Re: HOWTO: Control your QRPLabs RX from HDSDR on a PC

Erik Kaashoek

HOWTO: Wideband modifications of RX

Issue: The audio transformers are never exactly the same to side band surpression can not be aligned over a large audio frequency range
Solution: Remove the audio transformers connecting I/Q output directly to C1/C2 and AF Ground to to RX ground.

Issue: A ground loop can cause a lot of high frequency spikes in the audio due to USB currents.
Solution: To avoid ground the loop isolate PC USB->ATTiny85->SI5351 ground from the RX ground using a HF isolation transformer in the SI5351 output (torroid with 2 windings, each 5 turns, one connecting to the SI5351, other to the LO input of the RX). 

Issue: The RX audio output bandwith is rather limited making 192kHz audio input on the PC of limited added value
Solution: Replace C3, C4, C5 and C6 (original 100nF) with 2.2nF


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