I started with the log amp/detector with 3 MHz input,
and ended up with 28 transistors ( ! ).
Today a better choise is the AD606 or the cheaper
AD8407 from Analog Devices.
Filter assembly:
I made a 3 MHz BP filter with BW´s 100-300Hz-1-3-10-
30-100-300 kHz. Four xtals at 3.072 MHz were used for
BW´s up to 3 kHz. A four section LC-filter gave BW´s
from 10 to 300 kHz.
The LC-filter is based on four separate resonant
circuits with Q-multiplication. (Design from HP).
It is posssible to vary the BW without changing the gain.
When using the LC-filter the xtal-filter is bypassed.
The xtals operate in the series-resonant mode. By
varying the load impedance for each section the BW is
changed.
The final result is close to a factory made SA.
My SA had three frequency conversions.
Input 0-200 MHz
1 mixer (LO swept from 284 to 484 MHz)
1 IF= 284 MHz (BP-filter with three resonators)
2 mixer (Osc input 234 MHz. 117 MHz xtal osc
multiplied by two)
2 IF= 50 MHz (BP-filter with four resonators)
3 mixer (Osc input 47 MHz from LC-osc)
3 IF= 3.072 MHz
All mixers are similar to SBL-1 from Mini Circuits.
The filters were designed by info from " Handbook of
Filter Synthesis" by A. Zverev.
Important: BP-filters have spurious passbands far above the main
frequency. An additional low-pass filter in series
with the BP-filter can sometimes be useful to suppress
strange mixing products.
Gain distribution in a SA:
An input level to the first mixer of max -20 dBm (22.4 mV) shall give
approx. 100 mV to an AD606 log.det. for for full deflection on a scope
with 8 vertical divisions and 10dB/Div. This means a gain of 5.
With -20 dBm to the first mixer it is a good idea to have about
the same level to mixer 2 and 3. MAR amplifiers from Mini-Circuits
can be used to compensate for losses in the mixers and filters.
3 dB PAD´s can be distributed to improve matching.
First LO: 284-484 MHz
Obtaining low phase-noise in a VCO is difficult. A factory made VCO
is recommended. The control voltage must be free from noise. With
narrow BW´s phase noise turns up. An RC-network (3 k and 1uF to
ground) at the control input improves matter. A low-noise op-amp
in the sweep-circuit is a good idea.
Powersupply:
Magnetic fields can induce hum-voltages in the VCO-control voltage.
A toroidal mains-transformer is recommended.
It is a great learning experience to build a SA.
Feel free to ask questions.
Regards, Stein, LA7MI