eliminating the sum frequency in
Consider the case of checking
the harmonic output of a 3. 5 MHz
transmitter: The oscillator frequency
that will produce 1,013 MHz
difference at the fundamental
frequency is 1,016.5 MHz, the sum
frequency is 1,020 MHz and well
outside the bandpass. At frequencies
below 1 MHz, the sum and
difference will both be passed on to
the second mixer and its output will
contain four frequencies: the sum
and difference of the difference, and
the sum and difference of the sum.
The sum frequencies are in the 2
GHz range and not able to pass
through the 10. 7 MHz filter.
If the filter bandwidth is wide
enough to pass both difference
frequencies, there will be two signals
shown on the scope, but if the
bandwidth is narrow enough to
separate them, there will be only one
signal shown on the scope. If you are
going to analyze low frequencies, a
better approach is software and a
sound card for your computer. No
other equipment is needed.
A varactor diode can only tune
less than 2:1, so how is a sweep
oscillator made to cover one to 500
MHz? It is done by mixing two high
frequency oscillators. If one oscillator
is 1,000 MHz and the swept
oscillator is 1,001 to 1,500 MHz, the
difference is the output of one to
500 MHz. This would not be easy for
me — who never worked above 300
MHz — but some know how to do it.
To illustrate how it is done, there is a
schematic of a zero to 50 MHz VCO
QFor a future article, how
about including the entire
analog tachometer circuit
partially shown in the
January 2009 issue, page 27,
Figure 3, using the LM2907?
■ FIGURE 2
AThanks for the idea!
Although I designed an
LM2907 circuit, I didn’t
build it. What I built was a
WHEEL CIRCUMFERENCE = 10. 55 FT.
DRIVE SHAFT ROTATES 2 (31/2)/8 TIMES
FOR ONE WHEEL ROTATION, OTHER
WHEEL STATIONARY. ASSUME THAT IF
BOTH WHEELS ROTATE, DRIVE SHAFT
ROTATION WOULD BE 4 7/8 TIMES FOR
ONE WHEEL ROTATION.
MILE = 5280 FT, WHEEL ROTATES 500.47
TIMES, DRIVE SHAFT ROTATES 2439.81
IF I PUT 16 MAGNETS ON THE DRIVE
SHAFT, PULSES PER MILE = 39036.97.
AT 60 MPH, PULSES PER SECOND = 650.6
AT 6 MPH, PPS = 65.06
AT 5 MPH, PPS = 54. 22
AT 50 MPH, PPS =542.2
AT 70 MPH, PPS = 759.05
■ FIGURE 4
■ FIGURE 3
May 2009 33