EMRFD Message Archive 2271
Message Date From Subject 2271 2008-10-17 08:43:17 bkopski 250 MHz SA ADAPTOR ( SA-A) Reference: EMRFD, "Expanding Performance", 7.32
Years ago I built a variant of the popular W7ZOI / QST spectrum
analyzer which has been in use ever since. Mine was built to cover
100 MHz instead of the original 70 MHz because I wanted to include
the 70 MHz spectrum as used in radio control models. But I've always
wanted something "bigger and better" still having the realization
that no matter what the capability is, it will never be enough!
With the inspiration of the Reference, I have completed a swept
converter an "Adaptor" to put in front of the SA - which extends
the range to 250 MHz. The Adaptor itself and the SA + Adaptor combo
are shown in new photos in the K3NHI photo folder. The display shown
is that of a 25 MHz comb input to the SA-A at 25 MHz/grid.
Using mostly stuff on hand, the Adaptor block diagram consists of a
300 MHz 7 element input low pass ( CD "Hi-Lo" program), a TUFF-1, and
a POS 800W VCO for the LO. This is set up to convert ~0 to 250 MHz
input to 400 MHz which is then passed thru a 400 MHz triple tuned
filter. (The TTF was described earlier on this Group under K3NHI.)
The second converter is based on an NE602 with an LO input of 435 MHz
and includes a single tuned 35 MHz amplifier following the 602. The
resulting 35 MHz output is fed to the original SA now used as a fix-
tuned i.f. The overall Adaptor chain gain is set up for unity
transmission or 0 dB gain / loss.
Other details include using an SA-A interconnection scheme to remove
the SA ramp drive to its VCO and reroute it thru reshaping
(nonlinear) circuitry to drive the POS 800W with linear frequency
motion. At the same time the SA VCO drive is rerouted to an
adjustable DC control voltage in the Adaptor to tune the SA to 35
MHz. SA power is also used in the Adaptor. The Adaptor is activated
simply by turning its power switch ON and connecting its 35 MHz
output to the SA input.
Now, if I could just see to 500 MHz ..!!!