Crystal Filter

Board 1 || TV & FM, BC Filters || RF Amplifier || Bandpass Filters || First Mixer & Amplifier || Crystal Filter || Board 2

3.547MHz Crystal Filters

Crystal Filter/Crystal Oscillator Work in Unison
Adjusting Crystal Filter Bandwidth || 12 Volt Isolation and Shielding || Crystal Filter Inputs

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

Crystal Filter/Crystal Oscillator Work in Unison

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.

Adjusting Crystal Filter Bandwidth

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.

12 Volt Isolation and Shielding

All the 12 Volt lines were moved as far away from the crystal filter as possible. Since the 12 Volt line from the mixer/amplifier was subjected to the highest level of RF in the receiver, it was important to have the 12 Volt line isolated from the crystal filter.

Another isolation technique was to implement all the switching at the input of the crystal filters. The outputs are tied together with a 1:4 tapped transformer so that there is no 12 Volts near the output of the crystal filters.

Crystal filter shielding is important for a clean receiver, especially when using efficient outdoor antennas.

To further isolate the 12 Volts and stray signals from the crystal filter, a row of soldering pads to hold a shield, was installed between the switching circuit and the crystal filters.

To help crystal filter performance, shields can be installed between the first/second and second/third set of crystals. This provides maximum isolation for the output signal from the crystal filter for delivery to the second mixer. Soldering pads were placed at these locations to facilitate the mounting of the shields. The image below shows how the shields are installed.

This picture is from the ELR, but the shielding technique is exactly the same with the Blue Lightning Transceiver.

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.

Crystal Filter Inputs
Blue Lightning Transceiver

The diode swiching circuit shown above is called a Series-Shunt Switch using LEDs instead of PIN diodes normally used for this application.

This circuit was added because the two diode series switch used with the ELR with different crystal filter frequencies for each filter (3.547/4.000 MHz) did not have enough isolation when the same crystal filter frequency (3.547 MHz) was used for both filters in the BLT.

The first diode is the Series Switch which lets the signal pass when biased ON (LED on). When this LED is on, the second LED is off, the Shunt Switch, which shorts the signal going through it to ground. You want the second LED off in this case to allow the signal to pass freely into the filter.

When a filter is turned off, the first LED will be Off and the second LED will be ON, shorting the signal through to ground. Whatever passes through the first LED when OFF gets shorted to ground with the Shunt Switch, which is ON and passes any remaining signal to Ground.

A simple diagram of the Series_Shunt circuit is shown below. A single voltage is used to turn the dioded on/off, but notice the polarity of the diodes to the source of current. One is turned on at the same time the other is turned off.

The implemenation of this circuit in the receiver schematic shown above is a little more complicated in that the ground and V+ are done with the VN0106N3 switching circuit. The first LED of each filter (series switch) is tied to the second LED (shunt switch) of the other filter.

A circuit from Solid State Design, fig. 49, Page 91, shows input and output filter switches. In the case of the BLT, the input switch alone provided the necessary isolation and was easier to implement.

Some references for more information on PIN diode switches are the following:

How-and-why-to-use-pin-diodes-for-rf-switching

PIN diodes in RF switch applications

Pin_diode_handbook

References

"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!

The BLT Board 2 is the same as the ELR Board 2, so you will be directed to the ELR Board 2 Details

Send E-Mail || Amateur Radio Receivers || Blue Lightning Transceiver


Last Update: 11/8/2019
Web Author: David White, WN5Y
Free Website Translator