Norbert Kohns, DG1KPN
Intermediate Frequency Amplifier with Wide Range of Adjustment
VHF Communications 2/1996
Please note that none of the circuit or block diagrams, photographs, etc., are included here
Stimulated by the suggestions for expansion of the spectrum analyser from DK6ZK proposed in [1], I tried to find an integrated circuit which has the function of a regulated intermediate-frequency amplifier. During the search, I came upon a well-known component from Motorola, the MC1350 gain block, an integrated monolithic intermediate-frequency amplifier.
1. DESCRIPTION OF CIRCUIT
The biggest advantage of this circuit lies in the fact that both the amplification and the damping can be adjusted within wide limits using the AGC input. According to the data sheet [2], the power gain is 50dB and the AGC range -60dB.
Fig.1 shows the wiring diagram for the 10.7 MHz amplifier, following the data sheet. The amplifier input is automatically matched through a 50W resistance. The collector voltage for the output stage of the amplifier must be fed through the centre tapping of T1 to pins-1 and 8 of MC1350. The coupling coil transforms the impedance to 50W.
2. CIRCUIT ASSEMBLY
The transformer, T1, is wound onto a 6-mm. diameter coil former using 0.13 mm enameled copper wire. 24 turns are wound around the coil former and fixed on using mixed adhesive. 2.5 turns are wound over the primary winding in the vicinity of the centre tap as coupling, and these are also fixed on.
The inductance of the coil should be app. 6mH, so that it can be brought into resonance at 10.7 MHz with a 36pF capacitor.
Fig.2 shows the amplifier prototype, and Fig's.3 and 4 the layout and the component overlay for the printed circuit board, which should be soldered into a standard tinplate housing. The component side of the PCB is a continuous ground plane.
The top face of the printed circuit board is fully copper-lined. The bores are briefly countersunk, down to the earth connections, using a 3 mm. thick spherical milling head.
Signal inputs and outputs are fitted with SMA sockets. The 15V DC supply and the AGC voltage are fed through 1nF feedthrough capacitors.
3. PUTTING INTO OPERATION
Once all the components have been mounted on the printed circuit board, the supply voltage is applied and the AGC input is connected to earth (maximum amplification). Precisely 10.7 MHz is then fed in at app. -50dBm. The output should then be approximately 0dBm. T1 is tuned for maximum power output.
Fig.5 shows the resistance values for the voltage potential divider, which generates the AGC voltage required from an input voltage of 15V DC. This voltage must be variable between app. 5.5V DC and 7.3V DC.
The 22kW potentiometer of the spectrum analyser's frame wiring can be used for this purpose, by setting the supply for the 465 MHz intermediate-frequency amplifier to 15V DC permanently, as it no longer needs to be adjustable. So no additional potentiometer is required, which makes fitting into the spectrum analyser easier.
For a direct voltage of 5.5V DC at the AGC input, we obtain an amplification of app. 45dB. 0dB can be obtained at aproximately 6.7V DC, and app. 60dB damping at 7.3V DC.
4. INCORPORATION INTO THE SPECTRUM ANALYSER
This intermediate frequency amplifier circuit is very general in its range of application. Here, for example, it is incorporated into the spectrum analyser created by DB1NV. The intermediate frequency amplifier is simply looped in by means of a coaxial cable between the assemblies DB1NV 006, IF out, the quartz filter, DB1NV 008, and IF in.
T1 can now be fine tuned in the spectrum analyser at the maximum amplitude of the zero mark using the display.
When the spectrum analyser is operating normally, the gain potentiometer is set in such a way that, by and large, the noise level peaks precisely in the vertical position on the display. At this point, the amplification is precisely 0dB.
Starting from this setting, we can now increase signals by app. 40dB or reduce them by app. 50 - 60dB, depending on their power - practical testing will tell you far more about this than a long description from me!
With my spectrum analyser, I can identify signals satisfactorily, even at -120dBm, with maximum amplification from the 10.7 MHz amplifier and in a selected band width of 1 kHz.
5. COMPONENT LIST
not included here
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