[INRAD] PIN DIODES

[Charlie Mazoch Jr.]

Hi Tom:
  This PIN diode fad started out in the days when Hewlett Packard was 
making their series of PIN diodes. There were a lot of them installed 
into the FT-1000 Yaesu series. I installed a set into an Icom 735 and 
saw zero improvement.  I'm in full agreement with your remark about 
comments listed on W8JI's and the Sherwood sites. My beliefs concerning  
PIN switches in low signal receive path switching  is that as 
semiconductor barrier voltages increase so does electronic or resistor 
noise. The advantages of the PIN is that it acts as a variable resistor 
at RF and a 50 ma. forward bias may switch 1 ampere of RF  whereas in 
the PN junction diode this forward bias must be greater than the 
switching current. When one introduces electronic noise into a receivers 
front end by using inexpensive or sloppy design and then amplifies it 
100 + Dbm it's annoying in the least. I believe that as the Hf noise 
floor rises because of man made interference the equipment manufacturers 
don't put as much emphasis into low noise receiver design. I say this 
because if one looks at specifications of VHF and UHF equipment the MDS 
and noise figures seem better. I personally use Schottky diodes as low 
level signal path switches. Their barrier voltage is lower, 0.37 versus 
0.68 volts for hot carrier diodes. The GaAs devices offer much lower 
noise figure and their barrier voltage is less than silicon devices. 
This also leads to higher Ft's. I'm not an RF engineer but modification 
of my personal equipment has led to less electronic noise. This is by 
measuring MDS and noise versus signal. As stated in your e-mail relay 
switching is quieter than solid state if one utilizes quality devices. I 
believe W8JI's comment was that a 1N4148 PN junction diode was adequate 
for low level signal path switching. and a PIN diode offered no visible 
improvement on his measuring equipment. In fact he dares someone to 
prove him wrong. Not by ear but through testing with good equipment. His 
comments on linear amplifier tune up and VHF parasitic suppression in HF 
amplifiers are also worth reading.  Charlie  W5VIN