i.e., Contesting/DXing; Broadband Monitoring, SWLing, etc.). Optimizing for any one of these means compromising on the others, unless you add expensive complexity and cost.
The ALT-512 was design by experienced Contesters, for Contesters and DXers. Throughout its design phase, it was tested on-site at one of Europe's largest Multi-Multi Contest stations. Special attention was given to reception of weak signals in the presence of multiple strong signals.
Most SDR transceivers except the expensive multi-thousand dollar models use very broad 'Bandpass Filters' (BPF) in their front-end, covering multiple ham bands, as well as shortwave broadcast bands. Many radios have only one Low Pass Filter (LPF) at the top of the HF band. Both of these are typically 'broad as a barn door', allowing a large spectrum of frequency to be passed through to the digital circuitry without attenuation. The sum of all of these signals passing through to the digital circuitry easily overdrives the ADC and other digital components, causing signal distortion due to the receiver's front end 'crunching'!
The ALT-512 uses dedicated 'Bandpass Filters' (BPF) for the Ham Bands. Like an efficient pre-selector, these filters reject out-of-band signals efficiently (i.e., strong shortwave broadcast stations). This significantly reduces the signal strength of the sum of all of the RF applied to its digital circuitry. As a result, the radio can process a much higher level of in-band signals without crunching.
Most SDR transceivers use a single Analog to Digital Converter (ADC) to convert RF to voiceband for digital processing. This produces two signals, "I" and "Q", 90 degrees out of phase with each other, which are then processed in the DSP circuitry.
The ALT-512 applies the analog signals to a pair of ADC's, not just one. Each ADC produces an I and Q signal; one produces a +90 deg. shift, the other produces a -90 deg. shift. We call this 'Differential ADCs'. The benefit is, there are three different signals that can be processed in DSP, with 2 of them being 180 degrees out of phase. This enables greater noise reduction, which besides improving the singal to noise ratio, also enables using sharper (narrower) DSP filters without ringing. It also improves the notch-depth of the notch filter.
QUESTION 2: Why is the maximum supply voltage limited to 13.6 volts instead of 13.8 volts or even higher?
ANSWER 2: The simple answer is, for performance reasons.
This is explained in detail on a dedicated page, here: 13.6 Volts
QUESTION 3: Why does the ALT-512 not have a General Coverage Receiver like most other SDR transceivers have?
ANSWER 3: The simple answer is, it was designed for ultimate performance, rather than being an 'All-in-One' transceiver. Focusing on performance has the following advantages:
QUESTION 4: Why does the ALT-512 not have built-in features such as an ATU, battery pack, or CW paddle?
ANSWER 4: Although having these features built-in is advantages, it also limits flexibility, by limiting the operator to the features of one specific device.
QUESTION 5: What improvements does the ALT-512 have over its predecessor, the SKY-SDR?
ANSWER 5: There are numerous improvements in the hardware, software ergonomics and servicability:
QUESTION 6: What is the tuning rate of the ALT-512's VFO knob?
ANSWER 6: There are two answers to this question. The ALT-512 uses an opto-coupler with 254 steps per revolution. It has two modes of operation, Linear and Variable. These are selected in the software Menu #00 (Valcoder).
1) Plain Mode: The rate of change is linear and the step size is varied by software using the yellow 'STEP' button on the front panel. The step size is also mode-dependant, as follows:
2) Intelligent Mode: The rate of change varies with the speed at which you rotate the VFO knob. The faster you rotate it, the faster it changes frequency.
To be continued . . .