Well April Fool's Day turned out to be a constructive one for me after all. This morning after experimenting with Elsie and plotting the response of all the BPFs using firstly their nominal component values, then secondly using their L values adjusted to give a better response for each Band, I realised that the method I had used for measuring and calculating loss, which had been troubling me, was wrong. I should not in fact have terminated the input to the BPF with 50 ohms and then used the voltage Vin developed across it as the divisor when calculating the insertion loss from Vout. Instead I should have based the divisor on the voltage Vin developed across a 50 ohm load with only it connected to the output of the DDS with the BPF completely removed and disconnected. At first sight it might seem that this would produce an even worse result but in fact the reverse turns out to be true.
Since building my DDS and temporarily increasing drive to the 2N3866 for more output. I have been aware that although its output when measured on open circuit varies from a maximum of about 5 volts pk to pk at 2 MHz to a minimum of about 1.5 volts pk to pk at 20 MHz it does not behave similarly when loaded by 50 ohms, but instead it delivers a reasonably steady output of 1 volt pk to pk which is flat from about to 1 MHz to about 20 MHz. This would seem to imply that the internal Thevenin impedance of the DDS varies from about 200 ohms to 25 ohms over this frequency range. As a consequence the revised values used for Vin were taken from actual measurements made at each frequency of interest across a 50 ohm load before connecting the DDS to the BPF and measuring Vout across the 50 ohm BPF output terminating load. The loss calculated using these two values thus being the true insertion loss of the filter when introduced into the system. Measured results now compare much more favourably to what I had expected and that predicted by the software model even though I have so far not made any further adjustments to the coils in the BPF since their alignment on Wednesday. I do intend however doing an alignment check again before I fit the coils for 160 Meters to ascertain if I can improve the performance of the individual BPFs still further. The results obtained at present being as follows:
80 Meter Band
KHz Vin mV pp Vout mV pp Loss dB
3,300 1000 110 19.2
3,400 1000 340 9.4
3,500 1000 520 5.7
3,600 1000 520 5.7
3,700 1000 550 5.2
3,800 1000 420 7.5
3,900 1000 180 14.9
4,000 1000 90 20.9
40 Meter Band
KHz Vin mV pp Vout mV pp Loss dB
6,900 1080 310 10.8
7,000 1080 510 5.8
7,100 1080 570 4.9
7,200 1080 540 5.4
7,300 1080 360 8.6
7,400 1080 190 14.4
30 Meter Band
KHz Vin mV pp Vout mV pp Loss dB
9,900 1140 110 19.2
10,000 1140 250 12.4
10,100 1140 270 11.7
10,200 1140 150 17.1
20 Meter Band
KHz Vin mV pp Vout mV pp Loss dB
13,900 1220 400 9.6
14,000 1220 500 7.7
14,100 1220 630 5.7
14,200 1220 700 4.9
14,300 1220 620 5.8
14,400 1220 580 6.4
14.500 1220 370 8.4
17 Meter Band
KHz Vin mV pp Vout mV pp Loss dB
17,800 1100 190 15.4
17,900 1100 280 12.0
18,000 1100 340 10.2
18,100 1100 300 11.4
18,200 1100 230 13.6
18,300 1100 190 15.4
15 Meter Band
KHz Vin mV pp Vout mV pp Loss dB
20,900 920 190 13.6
21,000 920 260 11.1
21,100 920 340 8.6
21,200 920 340 8.6
21,300 900 300 9.6
21,400 900 250 11.0
21.500 900 220 12.4
12 Meter Band
KHz Vin mV pp Vout mV pp Loss dB
24,800 760 380 6.0
24,900 760 370 6.2
25,000 740 370 6.0
25,100 740 380 5.8
10 Meter Band
KHz Vin mV pp Vout mV pp Loss dB
28,000 640 215 9.4
28,500 640 250 8.2
29,000 640 350 5.4
29,500 640 350 5.4
29,700 640 265 7.7
30,000 640 175 11.4
I have posted some Elsie files using nominal and tuned inductor values pertaining to the above at http://groups.yahoo.com/group/BITX20/files/g3oth/ for comparison purposes. To view them you will need to download, save and then import them into the Elsie program when launched.
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