[INRAD] Switching diodes in roofing filter for the IC-775DSP

[William E. Sabin]

In my homebrew rcvr (see QRZ.COM, W0IYH) I use 1N4007 PIN diodes in the 
front end AGC circuits.  A problem with significant IMD occurs when these 
diodes (or any other PIN diodes or ordinary diodes) are just beginning to 
forward-conduct.  I use 3 diodes in series at each AGC location and my IMD 
measurements show that this eliminates the diodes as a source of *increased* 
IMD, regardless of their state of conduction.

I use MPN3404 PIN diodes to switch the 12 crystals in the first local 
oscillator because of their forward-biased storage time and low back-biased 
capacitance. I don't want the turned-off xtals to influence frequency via 
the turned-off diodes due to turned-off diode varactor effects. The 
interactions between the 12 xtals from 10.4 MHz to 38.4 MHz are negligible.

For the nine hamband RF filters I use miniature Radio Shack (RS 
275-241)relays which are inexpensive and very reliable (over many years zero 
failures in many low power applications). The simplicity of this method and 
the reduction of DC power consumption are desirable (the off-relays consume 
zero power).

Bill W0IYH

----- Original Message ----- 
From: "Charlie Mazoch Jr." <w5vin at earthlink.net>
To: "Anders Janis SM4RNA" <sm4rna at telia.com>; <list at inrad.net>
Sent: Thursday, November 24, 2005 9:29 AM
Subject: Re: [INRAD] Switching diodes in roofing filter for the IC-775DSP


> Hi:
>  If you read W8JI's notes on switching diodes the 1N4148 is just fine
> as a switch.