[Test-Equipment] Helper Instruments SM-1000

[Eric Lemmon]

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
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