This project is based on a design by Andy G6LBQ and employs a prototype PCB supplied by Sunil VU3SUA and Inductors supplied by Spectrum Communications in the UK and is primarily intended for use in the G6LBQ Multi Band BITX but could be used in conjunction with any ham band receiver or transceiver where a diode switched BPF is required.
I received the prototype PCB kindly donated by Sunil from India during thr first week of March 2012. As I had earlier already purchased the coils in a joint buy with Andy from Spectrum Communications in anticipation of the PCB being produced, I was quickly able to confirm that all coils and components I had on hand fitted perfectly to the PCB.
The special BA243 PIN diodes and NPO ceramic capacitors required to complete the build were then procured from local UK distributors and Ebay auctions using the values that had been computed by Andy and checked earlier by myself. These were delivered and fitted without difficulty to the board the following week. The PCB is laid out to accomodate 0.2 inch (5mm) spaced capacitors throughout and I used Ceramic NPO Disc capacitors for the lower filter values and Ceramic Multilayer NPO capacitors for the higher filter values and Ceramic Multilayer Y5V capacitors for coupling and de-coupling purposes.
Rather than use the 470 ohm and 390 ohm resistor value combination to forward bias the PIN diodes as recommended in Andy's BOM based on operation from 12 volts I chose instead to use 100 ohms and 330 ohms so that I could still operate and switch them at 10 mA from 5 volts provided by the output band switch control PIC utilised in my recently completed VU3CNS DDS.
Apart from fitting the coils used for 160 meters, my BPF PCB assembly is now complete and I have carried out some provisional tests this weekend using my VU3CNS DDS as a signal generator and Tektronix Model 465 Oscilloscope to align and evaluate it's performance as a stand alone unit.
To do this I terminated both input and output ports of the BPF with 50 ohm resistors and across each resistor connected x10 probes which in turn are connected to each of the 2 channel inputs of my scope. I then connected the output of my DDS set to 3.5 MHz to the input port of the BPF and applied a 5 volt DC signal to the 80 meter Band Selector Port and observed the resultant traces on the oscilloscope whilst adjusting each of the corresponding band coil cores for maximum output from the BPF. I repeated the procedure several times using signals up to 3.8 MHz from the DDS until a compromise was reached whereby the reponse of the BPF was as uniform as possible and attenuation least between the edges of the band.
The remaining bands were then aligned similarly.
Whilst performing this alignment I noticed interaction between all three of the band coils, adjustment of the centre coil seemed to have most effect on the output amplitude. However near resonance the input voltage amplitude of the BPF was also found to be notably more diminished and this was also found to be most effected by adjustment of the core nearest to the input, presumably both effects being as a result of loading on the DDS output due to the input impedance of the BPF becoming lower at resonance than that provided solely by the 50 ohms fixed resistor when the BPF is not near resonance.
I have taken this into account when computing the attenuation loss in the BPF for all the Bands aligned in this manner and the results are as follows.
80 Meter Band
KHz Vin mV pp Vout mV pp Loss dB
3,500 720 230 9.9
3,600 590 300 5.8
3,700 550 320 4.7
3,800 660 230 9.1
40 Meter Band
KHz Vin mV pp Vout mV pp Loss dB
7,000 800 250 10.2
7,100 710 320 6.9
7,200 680 320 6.6
7,300 760 240 9.9
30 Meter Band
KHz Vin mV pp Vout mV pp Loss dB
10,000 800 190 12.4
10,100 630 210 9.6
10,200 570 130 12.8
20 Meter Band
KHz Vin mV pp Vout mV pp Loss dB
14,000 860 510 4.6
14,100 790 600 2.4
14,200 800 620 2.2
14,300 860 550 3.9
14,400 900 450 6.0
17 Meter Band
KHz Vin mV pp Vout mV pp Loss dB
18,000 580 380 3.6
18,100 640 360 5.0
18,200 660 300 6.9
15 Meter Band
KHz Vin mV pp Vout mV pp Loss dB
21,000 480 190 8.0
21,100 430 230 5.5
21,200 440 250 4.9
21,300 480 240 6.0
21,400 500 200 8.0
21.500 480 160 9.6
12 Meter Band
KHz Vin mV pp Vout mV pp Loss dB
24,800 280 195 3.1
24,900 290 200 3.2
25,000 275 205 2.5
25,100 350 190 5.4
10 Meter Band
KHz Vin mV pp Vout mV pp Loss dB
28,000 325 150 6.7
28,500 215 190 1.1
29,000 270 215 1.9
29,500 365 180 6.2
29,700 365 150 7.7
The losses for some of the above bands seem to be higher than I would have expected whilst in other cases they seem to be lower, so I am open to any suggestions as to why this might or appear to be so and any comments regarding the validity of my measurement and alignment techniques would be appreciated.
The reason why I have not fitted the 160 Meter coils as yet is because according to theory ideally the windings on the coupling coils should each have an extra 3 turns as they are lacking on the standard coils chosen from those currently available from Spectrum Communications. Since these particular coils do not have screening cans fitted it would be comparitively easy to add the extra turns before the coils are fitted if later should I decide to do so.
In the meantime I would like to try in the next day or so to use the BPF in conjunction with an antenna and my G6LBQ transceiver PCB assembly and VU3CNS DDS to access it's performance in practice.
Finally a big thank you to Andy and Sunil without whose help this project would not have been possible.
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