Crystal Filter

Using a DDS VFO, normally you only have the option of only one offset. Therefore both crystal filters have to be the same frequency. In this case, 3.547 MHz filters are used. One with 39pf caps resulting in a wide filter which works well for SSB and casual listening and the other with 100pf caps resulting in a narrow filter for CW operation.

For information on this type of filter, check out "Designing and Building Simple Crystal Filters," by Wes Hayward, W7ZOI, QST, July 1987, P 24. These filters were made with the information from that article. References at the bottom of the page give more information on making crystal lattice filters to fine tune the responses of these filters.

The IRED2 is used only if you are using two different crystal filter frequencies. In this case with both being 3.547 MHz filters, the IRED2 is replaced with an LED.

The programming for the PIC DDS VFO has provisions for seperate offsets for each band, so if one wants to use two different crystal filter frequencies it is possible to do with changes in the programming to change the offsets according to the crystal frequency used with a specific band. For more information on this check out the PIC DDS VFO Program Code

With AM detection, a wider bandwith cyrstal filter is needed. A one 3.547 MHz crystal filter with two 39pf capacitors provides the bandwidth for AM listening.

A little tighter bandwidth with a two crystal/39pf capacitor filter also works but seems to cut off the highs somewhat.

The tighter bandwidth allows weaker stations to be received in some situations.

The Crystal Oscillator on Board 2 will be using a 4.000 MHz crystal oscillator for the Second Mixer to provide 455 kHz for the IF strip. The oscillator is locked on to the 4 MHz oscillator by not using the BPX-38 photo receiver. A 3.547 MHz crystal and the Phototransistor (BPX-38) are not used.

The following information explains the switching circuit:

Diode switching is used to switch the crystals at the oscillator. LEDs are used instead of diodes. The LEDs provide an indicator of which crystal is turned on, providing diagnosis of proper operation of the switching circuit.

The crystal ground circuit first goes through a capacitor, and then to the LED which grounds the crystal. A switching circuit using IRFU220s switch the crystals and LEDs.

The capacitor serves two purposes. The first one is to trim the frequency of the crystals so that the BFO frequency is the same when switching between the crystal filters. A value of 200pf was found to get the frequencies close. An optional trim cap (unmarked holes on the PCB) was added in parallel to the 4.000 crystal capacitor, to help get them exact if the operator desires.

The second purpose of the capacitor is to block the DC voltage, which turn on the LEDs, from getting to the crystal. A typical diode switching circuit using 1N914s use 1ma or less. The LEDs use 10ma.

Wes Hayward's article, "Designing and Building Simple Crystal Filters," (QST, July 1987, Page 24) provided a revolution in building high performance homebrew receivers. After this article, any homebrewer could build crystal filters that performed exactly the way he preferred with no math involved. Hayward's design was installed in the receiver exactly as he suggested in his article.

Identical capacitors (39pf and 100pf) were used in the filters and provide a wide and narrow bandwidth. Raising the value of the capacitors tightens the bandwidth and lowering the value widens the bandwidth. Since the impedance is difficult to change, his other formula, raising/lowering the impedance tightens/widens the bandwidth, can't be used as easily.

A very sharp bandwidth can be achieved with 300pf capacitors, but there is an increase in the loss of the filter with increased capacitance. For good CW reception, 100pf gave decent selectivity with minimun loss. For SSB reception 39pf is used in the wide filter. Either value is not critical.

If AM SWL reception is desired, one or two crystals in the filter are removed and low value capacitors are used to provide the wide bandwidth needed for AM detection.

One of the best articles on designing crystal ladder filters was published recently in QEX, "Crystal Parameter Measurement and Ladder Cyrstal-Filter Design", Sept/Oct 2003, by Randy Evans, KJ6PO. He may have the easiest program for determining capacitor values for a ladder filter. Input crystal values into a spreadsheet program available at the ARRL QEX download site. Spreadsheet program file is 0309evans.zip.

This article shows promise of improving the filters used in the receiver. Also, computer audio spectrum analyzers show the response of crystal filters and can help in improving the design.

The output of this board is the box labeled "Xtal Filter Out", with two soldering pads. The two pads are used to install a loop of wire for easy soldering and unsoldering during building and testing.

The "Xtal Filter Out" is attached to the "Xtal Filter In" inside the box of the Second Mixer on the second board.

Almost all the spurs in this receiver are the result of the VFO frequency getting into the second mixer. Always use shielded coax between the crystal filter output and the input of the second mixer to keep signal pickup to a minimum.

The inputs are switched by two LEDs in series. The two LEDs in series gave the best isolation of all the techniques tried and was easy to implement.

While experimenting with the two-LED switching circuit, it was found that LEDs have widely varying capacitance values that effects the loss through the LEDs. Also, the resistance used in the ground circuit of the LEDs makes a large difference. I found the resistance used and the capacitance of the LEDs have a relationship.

The first LED in the switch has the most effect on the loss of the switching circuit. A visible LED is used and the resistor value used is 1K, which makes for a bright indicator. The resistor value used with the second set is 470 ohm. Lower values increase isolation and lowers the impedance to the crystal filter.

For the lowest loss through the switching circuit, use a 2.2K resistor with the first LEDs and a 1K resistor with the second set. The resistors are in the ground circuit of the LEDs.

The red LEDs supplied with the kit (the first LED in the switch) have about 5-10pf capacitance. LEDs with low capacitance values provide better isolation between the filters.

The super-bright LEDs have a much higher capacitance - approximately 100 to 200pf. A direct replacement of the stock LEDs (bright red) with the Super-brights provides the lowest loss of any of the combinations. You can lower the resistance values to 470 ohms and have the same loss as with the stock LEDs. This provides better isolation with the super brights. This will also increase the brightness of the super-bright LEDs.

It appeared that the lower the capacitance value of the LEDs, a higher resistance value was needed to keep losses at a minimum. Higher capacitance LEDs with 2.2K resistors have the least loss, but the isolation goes down slightly.

"Crystal Parameter Measurement and Ladder Cyrstal-Filter Design", QEX, Sept/Oct 2003, by Randy Evans, KJ6PO. Claims to have the easiest program for determining capacitor values for a ladder filter. Input crystal values into a spreadsheet program available at the ARRL QEX download site. Spreadsheet program file is 0309evans.zip.

"Ladder Crystal Filter Design", by J. A. Hardcastle, November 1980, QST, Page 20.

"A Unified Approach to the Design of Crystal Ladder Filters", by Wes Hayward, W7ZOI, May 1982, QST, Page 21. This was the first article that hinted at the fact that regular computer crystals or color burst crystals could be used in a very effective homebrew crystal filter.

"Designing and Building Simple Crystal Filters," by Wes Hayward, W7ZOI, QST, July 1987, P 24. This article showed how using the same capacitance for each capacitor in a crystal ladder filter gave very good results, very inexpensively, and easily adjusted bandwidth.

Crystal ladder filters - This page contains JavaScript tools to design and simulate crystal ladder filters which are widely used in SSB transmitters and CW receivers. Could be used to fine tune the xtal filters used in the receiver. Excellent page!