BandPass Filter Board

Bandpass Filter Board - Iss 11

Band Pass Filter Test Jig.

Socket receptacles are soldered where the ferrite transformer pins and capacitors go for easy fitting and removal

>MW< Add pictures of T1/T2 winding>MW<

Features 

The bandpass filter board (BPF Board) is in some regards might seem like a simple thing. Don’t be fooled. It is a central and extremely important part of any radio design. See it as a funnel, two directional for RX and TX. If there are any obstructions in this tunnel it will reflect significantly on the performance of both receiver and transmitter. When testing or finding one particular band, say, is not working well (low output power, or poor performance on receive), suspect the BPF first!, and re-test it. For this reason it is worth a lot of effort to get this board right, and indeed there have been quite a few prototypes and a lot of time spent in perfecting this board.

• • Up to 16 BandPass filters:

  • The firmware determines which band fits in which BPF Board bay, but normally:

  • 1800 & 630 Mtrs LF Bands in bays 1 & 2

  • 160 through 10 Mtrs HF Bands in bays 3 – 12

  • 8, 6, 4 Mtr Bands in bays 13, 14, 15

  • 2 Meters VHF Bands in bay 16, using a small sub-board with air-spaced coils rather than ferrite transformers

  • (Not all countries are allowed to transmit on some of these bands bands, but can they can be used cross-band on receive)

• A band is selected with 4 bit parallel TTL level logic (0000 – 1111 Binary) supplied to this board from the CPU board.  This board can also be used in non HamPi Radio projects easily using this 4 bit binary interface, or by individual band selection by not fitting the two 74LS156 decoder ICs U1 and U2 and feeding the board with a band selection using the DIL Sockets.  This can be done using say a simple rotary switch or by other means.

• Band switching is achieved using both PIN Diodes (genuine ones) on lower frequency bands (Bands 1-12) and small relays on the four VHF bands (Bands 13-16).

• Each Filter  bay has its own LED indicator which indicates the Band selected. It is extremely helpful (and less frustrating) to be tuning the correct ferrite transformers!

• An 18dB Attenuator.

• 18dB Pre-amplifier.

• 2 notch filters for tuning out Local Oscillator (8MHz) leakage. This is very important on TX.

• 2 x Relays for selection of external LF and VHF modules. The HamPi will not have the ability to transmit on all 16 bands due to the PA bandwidth.  (At least initially) Currently the PA will transmit on 160 to 4Mtrs. To transmit on the two LF and the 2Mtrs bands these relays provide for external linear amplifiers on these bands.

Construction

Below is a parts list of the hand mounted parts for this board: (Use sliders in the tables to view any hidden text)

Mounting the Hand Soldered parts

Carefully solder in all hand fitted parts going by the table below:

1. Fit Coax connectors J2 (HF In/Out to PA Board) and J9 (RF In/Out to Radio Board) 

2. Fit the G6K relays. K11 and K12. These select the Through / Attenuator and Pre-Amplifier

3. Option. The LF and the 2mtr bands:

• If you do not want these bands then a) Fit J1, 2 pin 0.1″ pin strip and fit a push on Pin Header Shorting Block Connector, and do not fit relays K1 and K2, and do not fit SMA coax sockets J3 and J6.

• If you want the full features rig, then fit relays K1 and K2, and fit SMA coax sockets J3 and J6. Do not fit link J1

4. Option. The 8, 6, 4 and 2mtr bands:

• If you do not want a particular or all of the bands over 10mtrs, then some $$ can be saved by not fitting some or all of the relays K3 to K10. Part of the footprint on each of the PCB of these relays K3 to K10 is a solder link (they look like small D shaped backing onto each other. Carefully put a blob of solder across these solder links. (K3 & K4 are for 8mtr band, K5 & K6 for 6mtr band, K7 & K8 for 4mtr band and J9 & K10 for the 2mtr band)

5. Fit SW1, a 16 position 4 bit binary to 1 of 16 decimal rotary switch. This is used to select one filter at a time for testing / adjustment. This is a Surface Mount part.

6. Fit 2 x J310 surface mount JFET devices in the pre-amplifier

7. Wind and fit T1 & T2, 2 x ferrite binocular transformers. (Instructions and pictures are in the text below).

8. Fit all connectors and sockets, carefully observing the correct orientation. J11, J15, J1, J13  (2 pin strip), J15 (2 pin strip),  U1 & U2. (J13 and J15 are for testing /selecting the Attenuator (J13 ) or the Pre-amplifier (J15 ) RF path).

9. Fit IC1 or IC2, 78LS156 integrated circuits.

10. Fit Inductors L12 and L13. These are 1mH through hole parts (THT) parts. Carefully bend one of the wire ends and pass the two wires through the PCB and solder on the back side. (SMD inductors were found to not be suitable for the task). L12 is located close to U2 and L13 is located on the top of the board above bay 6/7

11. Fit Inductor L7 in a similar way to above. This is in the pre-amplifier section.

12. Fit J11, a 16 pin IDC Header. This eventually connects the BPF board to the CPU board with a 16 way ribbon cable.  This cable is not needed for testing / alignment. Hint: If you happen to solder this the wrong way around! No need to unsolder it to correct this error. Use strong pliers and carefully pull off the black plastic surround / body from J11 pins by pulling a little bit at a time each end, and simply turn it around and push it back on. (Guess who has done this at least once!)

13. Fit J10, a white 2 pin JST plug. This supplies 12V / 13.8V to the board for standalone testing, with no other HamPi boards needed.

14. It is suggested to not fit IFT1 and IFT2 10mm coils (cans) as yet. These are two notch filters that filter out any IF frequency leakage, of which there will be some. They are very important and must be eventually fitted before going live on air. You don’t want to interfere with anyone listening on your IF frequency (say 8MHz)!  Once the 40mtr and 30mtr bands are installed and working, then these notch filters are fitted and adjusted.

Notes on above band components:

1. The results are for the current ISS 11 board. Some cells of the table above are still blank, as work continues.

2. Capacitor choice is very  important and lossy capacitors will return poor results. Use Multi-Layer NPO / C0G SMD or through hole capacitors for best performance. Ceramic disk capacitors have been used on occasion but results vary.

3. The footprints for all band capacitors allow for both surface mounted parts and leaded parts. Both SMD and leaded capacitors can be soldered on the top or the bottom of the PCB which can be very convenient. Watch out for excessive loss.

4.  If the loss is more than in the table, something is wrong. Take time using the test jog until results are very similar or better than those above.

5.  Not all of these bands are available in all countries, and the frequency allocations can vary. This does not mean you cannot listen in on these frequencies, and work split band if you are unable to transmit on a particular band. Remember, as a Radio Amateur you are entirely responsible for where and what you transmit, and keeping to the regulations.

Test Jig preparation

The test jig is intended for testing only using through hole capacitors. Any NPO / C0G SMD capacitors can usually be relied upon to be of good quality and soldered straight on the main BPF board. Before using the test jig there is some preparation needed (if not already done as supplied):

1. J3, J4, J7, J8, & J9 are intended only for testing using the centre tap of the transformers. Centre taps are not used on any HamPi configurations, and therefore these can have links fitted. 

2. Using some short tinned copper wire links, on J3, J4, J7, J8, & J9 link pins 1 & 2 together and pins 4 & 5 together.

3. Fit 1.5mm diameter receptacle pins to all transformer PCB holes. There are 5 holes per transformer.

4. Fit smaller ??mm pins to the capacitor PCB holes. In general only two pins will be needed per capacitor. (Look carefully att he capacitor footprints. There is a single home, a space and then three more holes close together. These three holes are always connected together. This allows capacitors with different lead spacing to be connected neatly).

5. Solder in J1 & J2 pin headers. Each is 2 pins.  Normally fit a shorting link, unless it is the 220mtr band being tested where these links are removed to put T2 and T4 into circuit.

6. Solder on the two SMA coax connectors J5 & J6

The test jig is now ready for use

Testing and Alignment in the Test Jig

1. Decide what test equipment is to be used to test these bandpass filters. A spectrum analyser with tracking generator is probably the best choice, but not everyone owns such a luxury. (you might have a friend with one, or possibly the local Ham Radio club?).  A Mini-VNS is also adequate for the job, but more fiddly.

2. Calibration. With a spectrum analyser set the output to -20dBm (this is often the normal level anyway) and connect the two coax test cables needed together in series and to output and input and check that there is a minimum or no loss from the -20dBm level. With a VNA do a calibration procedure with both coax cables in series, and then save to a memory for later use.

3. Connect the test jig to the test equipment to be used using the mini coax cables.

4. Plug in the transformers and capacitors for a band to be tested. Unless testing the 2200mtr band this will be T1, T3, & T5, and C1 to C5.

5. Set the test equipment to the correct centre test frequency and a suitable bandwidth and the waveform should be on the screen.

6. Not carefully adjust the transformer cores to achieve an acceptable shape. USE ONLY THE CORRECT ADJUSTMENT TOOL on the transformers. The ferrite adjustment slugs are easy to damage and can be hard to remove and replace. BE VERY GENTLE when turning the slugs.

7. When the particular band is looking good, the parts can be transferred to the main BPF PCB in the correct Bay (1-16)

8. It is suggested to be one band / bay at a time, taking time and care to get it done correctly. (The 'Do it right, do it once principle'). Rework and fault finding can be very difficult and disappointing.

Initial BPF Board preparation

1. It is assumed here that the above construction has been done and all hand mounted parts fitted to the BPF Board.

2. Carefully inspect the main BPF board to ensure all the necessary parts have been correctly mounted.

3. Ensure the two 74LS156 TTL devices IC1 and IC2 are fitted in the DIL sockets U1 and U2 the correct way around.

4. Construct a test power lead and solder to a 2 pin JST power plug which will plug into J10 on the board. Be careful that the polarity is the correct way around (or the board will be damaged when power is applied!).  Look at the Circuit Diagram and PCB to get this correct. J10 is on the bottom right of the board and clearly marked with a + and - in silkscreen.

5. Apply power to J10.  Either 12V or 13.8V can be used.  (It is suggested that the power supply used for testing has a low current capability. As an example we use a 13.8V high current supply with a 7912 1A regulator in a small box to reduce the current. If the full 20 (or in the case here 43A) supply is used and something accidentally gets shorted expecting PCB tracking to light up with smoke!).

6. One of the Band bay LEDs will light up, giving reassurance that the supply is on and the board powered up.

7. By rotating SW1 the desired band bay can be selected and the corresponding LED will light up. (Note, Later on when the BPF Board is completed and connected to the CPU board with the 16 way ribbon cable this switch SW1 must be in position 0. The LED for Bay 4 will be on. This is the 'off' position and the BPF band can then be selected correctly by the CPU. If the CUP is connected and two LEDS are on this indicates SW! is ont in position 0).

Testing and Alignment of individual bands in the main BPF PCB

1. Remove the power.

2. Solder in the Band Pass 10mm transformers and associated capacitors into the correct Band / Bay.

3. Select the Band under test using SW1

4. Connect the two coax test cables to the test equipment. J2 is the signal injection and J9 the measurement output.

5. Ensure links J13 & J15 are not fitted. These are to select the Attenuator and Pr-Amplifier, and these are tested later on.

6. If relays K1 and K2 were not fitted, then ensure there is a link fitted on J1.

7. Apply power to the board and the measuring equipment should show the band waveform very similar to that seen on the test jig.

8. A very slight adjustment of the transformer cores may be needed to restore a good waveform shape.

9. The rest of the bands can now be set up and completed in a similar fashion.

Setup of the two IF Notch Filters

1. Power off

2. IF two Notch Filters utilise parts IFT38 +  C2 + C13 and IFT2 + C75 + C75 and are adjusted as follows.

3. Solder IFT1 and IFT2 in the correct locations. The 4 capacitors should already be fitted (SMD). If not fit these capacitors in the correct locations.

4. Power on the board and select the 40mtr band bandpass filter using SW1.

5. Monitor the waveform on the test equipment.  It might not look good, but do not despair!

6. A large IF Signal is now needed. This can be done with a Spectrum Analyser by increasing the output level, if possible, to 0dBm. Another way is to use a Signal Generator set to 8MHz (Or if the Radio Board has already been built and calibrated the actual IF Frequency. This is usually just below 8MHz, for example 7.998500MHz. This is a bit precise, and unnecessary as the notch filters set to 8MHz are not 'sharp enough' to make any difference).

7. Now carefully adjust IFT1 to reduce the signal observed to as little as possible (Notching out the IF 8MHx signal). Next to the same with IFT2 to reduce the signal even more. This might be hard to do as the other notch can be so good already the second notch cannot be seen. The way around this is to do the first notch and then using a pencil or narrow marker pen put a mark on the transformer can next to the slug groove. Then move the slug about a quarter of a turn. Nest adjust the other notch IFT to the minimum signal. Finally put the slug in the other IFT back to the mark made. All clear?  It will be when you do the procedure.

8. Once the two Notch Filters are tuned for the minimum signal on a Spectrum Analyser, with a 0dbm input the result should about -65dbm (not far above the noise floor)  and for a 10dbm input to around -59dbm.

And finally...

1.  Once all the bands are set up and the notch filters adjusted for the best IF rejection, there should never be a need to do any more adjustments to this board in the lifetime of the HamPiRadio on this board.

2. Attenuator and Pre-Amplifier testing. Set the test equipment to say the 20mtr band and also Set SW1 to 20mtrs.

3. Put a link on J13 and observe an 18dB drop in the signal level.

4. Remove link J13 and fit link J15. Observe an increase in signal level of around 18dB.

5. Remove link on J13.

6. Ensure SW1 is set to the 0 position.

7. The BandPass filter Board is now completed and ready for integration into the HamPi Radio

Above drawings of T1 & T2. (See notes on construction of T1 & T2)

Information Notes:

1. Note: Preamp is a supplied 12V directly from the CPU board, and the Through and Attenuator run on 5V sourced on this board – hence the different resistor values to maintain a similar current through the PIN diodes.

2. The 2200Mtr band has transformer values out of the ordinary, but by selecting different pins and in one case removing some turns standard values can be made to work.

3. The HP32 is a modified HP100 and not normally needed.  Yellow transformer. Open up the can by turning the yellow core clockwise. Keep turning and the metal outer can will be screwed off the core. Carefully cut the tiny wire to pin 3 and unwind ~12 turns until the wire ends at pin 2 (centre). Carefully unsolder or pull the wire off pin 2 and continue to unwind a further 10 turns. Solder the wire back onto pin 3. Lastly reassemble the can. In this way a centre tapped 100uH core is converted to a single core (no centre tap) 32uH core.

4. The HP Cores are numbered as follows. HP-PSSS, where HP Is for HamPi, the P for primary turns, the SSS for Secondary Turns on any winding bay: 1024= 1 primary and 0+2+4  secondary,  2224= 2 primary and 2+2+4 secondary, 3245= 3 primary and 2+4+5 secondary, 4356= 4 primary and 3+5+6 secondary.

5. The ID HPxxxx 10mm Ferrite Transformers (above) and available from this website are custom made for the HamPi project. These were specially developed to manufactured to achieve the best results and a lower price. Other transformers can be used such as TOKO (these can be hard to get now) or Spectrum Communications (UK based) has their own range, but they will have to be carefully chosen and tested.

6. The two IF Notch Filters (IFT1 & R2) on this BPF Board must be carefully tuned to the IF frequency to tune out any IF frequency leakage going into (RX) or out of (TX) the Radio Board. To do this introduce a large signal (0dbm or 10dbm) at the IF frequency (For example 7.9985MHz). Tune the two Notch Filters carefully for the minimum signal on a Spectrum Analyser. for 0dbm in this should tune down to about -65dbm (not far above the noise floor) or for a 10dbm input to around -59dbm.

7. The 11mtr and 10mtr bands are now combined as they are so close together. (Some countries such as New Zealand allow Radio Amateur radio use on certain frequencies)

8. The VHF bands (8, 6, 4 & 2mtr) can have a small sub-pcb boards that mount in place of what would be filters on the main BPF PCB. These 4 bands are in positions 13 to 16 on the BPF Board. These filters have open spaced wire coils and SMD capacitors. They will be supplied as unpopulated PCBs or Populated and tested. The 2M bandpass filter is now finalised. (Currently however it has only been found necessary to use a sub-board on the 2m band (position 16 on the board) as the 10mm transformer cans work adequately).

9. As supplied the HamPi operates with an Intermediate Frequency of 8MHz. Other IF frequencies are possible, but some hardware would need re-design by the constructor (different components only). The software is geared up and working for this. There is an added complexity when using a 10.7MHz IF (Or other IF frequency close to a HAM band) in that the skirt of the two notch filters on this board will affect the bandpass filter for 30 meters (10.1MHz). Since this is a low power band it won’t matter on TX too much as there will be enough drive, but in receive it could add extra loss to the filter. Tune these two notch filters exactly on the LO frequency, 10.7, or 9MHz etc. This stops any stray IF signal affecting the transmitter and also stops any stray IF being seen at the antenna and radiated on receive (and upsetting your old AM/FM radios in the house).

Ferrite Transformers

1. T1 (preamp) uses a ferrite BN43-2402 2t : 8t ct, using 0.2mm Enamelled Copper Wire (ECW). This means two turns on the primary, and 8 centre tapped (ct) on the secondary (4 turns + 4 turns). One turn is counted as the wire passing through one hole AND back through the other hole.

2. T2 (preamp) also uses a ferrite BN43-2402 10t ct : 2t, 0.2mm ECW. This means ten turns centre tapped (5 turns + 5 turns) on the primary, and 2 turns on the secondary.

3. Other transformer ferrite options have been banded around on the internet for similar pre-amps including BN61-202 (0.5dB Less NF) and BN-73-202 . I have used various ones larger than the BN43-2402 and they work OK, generally when using the type 43 ferrite material.  If you are experimenting remember to test for coverage on all the bands you want the pre-amp to work on.

4. In general a pre-amp will not be needed below 10MHz when a reasonable antenna is used.  On higher bands and less than ideal antennas a pre-amp can be of help when the band is not fully open.   A pre-amp will add some noise that will be heard in the audio. 

The LF 2200Mtrs / 160kHz Band 1

2200 Metres: A Unique and Exciting Challenge for Amateur Radio. The 2200 metre band is a true exception in amateur radio and stands apart from other bands. Regulations for its use vary considerably from country to country, so it’s important to check the local rules before operating.

There are some challenges:

• High Noise Levels – The 2200m band is extremely noisy, presenting a real operating challenge.

• Antenna Difficulties – Efficient antennas are hard to construct due to the long wavelength.

For example, the maths tells us: to build a half-wavelength antenna for 2200m (0.136 MHz), you would need:
Half-wavelength = (300 / 0.136 MHz) / 2 × 0.95 ≈ 1048 metres (0.95 is the reduction of the speed of light  (95%) in the copper / aluminium of the antenna)

That’s over a kilometre—clearly not practical for most operators! (Mind you the electric fence here is about 450mtrs long, and I have been eyeing it up lately!  Even though it is only about 1.5m off the ground it might tune up and work on WSPR… (When there are no livestock here and the electric fence can be turned off that is!)

But hey, don’t let that stop you. Don’t let the challenges put you off. Amateur radio is all about creativity, innovation and experimentation. You have probably been listening to Medium Wave and Long Wave for decades with your ‘Transistor’, and that all on an internal ferrite antenna.

The good news is that modern modes like WSPR (Weak Signal Propagation Reporter) open up exciting possibilities on the 2200m band. WSPR is designed specifically for challenging conditions: it lets amateurs achieve contacts and gather propagation data even with modest antennas, despite high noise and low signal levels.

2200Mtrs Filtering on the HamPiRadio

To operate on this band, some choices will need to be made, depending on the bandwidth you want to operate on. If you want the best filtering to remove noise, a very narrow bandwidth (WSPR, for instance, uses only 200 Hz) is the best choice. If the bandpass is designed for the full 60 kHz available (for instance in New Zealand), this can also be done, but opens the door for increased interference and noise.

The BPF and a Test Jig PCBs have both been laid out for experimentation, and no final component suggestions made at this point. The BPF has been trialled for the 130-140kHz narrower band and the full 130-190kHz band using 5th order Cherbyshev filter design.

These are some possibilities for configuration for the while 130-190kHz bandwidth. The first is a Butterworth configuration and the second a Chebychev configuration.  More to follow in the future no doubt, and it is hoped that contributions from builders.