Re: Wi-Spy Spectrum Analyzer

On Mon, 02 Jan 2006, in the Usenet newsgroup alt.internet.wireless, in article
<j8ljr19a03ql3h0njc76g1bdi8uisa26k1@xxxxxxx>, Jeff Liebermann wrote:

>ibuprofin@xxxxxxxxxxxxxxxxxxxxxx (Moe Trin) hath wroth:

>The channels are 1MHz wide. In theory, the receiver sensitivity is
>-90dBm while the overload point (on pg 27) is -10dBm (typ). That
>should yield a theoretical 80dB dynamic range. However, the RSSI
>indicator starts at -95dBm with 1-10 units and saturates at -40dBm
>with 28 to 31 units. That's only 55dB at best (probably less). With
>a 1MHz receiver, the base line noise level is probably about -85dBm
>(my guess) yielding only 45dB of dynamic range. Since I based my
>guess(tm) of dynamic range on the Wi-Spy signal level readings, which
>may or may not be accurate, the dynamic range may actually be larger.

In the 1970s, a "low cost" General Aviation avionics manufacturer got a
contract to build prototypes of a microwave landing system receiver. The
concept of MLS wanted a log IF, and the "high cost" General Aviation
avionics was using discrete log amps - about US$20 a piece, five per
receiver to get 96 dB of dynamic range. The "low cost" house design guy
was thumbing through a data book, and came across the RCA CA3089 FM IF
strip on a chip. It had a signal strength indication that (on the data
***) looked within about 2 dB of a nice log curve over 90 dB. Lab
tests on two or three samples confirmed, and they based their design on
that. In unit quantities, you can get this for under a buck and a half -
in quantity 1000, they probably give it away. Can you spell DIRT CHEAP?
You can see it coming, right? When they bought for production, the best
you could say was that the curve was monotonic. Huge flat spots separated
by near verticals. They wound up with plastic versions of the discrete,
at $6 a pop. They also had a fine time shoe-horning 6 mini-dips, 5 extra
chokes, 10 or so caps in to space formerly occupied by a single 16 pin DIP,
a toroid, and such. RCA got off the hook, because "that parameter is not
specified".

>Reminder. That was from the on-screen calibration markings, which
>might be wrong.

Sounds similar. RCA never thought about using the meter output as a log
IF (though it _sorta_ worked that way), and none but a house looking to
build something for really low costs would have tried. Actually, I gave
very high marks for the design attempt. It just didn't work out that way.

>The 1977 HP test equipment catalog prices the 8555 front end for
>$7700. Add $1475 for the 140T main frame and $3775 for the hi res IF
>section. That's what I use for a spectrum analyzer.

Sounds about right. Really _great_ product. Really expensive.

>Hmmm... There's a complete 141T on eBay for $680 total. Tempting.

I can't remember what we got as a replacement in 1992, but it was more
bells and whistles for a similar price. I'd never heard of the vendor,
but you know government purchasing regs.

>>Mini Circuits Lab used to make a nice low noise preamp - problem is the
>>onezy cost is probably more than you paid for the Wi-Spy.
>
>Those were MMIC's and had a lousy noise figure.

No, they had some nice ones, in addition to those three-terminal pieces
of crap.

>I make my own using a GaAs xsistor. However, if I can mount the radio
>very close to the antenna, it probably won't do much good.

True - then you have to add a step attenuator, and this is _really_ getting
expensive.

>> As long as the Mighty San Lorenzo is staying within it's banks (you guys
>> are hogging all the precip - we got a 10% chance tonight, which would be
>> the first rain since early November).

It didn't rain either, and there's none in the 7 day forecast now.

>My palatial office is right next to the levee in Santa Cruz. Glug...

At least it's not the Russian. Seems like every couple of years, that
place would flood to umpti-dump feet over flood stage, yet people would
buy and build right next to the river time after time, flood after flood.
I can't understand how any lender or insurance type would stand it.

Old guy
.

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