I am using 1N270 hot carrier diodes which are more sensitive than the past, small signal diodes, in QSD mixer designs. I borrowed the idea from the SPL-1 and TUF-1 mixers.
I was looking at trying to make something someone could put together really easy and cheap like to experiment with. I did a file on them and uploaded it. I have had to redo the original file text since when I did some test here and plotted the real life performance of the 1N270, I realized I could make the detector much more sensitive. And I discovered in the plots a very linear region of operation. The impedance of the diode in this region with regards to the changing current and voltage is 10 ohms. And that surprised me. Unusual also is that it maintains a constant pn junction capacitance of 8 pf? So it must have a small point contact junction? With a 0.2 volt signal bias change, the small signal current changes 20mA. Hence 10 ohms impedance. So it logically followed to design the QSD diode detector stage as a 50 ohm stage to follow a 50 phase splitter transformer, since the diode impedance was relatively low in this circuit. It terminates into a 10 ohm load across which you can connect a line level input of from 1k up to 50k. The max signal voltage at output is 0.2 volts. I will upload the better diode QSD circuit idea when I have it all calculated. The design data will help others set up this circuit. Anyways with that view in mind I decided to redo the QSD circuit concept for a 50 ohm input of 0.04 watts into the splitter. Which is around 60 dB of front end gain for average rf signals across the band. I am thinking someone might get by with 40 dB and others might try 80 dB of front end gain. But at the risk of more base band noise. 60 dB seems to be the ideal range of gain ahead of this circuit. The Phase splitter transformer now sees a 50 ohm circuit after it. It ends with a simple -3dB RC filter for 100 kHz. I am looking at the AD797 low noise amplifier and trying to determine its frequency response. If it is not well suited for rf then it would work well in the I/Q section. Some of you may know well how this will work in rf. I do not see enough data on its rf performance however. Some data sheets on the web should not be published without complete treatment of the product. It comes in standard DIP and surface mount packages. It is primarily used in oscilloscope amplifiers.
