With my University Degree complete, I had much more free time to work on my superheterodyne reciever. Armed with more knowledge than my 2025 self, I decided it was worth redesigning most of it. Beginning with the frontend amplifier, I decided to try the cascode amplifier once again.

I recently purchased a Roland Rubix22 USB audio interface second-hand for my PC, and being that it has XLR and 1/4 inch mic jacks, I thought it would be fun to have my hand at making a microphone. While looking into the different types of microphones that exist, I chose to try making a ribbon mic.

Working on my Vacuum Flourescent Display (VFD) filament driver, I realized I have no easy way to test the saturation current of my home made inductors. Characterizing the inductors core material is necessary to creating an efficient transformer, so I built a test unit based on the principles of inductance.

The previous cascode design was insufficient, so I redesigned it with much better results.

When testing the 20m superheterodyne reciever, there was a lot of noise generated by the previous frontend amplifier design, so I have decided to re-design the frontend amplifier. In this approach, I utilize the cascode configuration.

To amplify the IF portion of the 20m superheterodyne recievers signal chain, I though I would give the W7ZOI and K3NHI's termination insensitive amplifier design a try.

Still unsatisfied with my crystal filter, I decided to order some 8 MHz crystals. This decision ended up producing the best crystal filter for the reciever.

For the 20m superheterodyne reciever, I need two mixers for the RF-IF stage and IF-Audio stage. My initial diode ring mixer design functioned as intended, but this time I chose to make it more compact and to use different diodes. ~ I also built two this time.

With the new crystals characterized, I take on a new approach to designing a crystal filter.

To effectively build a Crystal filter, I characterized the new 4 MHz crystals I ordered. Using two methods, the NanoVNA-H with DiSlord firmware, and the G3UUR method.

After a dissapointing first attempt, I redesigned the crystal filter again. This time using a slightly different approach.

The next step in building my 20m superheterodyne reciever is the crystal filter. This filter will determine the selectivity of my radio's tuning and dynamic range. The goal is to make a crystal filter with a bandwidth of 3 kHz for SSB operation.

To prevent any strong out of band signals from overloading the frontend of my reciever, I designed a bandpass filter to place before the rf amplifier.

The next key component of my 20m Superheterodyne Reciever project was the mixer. In a previous post I did some experimentation with a Gilbert cell mixer, which yielded promising results, but there is a simpler approach that requires less components. This brings us to the double balanced diode ring mixer (or double balance mixer, DBM for short).

After the two previous designs, I decided to purchase some transistors rated for a higher frequency. This is a more pay-to-win approach to my previous issues, but works nonetheless. An additional bonus is the noise figure of the new transistors being significantly lower.

At my PC I usually use headphones and an FX-Audio DAC to listen to audio, but occasionally I will hook up my JBL partybox to play music with a speaker. The issue I was having was that the line level output from the DAC was at a fixed volume. So as a small, simple, 1 hour project, I made a stereo volume knob for the line level output.

Learning about the NanoVNA, I found that testing amplifiers without any attenuators resulted in terrible resutls. Because I want to test my amplifier designs using the NanoVNA, I made some simple 50 ohm attenuators.

Having acquired more knowledge on amplifier configurations and properties, I redesigned the front-end amplifier. This time using a common base as the first stage, then common emitter as a second stage, with a common collector at the output to match the impedance to 50 ohms for the VNA measurements.

With the ongoing superheterodyne reciever project, I designed and simulated a recieve amplifier to amplify the signals coming from the antenna. I have chosen to use a bipolar junction transistor in the common emitter configuration as my amplifier.

Lately I have been chipping away at the idea of creating a superheterodyne reciever (and potentially tranciever). One of the key blocks within the design of a superheterodyne reciever is a mixer. I have a number of options when it comes to mixer topologies, and among them I wanted to give the gilbert cell a look.