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Message-Id: <6.2.3.4.2.20060405185606.0dfa87d0@pop.tscm.com>
Date:	Fri, 14 Apr 2006 00:35:07 -0400
To:	"TSCM-L Professionals List" <TSCM-L2006@googlegroups.com>
From:	"James M. Atkinson" <jm...@tscm.com>
Subject: Re: [TSCM-L] Re: NLJD and cell phones
In-Reply-To: <1144272252.447029.307130@j33g2000cwa.googlegroups.com>
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This is why it is important to have more than one instrument to 
perform non-linear junction detection.

If you know the frequency of some part of the eavesdropping device, 
such as the control frequency of 433.92 MHz it is a simple matter of 
pulsing that signal out to the suspect area and then listen on the 
harmonics for it bouncing back. The 433.92 MHz signal will get though 
the filters and be reflected back at you, if the filtering on the 
final stages of the device is poor then you will get a non-linear 
response on the harmonics of interest (866, 1301, 1735 MHz, etc). 
However, if the filters in the bug are good, and the circuit is well 
designed you will not get a detectable signal on the harmonics, but 
will a good reflection on at 433.92 MHz.

A fixed band NLJD does a pretty good job of detecting imbedded 
devices when you are no more than a few inches away, and a fixed band 
NLJD is an extremely critical part of any TSCM project. But a fixed 
band NLJD is different that a fixed FREQUENCY NLJD. A fixed tuned 
unit will give you superior results on poor quality spyshop devices, 
raw circuit boards common with hobbyists, and devices that lack the 
extensive filtering and isolation present in more expensive bugs. An 
example of fixed tune NLJD is the original SuperScout that consisted 
of a transmitter on a single frequency that required that you open 
the case (which took an hour), then the tweaking of multiple pots, 
caps, and filters to just nudge the illumination frequency just a 
little. It then took 3 more hours of sacrificing small animals to the 
deity of your choice, and wave a chicken foot over the control box to 
coax the 'Scout to behave itself. One of the painful problems with a 
fixed tune NLJD is that a spy can very easily detect their use, and 
can develop some method to get the bug out before the sweep team 
finds the device. There is also the problem of a fixed tuned NLJD 
choking on interference on a congested frequency. Since many of the 
older NLJD's where fixed dead in the middle of the 902-928 band there 
was massive problems with other electronics operating dead on the 
same frequency. To counter this you had to re-tune your 'Scout a 
little above or a little below this %+**&^ point.

A tunable NLJD is much more desirable instrument, but you do take a 
minor performance hit. On the other hand, with modern electronics a 
tunable NLJD can be made extremely inexpensively for one of the ISM 
bands like 902-928, although there will be a huge expense in the 
mechanical parts of the NLJD. If someone was so interested they could 
actually build a fairly effective NLJD for less than $200 in 
electronic parts by using the circuits from a 900 MHz/2.4 GHz 
cordless phone, but the milling of the enclosure and mechanical parts 
would cost much more.

A swept tuned NLJD is the ideal solution, but most TSCM folks lack 
the capability of obtaining the equipment to do this, and even if 
they did assemble the equipment it would only be practical to use for 
short durations, and small areas due to potential interference 
issues. A swept tuned unit is best used where the thing being tested 
is placed inside a shielded enclosure, or where the testing 
environment is highly controlled such as in an underground or shielded SCIF.

A pulsed NLJD illumination circuit is desirable because the receiver 
circuit can be blanked out only at the exact moment the transmitter 
is lighting up the target. In this manner the receivers can be made 
to be much more sensitive, or at least create the illusion 
of  increased sensitivity.

A mutli-band NLJD is more desirable than a switched band NLJD. In a 
multi-band unit you have three separate receivers that are phase 
locked to each other, which are phase locked to the illumination 
transmitter. In a switched or multi-band NLJD you transmit the 
illumination signal, listen with one receiver, then re-tune the 
receiver, transmit another signal, then tune back to the original signal.

-jma


At 05:24 PM 4/5/2006, contr...@yahoo.co.uk wrote:

>A releted general question
>what happens with NLJD's if there's a circuit very close that is
>
>A) exactly in resonance at the working-frequency of that NLJD
>B) almost/tending to resonance of the working-frequency of that NLJD
>
>I assume absorption or attenuation or similar takes place ?
>
>contranl


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