Measurements
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Measurements

Conditions Channel Signal dBm
for '5' on
the meter
Signal
dBm for
0dB SNR
Comment
Before changes Ch 2   -123.5 Coils had been previously tuned to channel 2
Moved mixer drain to centre-tap of first IF Ch 2   -122.2 Gain loss not measured, 1.3 dB loss in SNR
As above, but the next day Ch 2   -122.8 Shows relative accuracy achieved in these tests.
Restore mixer drain, but reduce RF gain by a 56K in parallel with R41 Ch 2   -123.2  
Tuned L1 and L3 to Ch 2 Ch 2   -122.8 Again, shows comparative accuracy - shows the fractions of a dB are only estimates.
Tuned L2 and L4 to Ch 1 Ch 2   -123.2 Possible interaction of the middle section filter
  Ch 3   -123.2 The next three measurements ...
  Ch 4   -123.2 ... show variation of SNR across the band, ...
  Ch 5   -122.9 ... but tell us nothing about the gain.
Carefully retuned L1..L4 to Ch 1 Ch 1   -123.5  
  Ch 2   -122.8  
  Ch 3   -123.5  
  Ch 4   -123.5  
  Ch 5   -123  
Final tweak of tuning, make all circuits resonate just below channel 1. Ch 1 -107 -123.6 With old filter: -105/-121.0
  Ch 2 -106.2 -122.7 -104/-120.2
  Ch 3 -105.8 -123 -103.6/-120.5
  Ch 4 -104.5 -123 -102.5/-120.6
  Ch 5 -102 -122.5 -99/-119.5
Restore full RF gain to check Ch 3 -106.5   with old filter -104.5, conclude that 56K had reduced gain by about 1dB.
Try 27K in parallel with R41 Ch 1 -106 -123.2 SNR loss is acceptable, but would like to get more reduction yet in RF gain to approach the reduction when the old filter was in use.
  Ch 2 -105 -122.5  
  Ch 3 -104 -123  
  Ch 4 -103.2 -122.8  
  Ch 5 -100.8 -122  
Try 22K in parallel with R41 Ch 1 -105 -123 SNR loss 0.5dB, gain loss ~2.5dB
  Ch 2 -104 -122.3  
  Ch 3 -103.3 -122.9 SNR loss 0.6dB, gain loss ~3.2dB
  Ch 4 -102.7 -122.5  
  Ch 5 -99.7 -121.8 SNR loss 1.2dB, gain loss ~6.8 dB

Test conditions

  • Generator: Marconi TF2015, uncalibrated but agreed with spectrum analyser
  • Signal: 5KHz deviation frequency modulated
  • Noise meter: specially written program, filtering generator modulation frequency
  • Channels: Ch1 = 137.30, Ch2 = 137.40, Ch3 = 137.50, Ch4 = 137.62, Ch5 = 137.85 MHz
 

 
Copyright © David Taylor, Edinburgh   Last modified: 2015 Jan 18 at 09:32