The IF Block

I’ve long wanted to develop a set of re-usable building blocks for making homebrew radios. My vision includes a switched RF input bandpass filter block, a high-performance 1st mixer, a Tx/Rx IF block, an audio power amp and a control module that will include the Si5351 (for both LO’s) as well as an LCD display and user controls. By mixing and matching different incarnations of these blocks, many variations of radios can be built. My work is based on extensive use of the FST3257 CMOS switch for the mixer, BiDi amplifiers, and product detector/balanced modulator. I just finished testing and optimizing a homebrew PCB for the IF subsystem that I’m now calling “UTIF” (Universal Transceiver IF System). The block diagram is shown below. The schematic will follow as soon as I clean it up to reflect my latest mods:
The UTIF takes care of everything after the crystal filter and up to (but not including) the audio power amplifier. It uses just 3 chips, a few transistors and a bunch of resistors and  capacitors. There is also one easy-to-wind binocular core transformer and a couple of low-power regulators so that it can be run from 12VDC. LO drive (BFO) is supplied from an Si5351 or an Si570. Initial test results are very encouraging. Here are some preliminary specs as-tested:
IF Frequency                 9 Mhz; re-tunable up to 30 Mhz
IF Input impedance        50 ohms
Sensitivity                    Approx 0.3uv for 10dB S/N at audio output (2.5 khz BW)
AGC                            Approx 90 dB, including AGC and audio limiting
Audio Out                    Approx 2Vpp into 1K load
Mic Input                    Approx 5mv for full saturated DSB out
DSB Carrier Balance      Better than 45 dB
DSB Tx Out                 Approx +3dbm per sideband
CW Tx Out                   Approx 3 dbm via modulator DC-unbalance method
Current (12VDC)           Approx 45 ma Rx; approx 60 ma Tx
These specs are, of course, very preliminary and subject to change. There are still some details that have to be cleaned up, but next step will be to lay out a “proper” PCB that can be reproduced by a low-cost PCB fab and then release the design files so others can buy and/or modify the design. I’d like to think that much of the great control system work done by the group over at Minima  will be applicable. This will all be posted here as things progress..

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