Corrections made to previous Measurements & Calculations on April Fool's Day

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.          

Analysis of my BPF using Elsie

I havent done anything practical with my BPF since my last Blog, instead I have been scratching my head for reasons why with the exception of 12 and 10 Meters my measurements result in insertion loss values so much higher than I had expected. Either I havent tuned the BPFs correctly or the coils and / or capacitors I have used have a much lower Q than the norm, or the switching diodes have a much higher ohmic on value @ 10 mA than specified. I am not aware of whether there is a "right" way of going about tuning the BPF, the method I used was to use a succession of tweaks on all coils which because of interaction was repeated several times until I achieved a condition that provided a simultaneous minimum insertion loss at both band edges. The Qs of coils and capacitors I have used are quoted as being typically 85 and 2000, and the on resistance of diodes better than 1 ohm. I have not measured any of these independently, but I did try varying the DC bias current through the diodes from 5 to 20 mA and could barely perceive any change on the filter performance, so I doubt if they are responsible.
Although I was previously aware of Jim Tonne's Electrical Filter Software and had downloaded his free version of Elsie http://www.tonnesoftware.com/elsie.html some time ago I had never used it before today, so this morning I decided to explore it further to see if I could analyse my BPF using his software model. I discovered that by using the Manual Parts Entry feature in his program I was able to do this quite easily using the Schematic, Entry, Analysis, Plot and Tune features provided. As an example I have saved a file of a simulation of the 20 Meter Band Pass Filter which I have called 20MeterTuned.LCT at http://groups.yahoo.com/group/BITX20/files/g3oth/ which can be opened and viewed after launching the Elsie program. I am most impressed by this program, and am now going to model the other BPFs using it. The predicted results for the 20 Meter Band are better than I measured, even after substituting Qs as low as 50 and 500 for coils and capacitors respectively, so I suspect there must be something wrong with my previous tuning technique after all.
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