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Old 1st Aug 2021, 5:41 am   #41
Radio Wrangler
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Join Date: Mar 2012
Location: Fife, Scotland, UK.
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Default Re: DIY FM tuner with 6CW4 nuvistor and ECC85

Quadrature signal path techniques can, in theory, convert and demodulate anything. There are plenty of articles and app notes on what they can do.

Information on their limitations is less common, but crucially important.

They are related to 'phasing method' SSB receivers but are handling wider bandwidths. So the unwanted sideband is full of unwanted signals and needs to be rejected. Rejection is done by cancellation not filtering. Cancellation is a subtractive process. If the I and Q paths are split in the analogue domain you suddenly find yourself needing superb matching between the two channels, and precision phase shifters.

If your channels are only matched to 1% in gain, you only get 40dB suppression of the unwanted stuff. Similarly if your precision in phase terms is one degree, then that's also about 40dB suppression. This is going to take better than 1% resistors and amazing matching of RF amplifiers, mixers etc.

Software calibration can correct this and give you 10 to 20dB improvement, but your receiver is still at a big disadvantage to an classic analogue filtered version.

The escape route is to digitise the incoming signal with only one ADC, and then to play I & Q games entirely in the digital domain. You can have superb precision here and get superb cancellation. I & Q techniques are popular in DSP because they economise on the amount of processor power needed to do the necessary tasks.

There are receivers coming out where the whole 0-30MHz band is digitised in one fell swoop. They are definitely catching on. Their Achilles heel is that the ADC has to be scaled to handle the total peak voltage of all signals combined. Doing this pushes small signals towards the noise floor. They need decent preselector filters to try to reduce the total voltage hitting the ADC so they can be better scaled to handle individual signals.

They're not bad nowadays and the Icom IC-7300 is proving very popular. Their top end transceivers use analogue conversions and narrow filters ahead of their ADC, as well as preselectors. Look at the IC-7700 and IC-7800 families.

David
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