Cascode IF Amplifier With AGC For The G6LBQ Irwell HF Transceiver

 

HF Transceiver IF Amplifier Module

"Cascode IF Amplifier & AGC For The G6LBQ Irwell HF Transceiver Project"

In this blog post I present details of  my Cascode IF amplifier with AGC. The IF amplifier constitutes as a practical module for my HF multi-mode radio transceiver that I call the "Irwell" transceiver.

My version of the Cascode IF amplifier is based on Wes Hayward's (W7ZOI) & Jeff Damm's (WA7MLH) as presented in QST magazine December 2007.

The Cascode IF amplifier is fairly simple yet it offers high performance and low noise when compared to IC amplifiers like the MC1350P, a device that has often featured in home-brew radio & transceiver builds though now obsolete.

The IF amplifier contains three hybrid cascode transistor pairs comprising of a J310 JFET transistor and a 2N3904  general purpose NPN transistor. The gain of the IF amplifier with the AGC switched off is 60dB.

Here is the schematic diagram for my version of the cascode IF amplifier.

Click image to enlarge to full size! 

The three cascode stages offer 100dB of AGC range, this is made possible by applying a positive bias to the source of the J310 FET's using three 1N4148 signal diodes, without this bias voltage the AGC range would be limited.

The first two cascode stages contain tuned output circuits, the output of the third stage drives a broadband transformer with the secondary winding connecting to a differential transistor pair that feed the AGC detector diode and SSB/CW product detector output.

AM demodulation is done with an Analogue Devices AD8361 which is intended to be used as a mean-responding power detector for exact measurements of RF power. As an AM de-modulator the recovered audio sounds really nice and distortion free.

I designed a PCB for the IF amplifier using mainly surface mount components which are mostly 1206 size. Here's a CAD image of the PCB

Actual PCB Size is 100mm by 50mm

 And here's the PCB fully populated

Actual PCB Size is 100mm by 50mm

In use the IF amplifier performs well and the AGC is smooth in operation with all signals sounding equal in audio level.  I have now built 3 of the IF amplifiers boards and all have performed the same with no instabilities.

The cascode IF amplifier is an excellent building block for use in homebrew receivers and transceivers. Using the component values shown it will tune from 10MHz to 12 MHz but is easily adapted to other IF frequencies.

Whilst the cascode IF amplifier may not be cutting edge and appear a little dated it works well and despite experimenting with other devices and options I always end up back at the cascode!

I have also built a prototype IF amplifier using two low noise AD603 voltage controlled amplifiers, the AD603 is a fixed gain device with a built in 7 stage linear stepped attenuator. As the AD603 operates at a fixed gain noise and distortion are kept to a minimum making the device an excellent choice for IF subsystems.  My initial attempts at an AGC circuit to control the AD603's produced some instabilities so I need to spend some more time on this but for your interest here is an image of the prototype AD603 IF amplifier I built.

Actual PCB Size is 85mm by 42mm

The plan of intention with the Irwell transceiver project is such that nothing is set in stone and the modular approach allows for changes to be made whether it be to experiment or facilitate and upgrade. 

So am I done with the IF amplifier?   Hell no you can expect more IF amplifiers shenanigans from me in the future :)

Click Here For My Follow Up Cascode IF Amplifier article. 

Project files will be made available via the Groups.io platform by joining my G6LBQ community group where you can discuss my projects, ask questions and help others.

Joining my group is free just click on the button below.

Until next time... 

 

73's From Andy G6LBQ

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