Doing it by Design
Eric Edwards GW8LJJ describes an easy way to match crystals for ladder filters.
Eric Edwards GW8LJJ describes an easy way to match crystals for ladder filters.
This project uses a Colpitts oscillator and a novel circuit to ‘see’ the offset frequencies of the crystal (xtal) that has been selected as the nearest to the wanted frequency for use in a ladder IF filter. It came about after talks with a friend of mine, Ray G7BHQ. We were discussing IF filters and I mentioned that I have always used ready-made ones. He said that good results can be achieved with ladder filters but is time consuming in matching the crystals using an oscillator and frequency counter (meter).
I thought there must be an easier way to check for matched crystals than by observing the individual results on a frequency counter. I remembered a unit I had built up here some time ago to monitor my VFO frequency when using the home-brewed valve AM transmitter. It is called ‘Netometer11’ and the purpose of it was to monitor easily either with LEDs or a meter if the transmitter VFO had drifted high or low frequency without connecting to a frequency counter to see the change in frequency. Netometer11 does not measure the VFO frequency but it shows whether the VFO has drifted high or low in frequency.
Netometer11
The Netometer11 is a design by Dave GW4GTE and has been made a kit in its own right. (See reference section). It was produced for monitoring the VFO of AM transmitters and has four memory positions. Selecting any of these allow storage of the station’s most frequently used AM frequencies. Once the frequency is stored, which is a very simple operation using toggle switches, the transmitted carrier can be seen if any change is made with the transmitter’s VFO. There are two sensitivity settings with a low setting used for the AM transmitter VFO and the higher sensitivity that has much narrower frequency spacings of tens of Hertz, which makes it ideal for monitoring the difference in frequency of the crystal plugged into the test oscillator unit. It is this sensitivity mode of the Netometer that is used for crystal matching.
The Crystal Oscillator
The circuit used, Fig. 1, is a very well-known Colpitts type and is easy to build with or without a PCB. The circuit is basic and while suitable for crystal testing, when used for an oscillator such as a VFO, the output should be buffered with an emitter follower. The crystal is plugged into either the larger socket for the older HC6/U types or an 8-way (DIL) socket for the more common HC49 series. The circuit comprises a 2N3904 transistor with 10kΩ resistors to create the base bias. One of these resistors is connected from the base to the 12V rail and the other from the base connection to ground (0V).
In conjunction with the added crystal, oscillations are set up by the two capacitors (470pF), one connected to the base with the other end connected to the emitter of the transistor. The other capacitor is also connected to the emitter with the other end to ground. A bypass resistor (2.2kΩ) is also connected from the emitter to ground. The output of the oscillator is via a PCB type BNC socket. This allows for easy connection to the monitor (Netometer11) or a frequency counter (meter).
In Use
The 12V power supply is connected to the Netometer11 and to the ‘+’ pin on the crystal tester. The ground (0V) is also connected to both units. A BNC lead is connected between the crystal checker and Netometer11. A crystal is selected that is to be the reference frequency and this will have been measured on a frequency counter. Crystals are not all ‘spot-on’ frequency so a compromise has to be made by selecting a crystal that is as close to the wanted frequency as possible. This will be the reference frequency and the other crystals will be ‘matched’ to it.
Plug the reference crystal into the tester and switch on the 12V. The toggle switches on the Netometer11 can be set to any of the 0 or 1 positions. These represent the memory locations and are set using the bi
nary notation. Placing both the switches in the 0 position can be called memory 1. Following the binary code, one switch in the ‘0’ position and the other in the ‘1’ position has changed the memory location to two. A combination of 0 and 1 will provide four memory locations. This will be useful for checking up to four crystal frequencies without re-entering the frequency every time it is used. The memory is retained after switch-off and is ready for use the next time it is switched on.
The Netometer11, when used for the first time at switch-on and the crystal tester connected with a reference crystal, will show a flashing end (red) LED (if the coloured LEDs are as used as in the construction taken from the manual. See reference section). Push the button and the centre LED (green) will illuminate. Release the button and the centre along with one of the green LEDs either side will also illuminate.
The frequency of the reference crystal has now been set. Remove the crystal with the 12V still connected and the centre LED will be flashing slowly, indicating that it is waiting for a crystal to be tested. Plug in another crystal and the lit LEDs will show any frequency differences, Fig. 2. Selecting a range of crystals it can be seen at a glance if they are within the limits required either above or below the reference frequency without working out the differences seen on a frequency counter.
PCB
The PCB, Fig. 3, is of a simple design and is made on a single-sided FR4 board with a ground plane on the copper track side. There is only one transistor as mentioned in the oscillator paragraph and that is sufficient for this application. There are two crystal plug-in inputs to allow for the usual HC49 and HC6/U types. The larger crystal is plugged into a PCB header seen on the far left of the PCB and the smaller crystals are plugged into an eight-pin DIL socket. The output is a PCB fitted BNC connector and a BNC lead can be conveniently connected from that socket to the Netometer11 or frequency meter. The power supply is 12V and is connected to a pair of PCB pins. A completed PCB is shown at Fig. 4 and the working Netometer in use at Fig. 5.
Is there a kit
I can supply the PCB and all parts for the oscillator as shown on my usual ‘picking list’ available by contacting me at my email address.
Netometer11 is available as a complete kit as shown in the reference section and a complete manual is available from the same source. I am grateful to Dave GW4GTE for permission to use Netometer11 in this project.
References
• Netometer11: (Dave GW4GTE S9kits) www.s9plus.com
• Ladder filter design: Dishal2052
• PCB and picking list: Eric GW8LJJ
• Ray G7BHQ for the conversations and help during this project