|"High Performance" Pre-Amplifiers for 28, 50 & 70MHz Ham
Originally Published in Radio and Electronics World in September 1982
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The design uses a PCB, which is common to all three preamplifiers, the only differences being in the area of the input band-pass filters. The loss in these filters is typically 1-1.5dB which does add directly to the noise figure, but the reduction in out of band spurii was thought to be a significant advantage in view of the nature of the bands covered. When used in conjunction with the up converter the increase in sensitivity was from about lµV to 0.2µv for 12dB SINAD (FM). The gain (about 20dB) is all that is needed to overcome the conversion losses in the converter but can cause problems if used with an already sensitive receiver. Making R2 a variable resistor gives about 45dB of gain control. If this control is to be mounted away from the PCB, the leads should be well screened and decoupled.
The input is matched to Q1 by the input filter comprising LI, L2, Cl, C2, C3. The responses of the three filters are shown in Fig. 1,2 and 3. The input match is not particularly good as it has been sacrificed in favour of noise and out of band rejection performance. The gain may be altered by varying gate 1 voltage (by reducing the value of R2). The output is matched to 50R by the monofilar transformer Ti. The reactive component is not removed, but in practice the output return loss (VSWR) is adequate (about 8dB). 3SK88 and BF960 FETS were tried in the 6 and 4M versions and gave improved gain and noise figures but it wasn't thought worthwhile to specify these for most purposes.
|Construction and Testing
All the components are assembled onto the PCB. Any that require an earth connection should have this soldered directly to the earth plane. The uncommitted earth pads are for guidance only. Attach the tap to L1 as listed in Table 1. Set the cores of Ll and L2 to the positions indicated in Table 1. The use of ready-made coils means that the tuning should not be too far out. Connect a l0-12V power supply; the current drawn should be about 8-lOmA. Find a signal in the middle of the band and connect the preamp to the receiver. Peak the signal by adjusting Ll and L2. Repeat this with signals at the band edges. Recheck over the entire coverage of the preamp.
Take a 15cm length of 0.25mm wire. Wind one turn through a Fair-Rite 28-43002402 core. Form the tap by folding back and twisting together about 6mm of wire. Wind a further four turns onto the former.
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|Printed Circuit Board ArtworkThis should print 1:1 if printed at 300dpi. Toko components may be obtained from BEC Distribution in the UK. For other components, try some of the distributors listed here
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© 1982 G. Leighton © 2006 radioshop.co.uk