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

Fri Nov 25 11:16:32 CST 2005

Hi Thomas:
Very well described. The PIN acts as a pure variable resistance at RF 
and microwave frequencies thus one would not suffer the distortion as in 
a PN junction diode unless it is hard forward biased. The 1N4007 is a 
diffused junction diode consisting of a PN junction. It's depletion 
region acts as a barrier in a reverse biased condition.. But as you so 
accurately describe it does not offer the isolation of a PIN diode when 
used as an RF switch.     73's    Charlie   W5VIN

Thomas wrote:

> Bill,
>
> Disclaimer - by no means am I an expert on the subject of PIN diodes, 
> but I understand this to be the case:
>
> The PIN diode differs from standard PN junction diodes due to the 
> intentionally wide intrinsic layer between the P and N regions of the 
> diode. Yes, the reduced capacitance aids in the OFF state isolation, 
> but it is the minority carrier recombination time that is of real 
> value in RF switching applications.
>
> Appropriately applied PIN diodes have very long recombination times 
> compared to standard PN junction switching diodes. Most any switching 
> diode can be used in an RF switching application, but as you mention 
> they must be heavily forward biased to place their operation well into 
> the exponential forward biased region. Doing so allows a linear signal 
> to pass with minimal distortion. The PIN differs in that, because of 
> the intrinsic region with its long recombination time, appears more as 
> a low value resistance (<1 Ohm) than as a PN junction. The PIN diode 
> typically required much less forward current than a PN diode used in 
> the same RF switching application.
>
> The mistake that many hams make is assuming that any PIN diode will 
> work in most any application. The MPN3404 is rated for VHF switching 
> applications, because its recombination time is short; a few 
> microseconds. Yet, I often see the MPN 3404 used in HF RF and IF 
> applicaitions. Not to say that it won't act as an RF switch, but it 
> will not provide the isolation or low distortion of a more suitable HF 
> PIN device. By a more suitable HF PIN device, I suggest a device like 
> the HP5082-3081, with its 2500nS recombination time (1 tau).
>
> The reason people often use the IN4007 in PIN applications is it has a 
> doping profile quite similar to that of HF PIN diodes. Elecraft 
> successfully applies the IN4007 as an RF switch in their K2 
> transceiver design. They are able to achieve very good IMD performance 
> with this low cost device. That said, it is hard to beat a good RF 
> relay for isolation and IMD performance.
>
> '73, Thomas - AC7A
>
>
> Charlie Mazoch Jr. wrote:
>
>> Charlie Mazoch Jr. wrote:
>>
>>> Hi Bill:
>>>   The 1N4007 is a 1 amp 1000 PEV PN junction silicon diode mostly 
>>> used in HV power supplies. I agree about IMD when PN diodes are just 
>>> beginning to forward conduct. That's what diode mixer circuits are 
>>> all about. But if a diode is hard forward conducting as in switch 
>>> applications how does one get mixing?  It seems to me that one would 
>>> have to overcome this forward bias before mixing can occur. In my 
>>> Kenwood TS-2000 the unused band pass switching diodes have a reverse 
>>> bias applied. They use a resistor in an SMT transistor to accomplish 
>>> this novel bias scheme.
>>> I do agree about PIN diodes offering lower capacitance than a PN 
>>> junction diode in their reverse bias configuration. That's what the 
>>> intrinsic layer is all about.    73's
>>> Charlie  W5VIN
>>>
>>> William E. Sabin wrote:
>>>
>>>> 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.   
>>>>
>>>>
>>>>
>>>>
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