[Test-Equipment] Helper Instruments SM-1000

[Eric Lemmon]

John,

The modulation is direct FM, with deviation limited to about +/- 5 kHz,
just like "standard" land mobile radio.  The emissions are F1D and F2D.

My comment about using a very high frequency (800 MHz range) for
calibration was intended to make the point that errors are multiplied
when up-converted.  Let's suppose that your receiver is capable of
resolving a WWV signal at 10 MHz to an accuracy of 0.1 PPM, which is
VERY good.  That means that your 10 MHz oscillator is known to an
accuracy of 1 Hz.  Now, if you up-convert this to the 800 MHz band, a
factor of 80, the error at 800 MHz can be as much as 80 Hz.  That's far
too much "slop" at 800 MHz, and that is why I suggested using a cellular
signal that is accurate to 1 Hz in the 800 MHz band.

Whether or not a particular cellular telephone block is GPS disciplined
is usually governed by the technology used.  Older analog and TDMA
systems may or may not be GPS synchronized, but CDMA systems must be, in
order to keep each of the cell sites in step with each other for smooth
handoffs.

I really appreciate your response, and I hope that my suggestions are
found useful to you.

73, Eric Lemmon WB6FLY

John Kolb wrote:
> 
> Very Interesting.
> 
> With an Icom R-7000 receiver, I pick up a moderate strength beeping
> signal at
> 879.600, about 1 PPS. What sort of modulation is used? AM so there's a
> constant carrier?
> 
> Would it be reasonable to take a 10 MHz time base, divide down to 100 kHz,
> make a skinny pulse rich in harmonics and adjust the time base so the
> 8796 th
> harmonic is zero beat with the signal?
> 
> John
> 
> Eric Lemmon wrote:
> 
> >Jim,
> >
> >I owned an SM-1000 service monitor for a while, and I found a listing
> >for it in an old TESSCO catalog.  The published specification is +/- 0.5
> >PPM after a 24-hour warmup.  It should be left plugged in, even if not
> >turned on, in order to keep the oven running.
> >
> >It will work fine without a battery, but make certain the battery wires
> >are taped and stowed.  You did replace all of the metallic tape used to
> >make the seams RF-tight after you buttoned up the unit, didn't you?  A
> >lot of RF can leak out otherwise, corrupting measurements at very low RF
> >levels.
> >
> >One way to calibrate the master oscillator to a very high accuracy is to
> >tune in a GPS-disciplined control channel on one of your local 800 MHz
> >cellular telephone sites.  There are 21 control channels on each of two
> >blocks of assignments, Block A and Block B.  The Block A control
> >channels are at 30 kHz intervals beginning at 879.390 MHz and ending at
> >879.990 MHz.  The Block B control channels are at 30 kHz intervals
> >beginning at 880.020 MHz and ending at 880.620 MHz.  Each cell site will
> >have one control channel operating continuously, and the modulation
> >sounds like a musical buzz.  There is no voice on a control channel, so
> >there is no issue of eavesdropping.  One of the cellular blocks is
> >assigned to the local wireline carrier, and the other is assigned to a
> >non-wireline carrier.
> >
> >Put a short antenna on the receive input jack of your service monitor,
> >and search for the strongest control signal.  If each of the control
> >channels within a block show a similar frequency error, they are very
> >likely synchronized to GPS and the error is in your service monitor.
> >The SM-1000 has a small hole in the rear panel for adjusting the master
> >oscillator.
> >
> >The accuracy of the GPS-synchronized control channels is about 1 Hz, or
> >about 0.001 PPM.  You can't get that accuracy by tuning in WWV at the
> >bottom end of the service monitor's range.
> >
> >FYI, Helper Instruments was sold several years ago to Zetron, which
> >still sells some test equipment with the Helper label, but the SM-1000
> >was discontinued just before the sale.
> >
> >73, Eric Lemmon WB6FLY
> >
> >antqradio at juno.com wrote:
> >
> >
> >>Greetings
> >>First time post from a long time lurker.
> >>I have the above and just received a copy of the operators's
> >>instructions.  Now I should be able to properly operate this neat piece
> >>of test equipment.  The generator has output, the meters seem to work OK,
> >>so I assume it is working properly.  The problem is I don't have the over
> >>all specifications, they aren't in the instruction manual.  Helper
> >>doesn't list it as current production, in fact, it seems they doesn't
> >>support it any longer.
> >>
> >>So, what are the SM-1000 capabilities?
> >>What is the published frequency (XLO) stability?  Must be OK since it has
> >>a crystal oven!
> >>Is it OK to operate without the internal battery?  Mine was DOA so I
> >>removed it.
> >>
> >>Inquiring minds want to know!
> >>Jim
> >>Hot Springs, AR
> >>
> >>
> >
> >