[1000mp] IP3 on Rob Sherwood's site

[Bill Tippett]

K5UJ wrote:
 >Bill, I was responding to the PIN diode mod.

         OK...I thought that had been answered already.  When
both Inrad and W8JI say it is a myth, and when Sherwood
Engineering shows no improved MDS performance in a PIN
modified IC-781, that is "good enough" for me...Ulrich Rhode's
comments not withstanding.

                                         73,  Bill  W4ZV

INRAD's comments:

http://mailman.qth.net/pipermail/1000mp/2004-October/016498.html

----- Original Message -----
From: "International Radio" 
<<http://mailman.qth.net/mailman/listinfo/1000mp>inrad at rosenet.net>
To: "WD8ARZ" <<http://mailman.qth.net/mailman/listinfo/1000mp>wd8arz at 
ix.netcom.com>
Sent: Monday, April 19, 2004 2:05 PM
Subject: Re: New Kit Idea?? HP 5082-3081 diodes Upgrade


Bill,

First of all, this pin diode mod thing is a myth.  Pin diodes are not as
good a switch as regular switching diodes.   I think the input diodes are
already pin diodes (could be wrong without looking).
Once past the input filters there is no need to use pins.  Most people that
have done this mod report lower noise.  The reason for it however, is due to
the extra losses created by the higher resistance pin diodes.  The net
effect is a higher noise floor and poorer sensitivity.  People without a
good generator cannot tell this, of course.

Secondly, the attached message stating 10 dB reduction in internal noise
makes no sense as most of the noise is due to the 455 kHz IF and the pin
diodes are far away from that.  Unless of course his noise floor went up by
10 dB.  If everything worked as well as the theory suggests, there would be
NO change in internal noise.  The third order dynamic range would be
increased by 3 to 6 dB (not noticeable to the operator) and the second order
dynamic range could be improved by a substantial amount.  Second order means
two signals that add or subtract to give the tuned frequency.  i.e.  8 + 6 =
14, etc.  Most people don't have a problem with second order for several
reasons.  1. The 1000 MP is already good.  2.  The Mark V VRF preselector
eliminates the need as it makes the radios second order DR so high it can't
be measured without burning up the radio.  3.  Monoband antennas don't
respond to signals that far away from the tuned frequency.  And 4.
Contesters use external band pass filters which also eliminate the problem.

Thanks for the thought, however.  Watch our web site for an announcement of
a new roofing filter mod we are coming out with that will make a REAL
improvement in DR performance.

George
------------------------------


W8JI's comments:

http://www.w8ji.com/receivers.htm

Receiver Myths



PIN Diode Mods

Save your money. PIN diode replacement never has changed performance in any 
receiver I've listened to or measured. PIN diode mods don't change 
distortion, blocking, noise, or any other parameter. A normal signal diode 
with proper bias is just as good.

PIN diodes function as "RF switches" or "linear RF resistors" only when the 
carrier lifetime exceeds the period of an RF cycle by a large margin. Most 
of the diodes used in PIN mods do not have long enough carrier lifetime to 
even behave like a low-distortion linear resistance, let along a pure 
switch. They really aren't any better than manufacturers stock diodes, with 
the PIN diodes barely being linear at 30MHz let alone at 455kHz. The whole 
"PIN diode thing" would be laughable if it wasn't costing people money!


Sherwood Engineering's Measurements:

http://www.sherweng.com/table.html

Compare "Stock 781" to "781 PIN Diodes" down ~1/3 from top.

1.  MDS (sensitivity) is degraded 2 dB
2.  BDR is improved 3 dB
3.  IMD performance is degraded 6 dB

The slight improvement in BDR is definitely not worth
giving up 6 dB in IMD degradation and 2 dB sensitivity
in my opinion.