Re: DIY FM tuner with 6CW4 nuvistor and ECC85
A lot of receivers nowadays use I/Q mixdown from the incoming signal frequency into a pair of 1-pole lowpass filters, a bit of variable gain and then a pair of ADCs. ADI and all the usual semiconductor firms have had products out for some time. Last I looked, the Lime chips seemed amongst the best.
BUT
The dynamic range is rather limited. The synthesisers with on-chip VCOs and fairly basic Frac-N technology are noisy. Quite a bit of incoming bandwidth hits the ADCs. Also the cancellation of racing I and Q pthes against each other is limited. Fancy calibration helps, but not enough.
The HF transceiver currently set up in my radio shack up-converts to an IF in the 60MHz region (carefully more than twice 30MHz) where it has three sets of crystal filters (AM/FM, SSB, CW) then it mixes down to just above audio for its single ADC. Things do NOT go I/Q in the analogue sections, that happens only in nice, precise, digital processing. The synth is a multiloop one.
There is a preselector with switched inductors and a switched C, 2C, 4C, 16C etc network. All switched by relays!
David
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Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done
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