There are a couple of areas that has sparked my interest lately, I'm burning out with software projects.
- Parametric amplifiers, mixers, and oscillators.
- Tunnel diodes used as amplifiers and oscillators, supplies of available parts are all but gone. May be a moot issue if item #3 works out.
- Experiment with Lambda diodes as substitutes to Tunnel diodes in new designs.
- Anti-Parallel diode sub-harmonic mixers.
- Regenerative Receivers
- Use some of these older devices and techniques with Software Defined Radios.
- Linear RF Power Amplifiers
One project that I will try to do is creating a replacement for the the SoftRock radio using parametric mixers instead of FET commutating switches, the advantage besides simplicity is the frequency range possible with a parametric mixer which can be easily into the Microwave range.
Another area to be explored is using anti-parallel diodes as the mixers of a SDR receiver. Towards that end I have acquired a large NOS supply of Agilent (HP) dual matched diodes that are very fast. A quick prototype yielded a 6M receiver that worked quite well considering the time spent on it, shortly I will begin experimenting with a 2M version. Dual matched anti-parallel diodes as available for $1 that can reach 10's of GHz.
Another area to explore is using FET switches to implement an equivalent to the anti-parallel diodes, the advantages are that the clock circuitry can remain digital while the clocking speed is 1/2 the normal clocking speed
I have bought a good supply of 27pf, 45pf, 90pf, and 420pf high Q Varactors in lead packages which should be useful at HF, VHF, and UHF frequencies but I'm looking for an inexpensive source of higher capacity units( 220 pF ) for use at HF frequencies. One source that looks promising is Power Schoktty Diodes, I have about a dozen different devices that I need to test in an actual radio, they range in value from 80pF to 700pf at 1V bias while having 10,000V dv/dt capability so they should work great in the HF bands.
Other items that I have acquired lately in anticipation of having additional time and room available are several RF Power Transistors that I was able to buy in large quantities and are available in my store for less than $1 in quantity 1.