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CHANNEL GUARD
High-frequency Crystal IF Filters

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CHANNEL GUARD


These crystal filters for 10.695 MHz CB receiver IF circuits are designed to reduce bleedover interference. They work by narrowing the "skirt" selectivity around the center of the radio's IF passband. Our filter uses two very high-Q quartz crystals and a compensating amplifier to achieve this result.

All CBs have tuned transformers between the IF amplifier stages to help produce the receiver's overall selectivity. However the "Q" (Selectivity Factor) of such transformers isn't nearly as high as that of a crystal - The graph shows this difference. Both curves at the insertion point are about 8 kHz wide. But the typical IF transformer is much more broadbanded. Even at +/- 8 kHz away from center, the attenuation is still minimum, about - 18 dB. Strong adjacent channel signals can easily cause bleedover interference to the desired signal. Compare this broad curve to that of the crystal filter. The skirts are very narrow, and at the same +/- 8 kHz, the attenuation can be -60 dB or better. The result: a dramatic drop in adjacent channel interference.


FILTER PLACEMENT
The exact placement of any filter in the receiver IF chain is very important, and can make a difference in receiver performance. There are two schools of thought about its location:
1: Placing the filter very early in the IF strip, such as immediately following the first mixer , will help establish the overall receiver selectivity. Many CB rigs only use a single conversion process for both SSB and AM- Or they convert down a second time for AM and FM later in the circuit. So placing the filter early in the IF strip means that it will work in all modes. 2: Noise is normally generated within the IF amplifiers themselves. If the filter is placed further down the IF strip toward the Detector end, such noise can be reduced. This results in improved sensitivity as well as selectivity. Noise is broadbanded in nature and the less noise that gets through to the Detector, the better the Signal-to-Noise Ratio of the receiver.

A few of the better base rigs already use two IF filters. All AM/SSB rigs use a multipole crystal filter, which is switched to operate on both RX and TX. This filter is the large metal can about 2" x 1" that you see on the main chassis. For RX, it provides basic IF selectivity. For the SSB TX mode, it filters out one of the two sidebands. The second filter (if present) is a simple monolithic device which looks like a regular crystal, with three wires instead of two. "Monolithic" means that it has two quartz elements that share the same physical ground, in a single crystal holder. It's a cheap way of using two "crystals" in one can, instead of two individual crystals (which are sharper) like our filters use.

In most installations the best placement is early on, immediately following the First Mixer stage. The extra selectivity will reduce extremely strong signals which might otherwise desensitize the receiver IF strip.

INSTALLATION & APPLICATION NOTES
For proper performance, the filter must be correctly installed in the radio's IF strip. This normally involves nothing more than removing an interstage coupling capacitor and installing the Channel. Guard's INPUT and OUTPUT wires into the correct empty capacitor holes. Then the middle bare wire is grounded, and the RED wire is run to the +DC voltage source.

TYPICAL BANDPASS FILTER
Many rigs use 2 or 3 stage transformer coupling. Each connects with a small (2.5pF) capacitor. Remove this and install CG filter here.

TYPICAL IF AMP STAGE
Each IF stage that uses a transistor amplifier is usually coupled by a capacitor. Remove this and install CG filter here.

SPECIAL NOTE: AM-only MODELS
Many older CBs of the AM-only type use an 9.785MHz or 11.275 MHz IF, not 10.695 MHz. The CHANNEL GUARD won't work with the models.

INSTALLATION INSTRUCTIONS
The sketch at right shows the filter connections. With the parts side up and the RED wire on your right, the three bare wires are (left to right): INPUT, GROUND, OUTPUT Place the INPUT and OUTPUT wires into the two holes vacated by the coupling capacitor you removed. Solder the center GROUND wire to any convenient ground point. You can often tack it to the nearest shielded metal IF can. Also shown schematically is the general installation point between IF amplifier stages. Connect our RED wire to a convenient source of DC operating voltage, usually + 8 - 13.8 VDC. The easiest connection for this is directly at the switch lugs of the ON/OFF/VOLUME control. Connect to the lug on the switched side, not the input side nis way, no power is applied unless the radio is turned on.

1. Always ground the center filter wire to the chassis ground or common point. Any of the metal IF transformer cans will take solder, and these make a good hookup point. This ground connection also provides the -dc return path to power the filter; the filter's amp won't be powered up without it. -And grounding helps remove stray signals that cause oscillations or other interference to proper filter operation.

2. The following radios use non-standard IFs, for which we have no crystal filters: Browning Mark III, Mark IV, IVA, Tram D2011D201A. However the second IF is 455 kHz and our 455 filter works well here.

3. The following radios cannot use the high-frequency filter: Sears 934.382613831 , 3827, and equivalent in J.C. Penney chassis such as 981- 6241,6246, 6248. These use an 1 1.275 MHz IF which is not capacitively coupled. The filter can't be installed in the signal path without breaking the DC supply to the mixer or IF transistor. The 455 kHz filter can be used, but for AM only.

4. Many radios that are AM-only can use the 10.695 MHz as well as the 455 kHz filter. The crystal filter is much sharper than the ceramic 455 types. Almost all 40-channel PLL AMs use a 10.695 MHz first IF. The older 23-channel AMs with 23/14 MHz synthesizers have an 1 1.275 MHz IF. There's no Channel GUARD, but our 1 1.275 MHz crystal ($10) can still be added to improve IF selectivity.

5. It's very important that the Channel. GUARD doesn't increase the IF gain. Our filter is designed for a zero-loss, zero-gain effect. Excess gain shows up as: higher than normal S-Meter reading, background noise, buzzing, squealing, or audio distortion. In such cases, you can try lowering the gain of the amplifier a bit. Increase the value of the emitter resistor slightly. This is the resistor immediately below the transistor. We use a 560ohm (green-blue-brown) value. Try a 680kohm or even 1kohm to rebias the amplifier. If this doesn't work, you have some other problem.

6. Be sure the filter is installed only in the Receive signal path, never in the Transmit signal path. Correct points are already shown for the models on our list. For other radios, remember that the SSB TX signal passes through the radio IF filter, typically by use of switching diodes. If you install the CHANNEL GUARD in this path, you may slice off part of the transmitted sideband, resulting in distorted audio or off-frequency operation.

7. It's always a good idea to recheck the alignment of the IF transformer cans just before and after the filter installation point. Consult your schematic. Make sure these coils are still peaked for maximum signal strength, as indicated on the rig's S-Meter. Filter installation sometimes has an effect on the alignment of these IF cans.

8. Avoid shorts! Black sleewing is provided to slip over the bare filter wires. You may also need to put some electrical tape on the solder side of the filter PC board, if the filter has any possibility of touching other nearby metal parts.

9. Sometimes it's not possible to stand the filter up vertically from the radio's PC board. In such installations, bend the filter leads at right angles and don't cut them off For extra physical support, solder another bare wire to one of the crystals and a nearby transformer coil. This forms a bridge, with the filter supported at each of its long ends.

10. Never install the filter in the Noise Blanker signal path. Any sharp filtering there can stretch out the noise pulses, making the blanker useless. This is a common problem in all the Uniden-type SSB IF circuits, unless you follow our specific hookup list. These radios use a bandpass filter consisting of two or three capacitively coupled coils, right after the first Mixer stage. The NB output switch usually ties in between the first coil pair. Use the later coil pair, further along the IF strip.

11. We don't specifically support non-CB type "Ham" rigs like the Uniden HR2510, AR3500, or RCI 2950. However, these radios are all just glorified CBs, and even use the standard Uniden-type IF circuit. Our 10.695 MHz filter works wonders in these models!

TYPICAL CHANNEL GUARD FILTER
Use 2 pcs. standard 10.695MHz X-Tals
and a transistor for amplification.